Troubleshooting BGP

Troubleshooting BGP
Philip Smith <pfs@cisco.com>
NANOG 33
Las Vegas
30 January – 1 February 2005
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Presentation Slides

• Available on
ftp://ftp-eng.cisco.com
/pfs/seminars/NANOG33-BGP-Troubleshooting.pdf

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Assumptions

• Presentation assumes working knowledge of
BGP
Beginner and Intermediate experience of protocol

• Knowledge of Cisco CLI
Hopefully you can translate concepts into your own
router CLI

• If in any doubt, please ask!

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Agenda

• Fundamentals of Troubleshooting
• Local Configuration Problems
• Internet Reachability Problems

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Fundamentals:
Problem Areas

• First step is to recognise what usually
causes problems
• Possible Problem Areas:
Misconfiguration
Configuration errors caused by bad documentation,
misunderstanding of concepts, poor communication
between colleagues or departments

Human error
Typos, using wrong commands, accidents, poorly
planned maintenance activities
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Fundamentals:
Problem Areas

• More Possible Problem Areas:
“feature behaviour”
Or – “it used to do this with Release X.Y(a) but
Release X.Y(b) does that”

Interoperability issues
Differences in interpretation of RFC1771 and its
developments

Those beyond your control
Upstream ISP or peers make a change which has an
unforeseen impact on your network
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Fundamentals:
Working on Solutions

• Next step is to try and fix the problem
And this is not about diving into network and
trying random commands on random routers,
just to “see what difference this makes”
• The best procedure for “unfamiliar
problems” is to
Start at one place,
Deal with one symptom, and learn more
about it
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Fundamentals:
Working on Solutions

• Remember! Troubleshooting is about:
Not panicking
Creating a checklist
Working to that checklist
Starting at the bottom and working up

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Fundamentals:
Checklists

• This presentation will have references in
the later stages to checklists
They are the best way to work to a solution
They are what many NOC staff follow when
diagnosing and solving network problems
It may seem daft to start with simple tests
when the problem looks complex
But quite often the apparently complex can be
solved quite easily

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Fundamentals:
Tools

• Use system and network logs as an aid
• Record keeping:
Good and detailed system logs
Last known good configuration
History trail of working configurations and all
intermediate changes
Record of commands entered on routers and
other network devices

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Fundamentals:
Tools

• Familiarise yourself with the router’s
tools:
Is logging of the BGP process enabled?
(And is it captured/recorded off the router?)
Are you familiar with the BGP debug process
and commands (if available)
Check vendor documentation before switching on
full BGP debugging – you might get fewer surprises

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Fundamentals:
Tools

• Traffic and traffic flow measurement in the
network
Unexplained change in traffic levels on an
interface, a connection, a peering,…
Correlation of customer feedback on network
or connectivity issues…

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Agenda

• Fundamentals

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Local Configuration Problems

• Peer Establishment
• Missing Routes
• Inconsistent Route Selection
• Loops and Convergence Issues

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Peer Establishment

• Routers establish a TCP session
Port 179 – Permit in interface filters
IP connectivity (route from IGP)
• OPEN messages are exchanged
Peering addresses must match the
TCP session
Local AS configuration parameters

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Common Problems

• Sessions are not established
No IP reachability
Incorrect configuration
• Peers are flapping
Layer 2 problems

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Peer Establishment:
Diagram

1.1.1.1 2.2.2.2
iBGP
? R1 R2 eBGP
3.3.3.3
AS 1
R3
?
AS 2
R2#sh run | begin ^router bgp
router bgp 1
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 1

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Peer Establishment:
Symptoms

R2#show ip bgp summary
BGP router identifier 2.2.2.2, local AS number 1
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State
1.1.1.1 4 1 0 0 0 0 0 never Active
3.3.3.3 4 2 0 0 0 0 0 never Idle

• Both peers are having problems
State may change between Active, Idle and Connect

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Peer Establishment

• Is the Local AS configured correctly?
• Is the remote-as assigned correctly?
• Verify with your diagram or other documentation!
Local AS

R2# iBGP Peer
router bgp 1
eBGP Peer

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Peer Establishment:
iBGP

• Assume that IP connectivity has been checked
• Check TCP to find out what connections we are accepting
R2#show tcp brief all
TCB Local Address Foreign Address (state)
005F2934 *.179 3.3.3.3.* LISTEN
0063F3D4 *.179 1.1.1.1.* LISTEN

We Are Listening for TCP Connections for Port 179 for the
Configured Peering Addresses Only!

R2#debug ip tcp transactions
TCP special event debugging is on
R2#
TCP: sending RST, seq 0, ack 2500483296
TCP: sent RST to 4.4.4.4:26385 from 2.2.2.2:179

Remote Is Trying to Open the Session from 4.4.4.4 Address…
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Peer Establishment:
iBGP

What about Us?
R2#debug ip bgp
BGP debugging is on
R2#
BGP: 1.1.1.1 open active, local address 4.4.4.5
BGP: 1.1.1.1 open failed: Connection refused by remote host

We Are Trying to Open the Session from 4.4.4.5 Address…
R2#sh ip route 1.1.1.1
Routing entry for 1.1.1.1/32
Known via "static", distance 1, metric 0 (connected)
* directly connected, via Serial1
Route metric is 0, traffic share count is 1

R2#show ip interface brief | include Serial1
Serial1 4.4.4.5 YES manual up up

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Peer Establishment:
iBGP

• Source address is the outgoing interface towards the
destination but peering in this case is using loopback
interfaces!
• Force both routers to source from the correct interface
• Use “update-source” to specify the loopback when
loopback peering

R2#
router bgp 1
neighbor 1.1.1.1 update-source Loopback0

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Peer Establishment:
iBGP – Summary

• Assume that IP connectivity has been checked
Including IGP reachability between peers
• Check TCP to find out what connections we are accepting
Check the ports and source/destination addresses
Do they match the configuration?

• Common problem:
iBGP is run between loopback interfaces on router (for
stability), but the configuration is missing from the router ⇒
iBGP fails to establish
Remember that source address is the IP address of the
outgoing interface unless otherwise specified

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Peer Establishment:
Diagram

1.1.1.1 2.2.2.2
iBGP
R1 R2 eBGP
3.3.3.3
AS 1
R3
?
AS 2

• R1 is established now
• The eBGP session is still having trouble!

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Peer Establishment:
eBGP

• Trying to load-balance over multiple links to the
eBGP peer
• Verify IP connectivity
Check the routing table
Use ping/trace to verify two way reachability
R2#ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms

• Routing towards destination correct, but…
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Peer Establishment:
eBGP

R2#ping ip
Target IP address: 3.3.3.3
Extended commands [n]: y
Source address or interface: 2.2.2.2
.....
Success rate is 0 percent (0/5)

• Use extended pings to test loopback to loopback
connectivity
• R3 does not have a route to our loopback, 2.2.2.2

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Peer Establishment:
eBGP

• Assume R3 added a route to 2.2.2.2
• Still having problems…
R2#sh ip bgp neigh 3.3.3.3
BGP neighbor is 3.3.3.3, remote AS 2, external link
BGP version 4, remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:04, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Route refresh request: received 0, sent 0
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
BGP table version 1, neighbor version 0
Index 2, Offset 0, Mask 0x4
0 accepted prefixes consume 0 bytes
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 0; dropped 0
Last reset never
External BGP neighbor not directly connected.
No active TCP connection

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Peer Establishment:
eBGP

R2#
router bgp 1
neighbor 3.3.3.3 ebgp-multihop 2

• eBGP peers are normally directly connected
By default, TTL is set to 1 for eBGP peers
If not directly connected, specify ebgp-multihop

• At this point, the session should come up

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Peer Establishment:
eBGP


Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State
3.3.3.3 4 2 10 26 0 0 0 never Active

• Still having trouble!
Connectivity issues have already
been checked and corrected

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Peer Establishment:
eBGP

R2#debug ip bgp events
14:06:37: BGP: 3.3.3.3 open active, local address 2.2.2.2
14:06:37: BGP: 3.3.3.3 went from Active to OpenSent
14:06:37: BGP: 3.3.3.3 sending OPEN, version 4
14:06:37: BGP: 3.3.3.3 received NOTIFICATION 2/2
(peer in wrong AS) 2 bytes 0001
14:06:37: BGP: 3.3.3.3 remote close, state CLOSEWAIT
14:06:37: BGP: service reset requests
14:06:37: BGP: 3.3.3.3 went from OpenSent to Idle
14:06:37: BGP: 3.3.3.3 closing

