BGP (border gateway routing protocol)

BGP (Border Gateway Routing Protocol)
BGP (Border Gateway Routing Protocol) is a standardized exterior gateway protocol designed to
exchange routing and reachability information between autonomous systems (AS) on the Internet. The
Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets
configured by a network administrator, and are involved in making core routing decisions.
BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing
protocol employed on the Internet.
Basics of BGP
1. BGP is the path-vector protocol that provides routing information for autonomous systems on
the Internet via its AS-Path attribute.
2. BGP is a Layer 4 protocol that sits on top of TCP. It is much simpler than OSPF, because it doesn’t
have to worry about the things TCP will handle.
3. Peers that have been manually configured to exchange routing information will form a TCP
connection and begin speaking BGP. There is no discovery in BGP.
4. Medium-sized businesses usually get into BGP for the purpose of true multi-homing for their
entire network.
Figure 1 Basic Topology of BGP

5. An important aspect of BGP is that the AS-Path itself is an anti-loop mechanism. Routers will not
import any routes that contain themselves in the AS-Path.
Current Version
The current version of BGP is version 4 (BGP4) codified in RFC 4271 since 2006. Early versions of the
protocol are widely considered obsolete and are rarely supported.
Types of BGP
There are different terms used when describing BGP. These including:
1. Internal BGP (iBGP) operates inside an autonomous System (AS).
2. External BGP (eBGP), which is also known as an interdomain routing protocol, operates outside
an AS and connects one AS to another. These terms are just used to describe the same protocol
just the area of operation is what differs.
Figure 2 Types of BGP

Uses
Most Internet service providers must use BGP to establish routing between one another (especially if
they are multihomed). Compare this with Signaling System 7 (SS7), which is the inter-provider core call
setup protocol on the PSTN.
Very large private IP networks use BGP internally. An example would be the joining of a number of large
OSPF (Open Shortest Path First) networks where OSPF by itself would not scale to size. Another reason
to use BGP is multihoming a network for better redundancy, either to multiple access points of a single
ISP or to multiple ISPs.
BGP AD
 EBGP-20
 IBGP-200
Characteristics of BGP-4
The key features of BGP-4 include and not limited to these:
1. It is an advanced distance-vector protocol.
2. BGP sends full routing updates at the start of the session, trigger updates are sent afterward.
3. BGP maintains connection by sending