• If an error is detected, a notification is sent and the session
is closed
• R3 is configured incorrectly
Has “neighbor 2.2.2.2 remote-as 10”
Should have “neighbor 2.2.2.2 remote-as 1”
• After R3 makes this correction the session should come up

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Peer Establishment:
eBGP – Summary

• Remember to allow TCP/179 through edge filters
access-list 100 permit tcp host 3.3.3.3 eq 179 host 2.2.2.2
access-list 100 permit tcp host 3.3.3.3 host 2.2.2.2 eq 179

• Be very careful with multihop eBGP
Check IP connectivity (local and remote routing tables)
Remember to source updates from loopback
Watch for filters anywhere in the path
TTL must be at least 2 for ebgp-multihop between
directly connected neighbours
Use TTL value carefully

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Peer Establishment:
Passwords

• Using passwords on iBGP and eBGP sessions
Link won’t come up
Been through all the previous troubleshooting steps


Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
3.3.3.3 4 2 10 26 0 0 0 never Active

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Peer Establishment:
Passwords
R2#
router bgp 1
neighbor 3.3.3.3 password 7 05080F1C221C

• Configuration on R2 looks fine!
• Check the log messages – enable “log-neighbor-changes”
%TCP-6-BADAUTH: No MD5 digest from 3.3.3.3:179 to
2.2.2.2:11272
2.2.2.2:11272
2.2.2.2:11272

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Peer Establishment:
Passwords
R3#
router bgp 2

• Check configuration on R3
Password is missing from the eBGP configuration

• Fix the R3 configuration
Peering should now come up!
But it does not

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Peer Establishment:
Passwords

• Let’s look at the log messages again for any clues
R2#
%TCP-6-BADAUTH: Invalid MD5 digest from
3.3.3.3:11024 to 2.2.2.2:179
3.3.3.3:11024 to 2.2.2.2:179
3.3.3.3:11024 to 2.2.2.2:179

• We are getting invalid MD5 digest messages – password
mismatch!
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Peer Establishment:
Passwords

• We must have typo’ed the password on
one of the peering routers
Fix the password – best to re-enter password
on both routers
eBGP session now comes up

%TCP-6-BADAUTH: Invalid MD5 digest from 3.3.3.3:11027
to 2.2.2.2:179
%BGP-5-ADJCHANGE: neighbor 3.3.3.3 Up

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Peer Establishment:
Passwords – Summary

• Common problems:
Missing password – needs to be on both ends
Cut and paste errors – don’t!
Typographical & transcription errors
Capitalisation, extra characters, white space…
• Common solutions:
Check for symptoms/messages in the logs
Re-enter passwords using keyboard, from scratch
– don’t cut&paste
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Flapping Peer:
Common Symptoms

AS 1 AS 2

R1 eBGP R2

Layer 2

• Symptoms – the eBGP session flaps
• eBGP peering establishes, then
drops, re-establishes, then drops,…
NANOG 33 38

Flapping Peer

• Ensure BGP neighbour logging is enabled
no logs → no clue what is going on
• R1 and R2 are peering over some 3rd party L2 network

R2#
%BGP-5-ADJCHANGE: neighbor 1.1.1.1 Down BGP Notification sent
%BGP-3-NOTIFICATION: sent to neighbor 1.1.1.1 4/0 (hold time
expired) 0 bytes
R2#show ip bgp neighbor 1.1.1.1 | include Last reset
Last reset 00:01:02, due to BGP Notification sent, hold time
expired

• We are not receiving keepalives from the other side!

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Flapping Peer

• Let’s take a look at our peer!

2.2.2.2 4 2 53 284 10167 0 97 00:02:15 0

R1#show ip bgp summary | begin Neighbor
2.2.2.2 4 2 53 284 10167 0 98 00:03:04 0

• Hellos are stuck in OutQ behind update packets!
• Notice that the MsgSent counter has not moved

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Flapping Peer
R1#ping 2.2.2.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/21/24 ms

R1#ping ip
Target IP address: 2.2.2.2
Repeat count [5]:
Datagram size [100]: 1500
Timeout in seconds [2]:
Extended commands [n]:
Sweep range of sizes [n]:
.....
Success rate is 0 percent (0/5)

• Normal pings work but a 1500byte ping fails?
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Flapping Peer:
Diagnosis and Solution

• Diagnosis
Keepalives get lost because they get stuck in the router’s
queue behind BGP update packets.
BGP update packets are packed to the size of the MTU –
keepalives and BGP OPEN packets are not packed to the size
of the MTU ⇒ Path MTU problems
Use ping with different size packets to confirm the above –
100byte ping succeeds, 1500byte ping fails = MTU problem
somewhere
• Solution
Pass the problem to the L2 folks – but be helpful, try and
pinpoint using ping where the problem might be in the
network

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Flapping Peer:
Other Common Problems

• Remote router rebooting continually (typical with a 3-5
minute BGP peering cycle time)
• Remote router BGP process unstable, restarting
• Traffic Shaping & Rate Limiting parameters
• MTU incorrectly set on links, PMTU discovery disabled on
router
• For non-ATM/FR links, instability in the L2 point-to-point
circuits
Faulty MUXes, bad connectors, interoperability problems,
PPP problems, satellite or radio problems, weather, etc. The
list is endless – your L2 folks should know how to solve them
For you, ping is the tool to use

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Flapping Peer:
Fixed!

AS 1 AS 2

R1 eBGP R2

Layer 2

Small Packets
Large Packets

• Large packets are ok now
• BGP session is stable!
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• Missing Routes

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Quick Review

• Once the session has been established,
UPDATEs are exchanged
All the locally known routes
Only the bestpath is advertised
• Incremental UPDATE messages are
exchanged afterwards

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Quick Review

• Bestpath received from eBGP peer
Advertise to all peers
• Bestpath received from iBGP peer
Advertise only to eBGP peers
A full iBGP mesh must exist
(Unless we are using Route Reflectors)

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Missing Routes

• Route Origination
• UPDATE Exchange
• Filtering
• iBGP mesh problems

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Missing Routes:
Route Origination

• Common problem occurs when putting
prefixes into the BGP table
• BGP table is NOT the RIB
(RIB = Routing Information Base – a.k.a the
Routing Table)
BGP table, as with OSPF table, ISIS table,
static routes, etc, is used to feed the RIB, and
hence the FIB
Each routing protocol has a different priority
or “distance”

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Missing Routes:
Route Origination

• To get a prefix into BGP, it must exist in
another routing process too, typically:
Static route pointing to customer (for customer
routes into your iBGP)
Static route pointing to Null (for aggregates
you want to put into your eBGP)

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Route Origination:
Example I

• Network statement
R1# show run | include 200.200.0.0
network 200.200.0.0 mask 255.255.252.0

• BGP is not originating the route???
R1# show ip bgp | include 200.200.0.0
R1#

• Do we have the exact route?
R1# show ip route 200.200.0.0 255.255.252.0
% Network not in table

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Route Origination:
Example I

• Nail down routes you want to originate
ip route 200.200.0.0 255.255.252.0 Null0 254

• Check the RIB
R1# show ip route 200.200.0.0 255.255.252.0
200.200.0.0/22 is subnetted, 1 subnets
S 200.200.0.0 [1/0] via Null 0

• BGP originates the route!!
R1# show ip bgp | include 200.200.0.0
*> 200.200.0.0/22 0.0.0.0 0 32768

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Route Origination:
Example II

• Trying to originate an aggregate route
aggregate-address 7.7.0.0 255.255.0.0 summary-only

• The RIB has a component but BGP does not
create the aggregate???
R1# show ip route 7.7.0.0 255.255.0.0 longer
7.0.0.0/32 is subnetted, 1 subnets
C 7.7.7.7 [1/0] is directly connected, Loopback 0

R1# show ip bgp | i 7.7.0.0
R1#

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Route Origination:
Example II

• Remember, to have a BGP aggregate you need a
BGP component, not a RIB component
R1# show ip bgp 7.7.0.0 255.255.0.0 longer
R1#

• Once BGP has a component route we originate
the aggregate
network 7.7.7.7 mask 255.255.255.255

R1# show ip bgp 7.7.0.0 255.255.0.0 longer
*> 7.7.0.0/16 0.0.0.0 32768 i
s> 7.7.7.7/32 0.0.0.0 0 32768 i

• s means this component is suppressed due to the
“summary-only” argument

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Troubleshooting Tips

• BGP Network statement rules
Always need an exact route (RIB)
• aggregate-address looks in the BGP table,
not the RIB
• “show ip route x.x.x.x y.y.y.y longer”
Great for finding RIB component routes
• “show ip bgp x.x.x.x y.y.y.y longer”
Great for finding BGP component routes

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Missing Routes

• UPDATE Exchange
• Filtering

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Missing Routes:
Update Exchange

• Ah, Route Reflectors…
Such a nice solution to help scale iBGP
But why do people insist in breaking the rules all the
time?!

• Common issues
Clashing router IDs
Clashing cluster IDs

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Missing Routes:
Example I

• Two RR clusters
• R1 is a RR for R3
• R2 is a RR for R4
R1 R2

• R4 is advertising
7.0.0.0/8 R3 R4

• R2 has the route but
R1 and R3 do not?

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Missing Routes:
Example I

• First, did R2 advertise the route to R1?
R2# show ip bgp neighbors 1.1.1.1 advertised-routes
BGP table version is 2, local router ID is 2.2.2.2
Network Next Hop Metric LocPrf Weight Path
*>i7.0.0.0 4.4.4.4 0 100 0 i

• Did R1 receive it?
R1# show ip bgp neighbors 2.2.2.2 routes
Total number of prefixes 0

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Missing Routes:
Example I

• Time to debug!!
access-list 100 permit ip host 7.0.0.0 host 255.0.0.0
R1# debug ip bgp update 100

• Tell R2 to resend his UPDATEs
R2# clear ip bgp 1.1.1.1 out

• R1 shows us something interesting
*Mar 1 21:50:12.410: BGP(0): 2.2.2.2 rcv UPDATE w/ attr:
nexthop 4.4.4.4, origin i, localpref 100, metric 0,
originator 100.1.1.1, clusterlist 2.2.2.2, path , community
, extended community
*Mar 1 21:50:12.410: BGP(0): 2.2.2.2 rcv UPDATE about
7.0.0.0/8 -- DENIED due to: ORIGINATOR is us;

• Cannot accept an update with our Router-ID as the
ORIGINATOR_ID. Another means of loop detection
in BGP
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Missing Routes:
Example I – Summary

• R1 is not accepting the route when R2 sends it
on from its client, R4
R1 and R4 have the same router ID!
If R1 sees its own router ID in the originator attribute in
any received prefix, it will reject that prefix
This is how a route reflector attempts to avoid routing
loops
• Solution
do NOT set the router ID by hand unless you have a very
good reason to do so and have a very good plan for
deployment
Router-ID is usually calculated automatically by router

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Missing Routes:
Example II

• One RR cluster
R1 R2
• R1 and R2 are RRs
• R3 and R4 are RRCs
• R4 is advertising R3 R4
7.0.0.0/8
R2 has it
R1#show run | include cluster
R1 and R3 do not bgp cluster-id 10
R2#show run | include cluster
bgp cluster-id 10

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Missing Routes:
Example II

• Same steps as last time!
• Did R2 advertise it to R1?
R2# show ip bgp neighbors 1.1.1.1 advertised-routes
Origin codes: i - IGP, e - EGP, ? – incomplete
*>i7.0.0.0 4.4.4.4 0 100 0 i

• Did R1 receive it?
R1# show ip bgp neighbor 2.2.2.2 routes

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Missing Routes:
Example II

• Time to debug!!
access-list 100 permit ip host 7.0.0.0 host 255.0.0.0
R1# debug ip bgp update 100

• Tell R2 to resend his UPDATEs
R2# clear ip bgp 1.1.1.1 out

• R1 shows us something interesting
*Mar 3 14:28:57.208: BGP(0): 2.2.2.2 rcv UPDATE w/ attr: nexthop
4.4.4.4, origin i, localpref 100, metric 0, originator 4.4.4.4,
clusterlist 0.0.0.10, path , community , extended community
*Mar 3 14:28:57.208: BGP(0): 2.2.2.2 rcv UPDATE about 7.0.0.0/8 --
DENIED due to: reflected from the same cluster;

• Remember, all RRCs must peer with all RRs in a
cluster; allows R4 to send the update directly to R1

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Missing Routes:
Example II – Summary

• R1 is not accepting the route when R2 sends it on
If R1 sees its own router ID in the cluster-ID attribute in any
received prefix, it will reject that prefix
How a route reflector avoids redundant information

• Reason
Early documentation claimed that RRC redundancy should
be achieved by dual route reflectors in the same cluster
This is fine and good, but then ALL clients must peer with
both RRs, otherwise examples like this will occur
• Solution
Use overlapping Route Reflector Clusters for redundancy,
stay with defaults

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• “show ip bgp neighbor x.x.x.x advertised”
Lets you see a list of NLRI that you sent a peer
Note: The attribute values shown are taken from the BGP
table; attribute modifications by outbound route-maps will
not be shown
• “show ip bgp neighbor x.x.x.x routes”
Displays routes x.x.x.x sent to us that made it through our
inbound filters
• “show ip bgp neighbor x.x.x.x received”
Can only use if “soft-reconfig inbound” is configured
Displays all routes received from a peer, even those that
were denied

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“soft-reconfiguration”

• Ideal for troubleshooting problems with inbound
filters and attributes
• “show ip bgp neighbor x.x.x.x routes”
alpha#sh ip bgp neigh 192.168.12.1 routes
*>i1.0.0.0 192.168.12.1 0 50 0 i
*>i222.222.0.0/19 192.168.5.1 200 0 3 4 i

• “show ip bgp neighbor x.x.x.x received”
alpha#sh ip bgp neigh 192.168.12.1 received-routes
* i1.0.0.0 192.168.12.1 0 100 0 i
* i169.254.0.0 192.168.5.1 0 100 0 3 i
* i222.222.0.0/19 192.168.5.1 100 0 3 4 i
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Missing Routes

• UPDATE Exchange
• Filtering

NANOG 33 68

Update Filtering

• Type of filters
Prefix filters
AS_PATH filters
Community filters
Route-maps
• Applied incoming and/or outgoing

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Missing Routes
Update Filters

• Determine which filters are applied
to the BGP session
show ip bgp neighbors x.x.x.x
show run | include neighbor x.x.x.x
• Examine the route and pick out the
relevant attributes
show ip bgp x.x.x.x
• Compare the attributes against the filters

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Missing Routes
Update Filters

10.0.0.0/8 ??? 10.0.0.0/8

R1 R2

• Missing 10.0.0.0/8 in R1 (1.1.1.1)
• Not received from R2 (2.2.2.2)

R1#show ip bgp neigh 2.2.2.2 routes


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Missing Routes
Update Filters

• R2 originates the route
• Does not advertise it to R1
R2#show ip bgp neigh 1.1.1.1 advertised-routes

R2#show ip bgp 10.0.0.0
BGP routing table entry for 10.0.0.0/8, version 1660
Paths: (1 available, best #1)
Not advertised to any peer
Local
0.0.0.0 from 0.0.0.0 (2.2.2.2)
Origin IGP, metric 0, localpref 100, weight 32768, valid,
sourced, local, best

NANOG 33 72

Missing Routes
Update Filters

• Time to check filters!
• ^ matches the beginning of a line
• $ matches the end of a line
• ^$ means match any empty AS_PATH
• Filter “looks” correct
R2#show run | include neighbor 1.1.1.1
neighbor 1.1.1.1 filter-list 1 out

R2#sh ip as-path 1
AS path access list 1
permit ^$

NANOG 33 73

Missing Routes
Update Filters

R2#show ip bgp filter-list 1

R2#show ip bgp regexp ^$
Status codes: s suppressed, d damped, h history, * valid,
> best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

*> 10.0.0.0 0.0.0.0 0 32768 i

• Nothing matches the filter-list???
• Re-typing the regexp gives the expected output

NANOG 33 74

Missing Routes
Update Filters

• Copy and paste the entire regexp line from the
configuration

Nothing matches again! Let’s use the up arrow key to see where the
cursor stops

End of Line Is at the Cursor

• There is a trailing white space at the end
• It is considered part of the regular expression

NANOG 33 75

Missing Routes
Update Filters

• Force R2 to resend the update after the filter-list
correction
• Then check R1 to see if it has the route

R2#clear ip bgp 1.1.1.1 out


• R1 still does not have the route
• Time to check R1’s inbound policy for R2

NANOG 33 76

Missing Routes
Update Filters
R1#show run | include neighbor 2.2.2.2
neighbor 2.2.2.2 route-map POLICY in
R1#show route-map POLICY
route-map POLICY, permit, sequence 10
Match clauses:
ip address (access-lists): 100 101
as-path (as-path filter): 1
Set clauses:
Policy routing matches: 0 packets, 0 bytes
R1#show access-list 100
Extended IP access list 100
permit ip host 10.0.0.0 host 255.255.0.0
permit ip 200.1.0 0.0.0.255 host 255.255.255.0
R1#show ip as-path 1
permit ^12$
NANOG 33 77

Missing Routes
Update Filters
10.0.0.0/8 ??? 10.0.0.0/8

R1 R2

• Confused? Let’s run some debugs
Standard IP access list 99
permit 10.0.0.0

R1#debug ip bgp 2.2.2.2 update 99
BGP updates debugging is on for access list 99 for neighbor 2.2.2.2

R1#
4d00h: BGP(0): 2.2.2.2 rcvd UPDATE w/ attr: nexthop 2.2.2.2,
origin i, metric 0, path 12
4d00h: BGP(0): 2.2.2.2 rcvd 10.0.0.0/8 -- DENIED due to: route-map;

NANOG 33 78

Missing Routes
Update Filters
R1#sh run | include neighbor 2.2.2.2
neighbor 2.2.2.2 route-map POLICY in
R1#sh route-map POLICY
route-map POLICY, permit, sequence 10
Match clauses:
ip address (access-lists): 100 101
as-path (as-path filter): 1
Set clauses:
Policy routing matches: 0 packets, 0 bytes
R1#sh access-list 100
R1#sh access-list 101
permit ip 200.1.1.0 0.0.0.255 host 255.255.255.0
R1#sh ip as-path 1
permit ^12$

NANOG 33 79

Missing Routes
Update Filters

• Wrong mask! Needs to be /8 and the ACL allows a /16 only!

• Should be

• Use prefix-list instead, more difficult to make a mistake
ip prefix-list my_filter permit 10.0.0.0/8

• What about ACL 101?
Multiple matches on the same line are ORed
Multiple matches on different lines are ANDed

• ACL 101 does not matter because ACL 100 matches
which satisfies the OR condition

NANOG 33 80

Update Filtering:
Summary

• If you suspect a filtering problem, become
familiar with the router tools to find out
what BGP filters are applied
• Tip: don’t cut and paste!
Many filtering errors and diagnosis problems
result from cut and paste buffer problems on
the client, the connection, and even the router

NANOG 33 81

Update Filtering:
Common Problems

• Typos in regular expressions
Extra characters, missing characters, white space, etc
In regular expressions every character matters, so
accuracy is highly important
• Typos in prefix filters
Watch the router CLI, and the filter logic – it may not be
as obvious as you think, or as simple as the manual
makes out
Watch netmask confusion, and 255 profusion – easy to
muddle 255 with 0 and 225!

NANOG 33 82

Missing Routes
Community Problems

10.0.0.0/8 ??? 10.0.0.0/8

R1 R2

• Missing 10.0.0.0/8 in R1 (1.1.1.1)
• Not received from R2 (2.2.2.2)



NANOG 33 83

Missing Routes
Community Problems

• R2 originates the route
Local
0.0.0.0 from 0.0.0.0 (2.2.2.2)
Origin IGP, metric 0, localpref 100, weight 32768, valid,
sourced, local, best

• But the community is not set
Would be displayed in the “show ip bgp” output

NANOG 33 84

Missing Routes
Community Problems

• Fix the configuration so community is set
R2#show run | begin bgp
router bgp 2
network 10.0.0.0 route-map set-community
...
route-map set-community permit 10
set community 2:2 1:50

Local
0.0.0.0 from 0.0.0.0 (2.2.2.2)
Origin IGP, metric 0, localpref 100, weight 32768,
valid, sourced, local, best
Community 2:2 1:50
NANOG 33 85

Missing Routes
Community Problems

• R2 now advertises prefix with community to R1
• But R1 still doesn’t see the prefix
R1 insists there is nothing wrong with their configuration



• Configuration verified on R2
• No filters blocking announcement on R2
• So what’s wrong?
NANOG 33 86

Missing Routes
Community Problems
• Check R2 configuration again!
router bgp 2
network 10.0.0.0 route-map set-community
neighbor 1.1.1.1 prefix-list my-agg out
neighbor 1.1.1.1 prefix-list their-agg in
!
ip prefix-list my-agg permit 10.0.0.0/8
ip prefix-list their-agg permit 20.0.0.0/8
!
route-map set-community permit 10
set community 2:2 1:50
• Looks okay – filters okay, route-map okay
• But forgotten “neighbor 1.1.1.1 send-community”
Cisco IOS does NOT send communities by default
NANOG 33 87

Missing Routes
Community Problems

• R2 now advertises prefix with community to R1
• But R1 still doesn’t see the prefix
Nothing wrong on R2 now, so turn attention to R1
router bgp 1
neighbor 2.2.2.2 route-map R2-in in
neighbor 2.2.2.2 route-map R1-out out
!
ip community-list 1 permit 1:150
!
route-map R2-in permit 10
match community 1
set local-preference 150
NANOG 33 88

Missing Routes
Community Problems

• Community match on R1 expects 1:150 to be set on
prefix
• But R2 is sending 1:50
Typo or miscommunication between operations?
• R2 is also using the route-map to filter
If the prefix does not have community 1:150 set, it is dropped
– there is no next step in the route-map
Watch the route-map rules in Cisco IOS – they are basically:
if <match> then <set> and exit route-map
else if <match> then <set> and exit route-map
else if <match> then <set> etc…
Blank route-map line means match everything, set nothing
NANOG 33 89

Missing Routes
Community Problems

• Fix configuration on R2 to set community 1:150 on
announcements to R1
• Fix configuration on R1 to also permit prefixes not matching the
route-map – troubleshooting is easier with prefix-filters doing
the filtering

R1#show run | begin ^route-map
match community 1
set local-preference 150


* 10.0.0.0 2.2.2.2 0 0 2 i

NANOG 33 90

Missing Routes
Community Problems

• Watch route-maps
Route-map rules often catch out operators when they
are used for filtering
Absence of an appropriate match means the prefix will
be discarded
• Remember to configure all routers to send BGP
communities
Include it in your default template for iBGP
It should be iBGP default in a Service Provider Network
Remember that it is required to send communities for
eBGP too

NANOG 33 91

Missing Routes:
Common Community Problems

• Each router implementation has different
defaults for when communities are sent
Some don’t send communities
Others do for iBGP and not for eBGP
Others do for both iBGP and eBGP peers
• Watch how your implementation handles
communities
There may be implicit filtering rules
• Each ISP has different community policies
Never assume that because communities exist that
people will use them, or pay attention to the ones you
send

NANOG 33 92

Missing Routes:
General Problems

• Make and then Stick to simple policy rules:
Most router implementations have particular rules for
filtering of prefixes, AS-paths, and for manipulating BGP
attributes
Try not to mix these rules
• Rules for manipulating attributes can also be
used for filtering prefixes and ASNs
These can be very powerful, but can also become very
confusing

NANOG 33 93

Missing Routes

• UPDATE Exchange
• Filtering

NANOG 33 94

Missing Routes
iBGP Example I

• Symptom: prefixes seen across network,
but no connectivity
Prefixes learned from eBGP peer are passed
across iBGP mesh
But no connectivity to those prefixes

NANOG 33 95

Missing Routes
iBGP Example I

1.1.1.1 R5 2.2.2.2

R1 iBGP R2
eBGP
3.3.3.3 AS 1
R3 4.4.4.4
R4 B
eBGP
AS 3

A
10.10.0.0/24 • R3 customers can reach AS2
AS 2
• No other customers connected
to AS1 or AS3 can reach AS2

NANOG 33 96

Missing Routes
iBGP Example I

• Looking at R3
R3#show ip bgp
Status codes: * valid, > best, i - internal,
*> 3.0.0.0 10.10.10.10 0 2 5 i
*> 4.0.0.0 10.10.10.10 0 2 5 i
*> 10.10.0.0/24 10.10.10.10 0 2 i
*> 10.20.0.0/16 10.10.10.10 0 2 i

• Looking at R4
R4#show ip bgp
* i3.0.0.0 10.10.10.10 100 0 2 5 i
* i4.0.0.0 10.10.10.10 100 0 2 5 i
* i10.10.0.0/24 10.10.10.10 100 0 2 i
* i10.20.0.0/16 10.10.10.10 100 0 2 i

NANOG 33 97

Missing Routes:
iBGP Example I

• Notice that R3 reports the prefixes learned from
AS2
Paths are valid (*) and best (>)

• Notice that R4 reports the prefixes learned from
R3
Paths are valid (*) and internal (i)
But no best path
This is the clue…

NANOG 33 98

Missing Routes:
iBGP Example I

• Look at the BGP table entry:
R4#sh ip bgp 10.10.0.0/24
Paths: (1 available, no best path) The clues
2, (received & used)
10.10.10.10 (inaccessible) from 3.2.1.2 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal

• Look at the Routing Table entry
R4#sh ip route 10.10.0.0 255.255.255.0

• The next hop?
R4#sh ip route 10.10.10.10

NANOG 33 99

Missing Routes:
iBGP Example I – Diagnosis

• R4 does not use the 10.10.0.0/24 destination
because there is no valid next-hop
• Configuration on R3 has:
Either no routing information on how to reach the
10.10.10.10/30 point to point link
By forgetting to put the link into the IGP
Or not excluded external next-hops from the internal
network
By forgetting to set itself as the next-hop for all externally
learned prefixes on the iBGP session with R4

NANOG 33 100

Missing Routes:
iBGP Example I – Solution

• Make sure that all the BGP NEXT_HOPs are
known by the IGP
(whether OSPF/ISIS, static or connected routes)
If NEXT_HOP is also in iBGP, ensure the iBGP distance
is longer than the IGP distance
—or—
• Don’t carry external NEXT_HOPs in your network
Replace eBGP next_hop with local router address on
all the edge BGP routers
(Cisco IOS “next-hop-self”)

NANOG 33 101

Missing Routes
iBGP Example I – Solution

• R3 now includes the missing “next-hop-
self” configuration
• Looking at R4 now:
R4#show ip bgp
*>i3.0.0.0 3.3.3.3 100 0 2 5 i
*>i4.0.0.0 3.3.3.3 100 0 2 5 i
*>i10.10.0.0/24 3.3.3.3 100 0 2 i
*>i10.20.0.0/16 3.3.3.3 100 0 2 i

NANOG 33 102

Missing Routes
iBGP Example II

• Symptom: customer complains about
patchy Internet access
Can access some, but not all, sites connected
to backbone
Can access some, but not all, of the Internet

NANOG 33 103

Missing Routes
iBGP Example II

1.1.1.1 R5 2.2.2.2

R1 iBGP R2
eBGP
3.3.3.3 AS 1
R3 4.4.4.4
R4 B
eBGP
AS 3

A
• Customer connected to R1 can see
10.10.0.0/24 AS3, but not AS2
AS 2
• Also complains about not being able
to see sites connected to R5
• No complaints from other customers
NANOG 33 104

Missing Routes
iBGP Example II

• Diagnosis: This is the classic iBGP mesh
problem
The full mesh isn’t complete – how do we know this?
• Customer is connected to R1
Can’t see AS2 ⇒ R3 is somehow not passing routing
information about AS2 to R1
Can’t see R5 ⇒ R5 is somehow not passing routing
information about sites connected to R5
But can see rest of the Internet ⇒ his prefix is being
announced to some places, so not an iBGP origination
problem
NANOG 33 105

Missing Routes
iBGP Example II
R3#sh ip bgp sum | begin ^Neigh
1.1.1.1 4 1 200 20 32 0 0 3d10h Active
2.2.2.2 4 1 210 25 32 0 0 3d16h 15
4.4.4.4 4 1 213 22 32 0 0 3d16h 12
5.5.5.5 4 1 215 19 32 0 0 3d16h 0
10.10.10.10 4 2 2501 2503 32 0 0 3d16h 100
R3#

• BGP summary shows that the peering with router
R1 is down
Up/Down is 3 days 10 hours, yet active
Which means it was last up 3 days and 10 hours ago
So something has broken between R1 and R3

NANOG 33 106

Missing Routes
iBGP Example II

• Now check configuration on R1
R1#sh conf | b bgp
router bgp 1
neighbor iBGP-ipv4-peers peer-group
neighbor iBGP-ipv4-peers remote-as 1
neighbor iBGP-ipv4-peers update-source Loopback0
neighbor iBGP-ipv4-peers send-community
neighbor iBGP-ipv4-peers prefix-list ibgp-prefixes out
neighbor 2.2.2.2 peer-group iBGP-ipv4-peers

• Where is the peering with R3?
• Restore the missing line, and the iBGP with R3
comes back up
NANOG 33 107

Missing Routes
iBGP Example II
1.1.1.1 4 1 200 20 32 0 0 00:00:50 8
2.2.2.2 4 1 210 25 32 0 0 3d16h 15
4.4.4.4 4 1 213 22 32 0 0 3d16h 12
5.5.5.5 4 1 215 19 32 0 0 3d16h 0
10.10.10.10 4 2 2501 2503 32 0 0 3d16h 100
R3#

• BGP summary shows that no prefixes are being
heard from R5
This could be due to inbound filters on R3 on the iBGP
with R5
But there were no filters in the configuration on R3
This must be due to outbound filters on R5 on the iBGP
with R3
NANOG 33 108

Missing Routes
iBGP Example II

• Now check configuration on R5
R5#sh conf | b neighbor 3.3.3.3
neighbor 3.3.3.3 update-source loopback0
neighbor 3.3.3.3 prefix-list ebgp-filters out
neighbor 4.4.4.4 update-source loopback0
neighbor 4.4.4.4 prefix-list ibgp-filters out
!
ip prefix-list ebgp-filters permit 20.0.0.0/8
ip prefix-list ibgp-filters permit 10.0.0.0/8

• Error in prefix-list in R3 iBGP peering
ebgp-filters has been used instead of ibgp-filters
Typo – another advantage of using peer-groups!
NANOG 33 109

Missing Routes
iBGP Example II

• Fix the prefix-list on R5
• Check the iBGP again on R3
Peering with R1 is up
Peering with R5 has prefixes

• Confirm that all is okay with customer
1.1.1.1 4 1 200 20 32 0 0 00:01:53 8
2.2.2.2 4 1 210 25 32 0 0 3d16h 15
4.4.4.4 4 1 213 22 32 0 0 3d16h 12
5.5.5.5 4 1 215 19 32 0 0 3d16h 6
10.10.10.10 4 2 2501 2503 32 0 0 3d16h 100
R3#

NANOG 33 110


• Watch the iBGP full mesh
Use peer-groups both for efficiency and to
avoid making policy errors within the iBGP
mesh
Use route reflectors to avoid accidentally
missing iBGP peers, especially as the mesh
grows in size
• Watch the next-hop for external paths

NANOG 33 111


• Missing Routes

NANOG 33 112

Inconsistent Route Selection

• Two common problems with route selection
Inconsistency
Appearance of an incorrect decision
• RFC 1771 defines the decision algorithm
• Every vendor has tweaked the algorithm
http://www.cisco.com/warp/public/459/25.shtml
• Route selection problems can result from
oversights by RFC 1771

NANOG 33 113

Inconsistent Route Selection:
Example I

• RFC says that MED is not always compared
• As a result, the ordering of the paths can effect
the decision process
• For example, the default in Cisco IOS is to
compare the prefixes in order of arrival (most
recent to oldest)
This can result in inconsistent route selection
Symptom is that the best path chosen after each BGP
reset is different

NANOG 33 114

Example I

• Inconsistent route selection may cause problems
Routing loops
Convergence loops—i.e. the protocol continuously
sends updates in an attempt to converge
Changes in traffic patterns
• Difficult to catch and troubleshoot
In Cisco IOS, the deterministic-med configuration
command is used to order paths consistently
Enable in all the routers in the AS
The bestpath is recalculated as soon as the command
is entered

NANOG 33 115

Symptom I:
Diagram

AS 3 AS 10
10.0.0.0/8
R3
R2
MED 30
MED 20 RouterA

AS 2

AS 1
MED 0
• RouterA will have three paths
• MEDs from AS 3 will not be compared to R1
MEDs from AS 1
• RouterA will sometimes select the path from R1 as best and but may
also select the path from R3 as best
NANOG 33 116

Example I

RouterA#sh ip bgp 10.0.0.0
Paths: (3 available, best #3, advertised over iBGP, eBGP)
3 10
2.2.2.2 from 2.2.2.2
3 10
3.3.3.3 from 3.3.3.3
Origin IGP, metric 30, valid, external
1 10
1.1.1.1 from 1.1.1.1
Origin IGP, metric 0, localpref 100, valid, internal, best

• Initial State
Path 1 beats Path 2 – Lower MED
Path 3 beats Path 1 – Lower Router-ID
NANOG 33 117

Example I

1 10
1.1.1.1 from 1.1.1.1
3 10
2.2.2.2 from 2.2.2.2
3 10
3.3.3.3 from 3.3.3.3
Origin IGP, metric 30, valid, external, best

• 1.1.1.1 bounced so the paths are re-ordered
Path 1 beats Path 2 – Lower Router-ID
Path 3 beats Path 1 – External vs Internal
NANOG 33 118

Deterministic MED:
Operation

• The paths are ordered by Neighbour AS
• The bestpath for each Neighbour AS group is
selected
• The overall bestpath results from comparing the
winners from each group
• The bestpath will be consistent because paths
will be placed in a deterministic order

NANOG 33 119

Deterministic MED:
Result

1 10
1.1.1.1 from 1.1.1.1
3 10
2.2.2.2 from 2.2.2.2
3 10
3.3.3.3 from 3.3.3.3
Origin IGP, metric 30, valid, external

• Path 1 is best for AS 1
• Path 2 beats Path 3 for AS 3 – Lower MED
• Path 1 beats Path 2 – Lower Router-ID
NANOG 33 120

Deterministic MED:
Summary

• Always use “bgp deterministic-med”
• Need to enable throughout entire network at
roughly the same time
• If only enabled on a portion of the network
routing loops and/or convergence problems may
become more severe
• As a result, default behaviour cannot be changed
so the knob must be configured by the user

NANOG 33 121

Solution – Diagram

AS 3 AS 10
10.0.0.0/8
R3
R2
MED 30
MED 20 RouterA

AS 2

AS 1
MED 0
R1
• RouterA will have three paths
• RouterA will consistently select the path from R1 as best!

NANOG 33 122

Example II

AS 10 AS 20
• The bestpath changes
every time the peering R1 R2

is reset

R3

10 100
1.1.1.1 from 1.1.1.1
Origin IGP, metric 0, localpref 100, valid, external
20 100
2.2.2.2 from 2.2.2.2
Origin IGP, metric 0, localpref 100, valid, external, best

NANOG 33 123

Example II

20 100
2.2.2.2 from 2.2.2.2
Origin IGP, metric 0, localpref 100, valid, external
10 100
1.1.1.1 from 1.1.1.1

• The “oldest” external is the bestpath
All other attributes are the same
Stability enhancement!!—CSCdk12061—Integrated in 12.0(1)

• “bgp bestpath compare-router-id” will disable this
enhancement—CSCdr47086—Integrated in 12.0(11)S and 12.1(3)

NANOG 33 124

Example III

R1#sh ip bgp 11.0.0.0
100
1.1.1.1 from 1.1.1.1
Origin IGP, localpref 120, valid, internal
100
2.2.2.2 from 2.2.2.2

• Path 1 has higher localpref but path 2
is better???
• This appears to be incorrect…

NANOG 33 125

Example III
• Path is from an internal peer which means the path
must be synchronized by default
• Check to see if sync is on or off
R1# show run | include sync
R1#
• Sync is still enabled, check for IGP path:
R1# show ip route 11.0.0.0
• CSCdr90728 “BGP: Paths are not marked as not
synchronized”—Fixed in 12.1(4)
• Path 1 is not synchronized
• Router made the correct choice
NANOG 33 126

Inconsistent Path Selection

• Summary:
RFC1771 isn’t prefect when it comes to path selection –
years of operational experience have shown this
Vendors and ISPs have worked to put in stability
enhancements
But these can lead to interesting problems
And of course some defaults linger much longer than
they ought to – so never assume that an out of the box
default configuration will be perfect for your network

NANOG 33 127


• Missing Routes

NANOG 33 128

Route Oscillation

• One of the most common problems
• Main symptom is that traffic exiting the network
oscillates every minute between two exit points
This is almost always caused by the BGP NEXT_HOP
being known only by BGP
Common problem in ISP networks – but if you have
never seen it before, it can be a nightmare to debug
and fix
• Other symptom is high CPU utilisation for the
BGP router process

NANOG 33 129

Route Oscillation:
Diagram

R3

R1

AS 3 R2

142.108.10.2

AS 4
AS 12

• R3 prefers routes via AS 4 one minute
• BGP scanner runs then R3 prefers routes via AS 12
• The entire table oscillates every 60 seconds
NANOG 33 130

Route Oscillation:
Diagnosis

267 network entries and 272 paths using 34623 bytes of memory

R3#sh ip route summary | begin bgp
bgp 3 4 6 520 1400
External: 0 Internal: 10 Local: 0
internal 5 5800
Total 10 263 13936 43320

• Watch for:
Table version number incrementing rapidly
Number of networks/paths or external/internal
routes changing

NANOG 33 131

Route Oscillation:
Troubleshooting
• Pick a route from the RIB that has changed within
the last minute
• Monitor that route to see if it changes every minute
R3#show ip route 156.1.0.0
Known via "bgp 3", distance 200, metric 0
Routing Descriptor Blocks:
* 1.1.1.1, from 1.1.1.1, 00:00:53 ago
AS Hops 2, BGP network version 474

Advertised to non peer-group peers:
2.2.2.2
4 12
1.1.1.1 from 1.1.1.1 (1.1.1.1)
Origin IGP, localpref 100, valid, internal, best
12
142.108.10.2 (inaccessible) from 2.2.2.2 (2.2.2.2)

NANOG 33 132

Route Oscillation:
Troubleshooting
• Check again after bgp_scanner runs
• bgp_scanner runs every 60 seconds and validates
reachability to all nexthops
R3#sh ip route 156.1.0.0
* 142.108.10.2, from 2.2.2.2, 00:00:27 ago

R3#sh ip bgp 156.1.0.0
1.1.1.1
4 12
1.1.1.1 from 1.1.1.1 (1.1.1.1)
12
142.108.10.2 from 2.2.2.2 (2.2.2.2)
NANOG 33 133

Route Oscillation:
Troubleshooting

• Lets take a closer look at the nexthop
R3#show ip route 142.108.10.2
* 142.108.10.2, from 2.2.2.2, 00:00:50 ago

R3#show ip bgp 142.108.10.2
1.1.1.1
4 12
1.1.1.1 from 1.1.1.1 (1.1.1.1)
12
142.108.10.2 from 2.2.2.2 (2.2.2.2)

NANOG 33 134

Route Oscillation:
Troubleshooting

• BGP nexthop is known via BGP
• Illegal recursive lookup
• Scanner will notice and install the other path in the RIB
R3#sh debug
BGP events debugging is on
BGP updates debugging is on
IP routing debugging is on
R3#
BGP: scanning routing tables
BGP: nettable_walker 142.108.0.0/16 calling revise_route
RT: del 142.108.0.0 via 142.108.10.2, bgp metric [200/0]
BGP: revise route installing 142.108.0.0/16 -> 1.1.1.1
RT: add 142.108.0.0/16 via 1.1.1.1, bgp metric [200/0]

NANOG 33 135

Route Oscillation:
Troubleshooting

• Route to the nexthop is now valid
• Scanner will detect this and re-install the other path
• Routes will oscillate forever
R3#
BGP: scanning routing tables
BGP: ip nettable_walker 142.108.0.0/16 calling revise_route
BGP: nettable_walker 156.1.0.0/16 calling revise_route

NANOG 33 136

Route Oscillation:
Step by Step

R3

R1

AS 3 R2

142.108.10.2

AS 4
AS 12
• R3 naturally prefers routes from AS 12
• R3 does not have an IGP route to 142.108.10.2 which is the next-hop for
routes learned via AS 12
• R3 learns 142.108.0.0/16 via AS 4 so 142.108.10.2 becomes reachable
NANOG 33 137

Route Oscillation:
Step by Step

• R3 then prefers the AS 12 route for
142.108.0.0/16 whose next-hop is 142.108.10.2
• This is an illegal recursive lookup
• BGP detects the problem when scanner runs and
flags 142.108.10.2 as inaccessible
• Routes through AS 4 are now preferred
• The cycle continues forever…

NANOG 33 138

Route Oscillation:
Solution

• Make sure that all the BGP NEXT_HOPs are
known by the IGP
(whether OSPF/ISIS, static or connected routes)
If NEXT_HOP is also in iBGP, ensure the iBGP distance
is longer than the IGP distance
—or—
• Don’t carry external NEXT_HOPs in your network
Replace eBGP next_hop with local router address on
all the edge BGP routers
(Cisco IOS “next-hop-self”)

NANOG 33 139

Route Oscillation:
Solution

R3

R1

AS 3 R2

142.108.10.2

AS 4
AS 12

• R3 now has IGP route to AS 12 next-hop or R2 is using next-hop-self
• R3 now prefers routes via AS 12 all the time
• No more oscillation!!
NANOG 33 140


• High CPU utilisation in the BGP process is
normally a sign of a convergence problem
• Find a prefix that changes every minute
• Troubleshoot/debug that one prefix

NANOG 33 141


• BGP routing loop?
First, check for IGP routing loops to the BGP NEXT_HOPs

• BGP loops are normally caused by
Not following physical topology in RR environment
Multipath with confederations
Lack of a full iBGP mesh

• Get the following from each router in the loop path
The routing table entry
The BGP table entry
The route to the NEXT_HOP

NANOG 33 142

Convergence Problems

• Route reflector with RR
250 route reflector
clients
• 100k routes
• BGP will not
converge

NANOG 33 143

• Have been trying to converge for 10 minutes
• Peers keep dropping so we never converge?
RR# show ip bgp summary
20.3.1.160 4 100 10 5416 9419 0 0 00:00:12 Closing
20.3.1.161 4 100 11 4418 8055 0 335 00:10:34 0
20.3.1.162 4 100 12 4718 8759 0 128 00:10:34 0
20.3.1.163 4 100 9 3517 0 1 0 00:00:53 Connect
20.3.1.164 4 100 13 4789 8759 0 374 00:10:37 0
20.3.1.165 4 100 13 3126 0 0 161 00:10:37 0
20.3.1.166 4 100 9 5019 9645 0 0 00:00:13 Closing
20.3.1.167 4 100 9 6209 9218 0 350 00:10:38 0

• Check the log to find out why
RR#show log | i BGP
*May 3 15:27:16: %BGP-5-ADJCHANGE: neighbor 20.3.1.118 Down— BGP Notification sent
*May 3 15:27:16: %BGP-3-NOTIFICATION: sent to neighbor 20.3.1.118 4/0 (hold time expired) 0 byt
*May 3 15:28:10: %BGP-5-ADJCHANGE: neighbor 20.3.1.52 Down— BGP Notification sent
*May 3 15:28:10: %BGP-3-NOTIFICATION: sent to neighbor 20.3.1.52 4/0 (hold time expired) 0 byte

NANOG 33 144

• We are either missing hellos or our peers are not sending them
• Check for interface input drops

RR# show interface gig 2/0 | include input drops
Output queue 0/40, 0 drops; input queue 0/75, 72390 drops
RR#

• 72k drops will definitely cause a few peers to go down
• We are missing hellos because the interface input queue is very small
• A rush of TCP Acks from 250 peers can fill 75 spots in a hurry
• Increase the size of the queue

RR# show run interface gig 2/0
interface GigabitEthernet 2/0
ip address 7.7.7.156 255.255.255.0
hold-queue 2000 in

NANOG 33 145


• Let’s start over and give BGP another chance

RR# clear ip bgp *
RR#

• No more interface input drops
RR# show interface gig 2/0 | include input drops
Output queue 0/40, 0 drops; input queue 0/2000, 0 drops
RR#

• Our peers are stable!!
RR# show log | include BGP
RR#

NANOG 33 146


• BGP converged in 25 minutes
• Still seems like a long time
• What was TCP doing?
RR#show tcp stat | begin Sent:
Sent: 1666865 Total, 0 urgent packets
763 control packets (including 5 retransmitted)
1614856 data packets (818818410 bytes)
39992 data packets (13532829 bytes) retransmitted
6548 ack only packets (3245 delayed)
1 window probe packets, 2641 window update packets

RR#show ip bgp neighbor | include max data segment
Datagrams (max data segment is 536 bytes):

NANOG 33 147


• 1.6 Million packets is high
• 536 is the default MSS (max segment size) for a TCP connection
• Very small considering the amount of data we need to transfer


• Enable path mtu discovery
• Sets MSS to max possible value

RR#show run | include tcp
ip tcp path-mtu-discovery
RR#

NANOG 33 148


• Restart the test one more time
RR# clear ip bgp *
RR#

• MSS looks a lot better


NANOG 33 149


• TCP sent 1 million fewer packets
• Path MTU discovery helps reduce overhead by
sending more data per packet
RR# show tcp stat | begin Sent:
Sent: 615415 Total, 0 urgent packets
0 control packets (including 0 retransmitted)
602587 data packets (818797102 bytes)
9609 data packets (7053551 bytes) retransmitted
2603 ack only packets (1757 delayed)
0 window probe packets, 355 window update packets

• BGP converged in 15 minutes!
• More respectable time for 250 peers and 100k routes

NANOG 33 150

Summary/Tips

• Use ACLs when enabling debug
commands
• Ensure that BGP logging is switched on
• Ensure that deterministic MED’s are
enabled
• If the entire table is having problem pick
one prefix and troubleshoot it

NANOG 33 151

Agenda

• Fundamentals

NANOG 33 152

Internet Reachability Problems

• BGP Attribute Confusion
To Control Traffic in → Send MEDs and AS-PATH
prepends on outbound announcements
To Control Traffic out → Attach local-preference to
inbound announcements

• Troubleshooting of multihoming and transit is
often hampered because the relationship
between routing information flow and traffic flow
is forgotten

NANOG 33 153

BGP Path Selection Process

• Each vendor has “tweaked” the path
selection process
Know it for your router equipment – saves time
later
Especially applies with networks with more
than one BGP implementation present
Best policy is to use supplied “knobs” to
ensure consistency – and avoid steps in the
process which can lead to inconsistency

NANOG 33 154

MED Confusion

• Default MED on Cisco IOS is ZERO
It may not be this on your router, or your
peer’s router
• Best not to rely on MEDs for multihoming
on multiple links to upstream
Their default might be 2^32-1 resulting in your
hoped for best path being their worst path
“Workaround”, i.e. current good practice, is to
use communities rather than MEDs

NANOG 33 155

Community Confusion I

• Set community in a route-map does just that – it
overwrites any other community set on the prefix
Use additive keyword to add community to existing list
• Use Internet format for community (AS:xx) not
the 32-bit IETF format
32-bit format is hard for humans to comprehend
Whereas AS:xx format is more intuitive/recognisable

NANOG 33 156

Community Confusion II

• Cisco IOS never sends community by default
Some implementations send community by default for
iBGP peerings
Some implementations also send community by default
for eBGP peerings
• Never assume that your neighbouring AS will
honour your no-export community – ask first!
If you leak iBGP prefixes to your upstream for
loadsharing purposes, this could result in your iBGP
prefixes leaking to the Internet

NANOG 33 157

AS-PATH prepending

• 20 prepends will not lessen the priority of your
path any more than 10 prepends will – check it
out at a Looking Glass
The Internet is on average only 5 ASes deep, maximum
AS prepend most ISPs have to use is around this too
Know you BGP path selection algorithm

• Some ISPs use bgp maxas-path 15 to drop
prefixes with AS-paths longer than 15 ASNs

NANOG 33 158

Private ASNs

• Private ASes should not ever appear in the
Internet
• Cisco IOS remove-private-AS command does not
remove every instance of a private AS
e.g. won’t remove private AS appearing in the middle of
a path surrounded by public ASNs
www.cisco.com/warp/public/459/32.html
• Apparent non-removal of private-ASNs may not
be a bug, but a configuration error somewhere
else

NANOG 33 159

Troubleshooting Connectivity
Example I

AS 1 AS 3
192.168.1.0/24

R1 R3

R2
AS 2

• Symptom: AS1 announces 192.168.1.0/24 to AS2
but AS3 cannot see the network

NANOG 33 160

Example I

• Checklist:
AS1 announces, but does AS2 see it?
We are checking eBGP filters on R1 and R2. Remember
that R2 access will require cooperation and assistance
from your peer

Does AS2 see it over entire network?
We are checking iBGP across AS2’s network
(unneeded step in this case, but usually the next
consideration). Quite often iBGP is misconfigured,
lack of full mesh, problems with RRs, etc.

NANOG 33 161

Example I

• Checklist:
Does AS2 send it to AS3?
We are checking eBGP configuration on R2. There may be
a configuration error with as-path filters, or prefix-lists, or
communities such that only local prefixes get out

Does AS3 see all of AS2’s originated prefixes?
We are checking eBGP configuration on R3. Maybe AS3 does
not know to expect prefixes from AS1 in the peering with
AS2, or maybe it has similar errors in as-path or prefix or
community filters
NANOG 33 162

Example I

• Troubleshooting connectivity beyond
immediate peers is much harder
Relies on your peer to assist you – they have
the relationship with their BGP peers, not you
Quite often connectivity problems are due to
the private business relationship between the
two neighbouring ASNs

NANOG 33 163

Example II

AS 1 AS 3
203.51.206.0

R1 R3

The Internet

• Symptom: AS1 announces 203.51.206.0/24 to its
upstreams but AS3 cannot see the network

NANOG 33 164

Example II

• Checklist:
AS1 announces, but do its upstreams see it?
We are checking eBGP filters on R1 and upstreams.
Remember that upstreams will need to be able to help
you with this

Is the prefix visible anywhere on the Internet?
We are checking if the upstreams are announcing the
network to anywhere on the Internet. See next slides
on how to do this.

NANOG 33 165

Example II

• Help is at hand – the Looking Glass
• Many networks around the globe run Looking
Glasses
These let you see the BGP table and often run simple
ping or traceroutes from their sites
www.traceroute.org for IPv4
www.traceroute6.org for IPv6
• Next slides have some examples of a typical
looking glass in action

NANOG 33 166

Example II

• Hmmm….
• Looking Glass can see 202.173.144.0/21
This includes 202.173.147.0/24
So the problem must be with AS3, or AS3’s
upstream
• A traceroute confirms the connectivity

NANOG 33 169

Example II

• Help is at hand – RouteViews
• The RouteViews router has BGP feeds from
around 60 peers
www.routeviews.org explains the project
Gives access to a real router, and allows any provider
to find out how their prefixes are seen in various parts
of the Internet
Complements the Looking Glass facilities
• Anyway, back to our problem…

NANOG 33 171

Example II

• Checklist:
Does AS3’s upstream send it to AS3?
We are checking eBGP configuration on AS3’s upstream.
There may be a configuration error with as-path filters, or
prefix-lists, or communities such that only local prefixes get
out. This needs AS3’s assistance.

Does AS3 see any of AS1’s originated prefixes?
We are checking eBGP configuration on R3. Maybe AS3 does
not know to expect the prefix from AS1 in the peering with its
upstream, or maybe it has some errors in as-path or prefix or
community filters

NANOG 33 172

Example II

• Troubleshooting across the Internet is harder
But tools are available

• Looking Glasses, offering traceroute, ping and
BGP status are available all over the globe
Most connectivity problems seem to be found at the
edge of the network, rarely in the transit core
Problems with the transit core are usually intermittent
and short term in nature

NANOG 33 173

Example III

The Internet

AS 2 AS 3

R2 R3

R1

AS 1

• Symptom: AS1 is trying to loadshare between its upstreams,
but has trouble getting traffic through the AS2 link
NANOG 33 174

Example III

• Checklist:
What does “trouble” mean?
• Is outbound traffic loadsharing okay?
Can usually fix this with selectively rejecting prefixes,
and using local preference
Generally easy to fix, local problem, simple application
of policy
• Is inbound traffic loadsharing okay?
Errummm, bigger problem if not
Need to do some troubleshooting if configuration with
communities, AS-PATH prepends, MEDs and selective
leaking of subprefixes don’t seem to help
NANOG 33 175

Example III

• Checklist:
AS1 announces, but does AS2 see it?
We are checking eBGP filters on R1 and R2. Remember
that R2 access will require cooperation and assistance
from your peer

Does AS2 see it over entire network?
We are checking iBGP across AS2’s network. Quite
often iBGP is misconfigured, lack of full mesh,
problems with RRs, etc.

NANOG 33 176

Example III

• Checklist:
Does AS2 send it to its upstream?
We are checking eBGP configuration on R2. There may be
a configuration error with as-path filters, or prefix-lists, or
communities such that only local prefixes get out

Does the Internet see all of AS2’s originated
prefixes?
We are checking eBGP configuration on other Internet
routers. This means using looking glasses. And trying to find
one as close to AS2 as possible.

NANOG 33 177

Example III

• Checklist:
Repeat all of the above for AS3

• Stopping here and resorting to a huge prepend
towards AS3 won’t solve the problem
• There are many common problems – listed on
next slide
And tools to help decipher the problem

NANOG 33 178

Example III

• No inbound traffic from AS2
AS2 is not seeing AS1’s prefix, or is blocking it in
inbound filters
• A trickle of inbound traffic
Switch on NetFlow (if the router has it) and check the
origin of the traffic
If it is just from AS2’s network blocks, then is AS2
announcing the prefix to its upstreams?
If they claim they are, ask them to ask their upstream
for a “show ip bgp” output – or use a Looking Glass to
check

NANOG 33 179

Example III

• A light flow of traffic from AS2, but 50% less than
from AS3
Looking Glass comes to the rescue
LG will let you see what AS2, or AS2’s upstreams are
announcing
AS1 may choose this as primary path, but AS2
relationship with their upstream may decide
otherwise
NetFlow comes to the rescue
Allows AS1 to see what the origins are, and with the
LG, helps AS1 to find where the prefix filtering culprit
might be

NANOG 33 180

Example IV

The Internet

AS 2 AS 3

R2 R3

R1

AS 1

• Symptom: AS1 is loadsharing between its upstreams, but
the traffic load swings randomly between AS2 and AS3
NANOG 33 181

Example IV

• Checklist:
Assume AS1 has done everything in this
tutorial so far
All the configurations look fine, the Looking Glass
outputs look fine, life is wonderful… Apart from those
annoying traffic swings every hour or so

L2 problem? Route Flap Damping?
Since BGP is configured fine, and the net has been
stable for so long, can only be an L2 problem, or
Route Flap Damping side-effect

NANOG 33 182

Example IV

• L2 – upstream somewhere has poor
connectivity between themselves and the
rest of the Internet
Only real solution is to impress upon
upstream that this isn’t good enough, and get
them to fix it
Or change upstreams

NANOG 33 183

Example IV

• Route Flap Damping
Many ISPs implement route flap damping
Many ISPs simply use the vendor defaults
Vendor defaults are generally far too severe
There is real concern that the “more lenient” RIPE-229
values are too severe
Opinion is growing that flap damping does more harm
than good
e.g. www.cs.berkeley.edu/~zmao/Papers/sig02.pdf

• Again Looking Glasses come to the operator’s
assistance

NANOG 33 184

Example IV

• Several Looking Glasses allow the operators to
check the flap or damped status of their
announcements
Many oscillating connectivity issues are usually caused
by L2 problems
Route flap damping will cause connectivity to persist
via alternative paths even though primary paths have
been restored
Quite often, the exponential back off of the flap
damping timer will give rise to bizarre routing
Common symptom is that bizarre routing will often clear
away by itself

NANOG 33 186

Troubleshooting Summary

• Most troubleshooting is about:
• Experience
Recognising the common problems
• Not panicking
• Logical approach
Check configuration first
Check locally first before blaming the peer
Troubleshoot layer 1, then layer 2, then layer 3, etc

NANOG 33 187

Troubleshooting Summary

• Most troubleshooting is about:
• Using the available tools
The debugging tools on the router hardware
Internet Looking Glasses
Colleagues and their knowledge
Public mailing lists where appropriate

NANOG 33 188

Closing Comments

• Presentation has covered the most common
troubleshooting techniques used by ISPs today
• Once these have been mastered, more complex
or arcane problems are easier to solve
• Feedback and input for future improvements is
encouraged and very welcome

NANOG 33 189

Presentation Slides

• Available on
ftp://ftp-eng.cisco.com
/pfs/seminars/NANOG33-BGP-Troubleshooting.pdf

NANOG 33 190

Troubleshooting BGP
The End! ☺

NANOG 33 191

Troubleshooting BGP

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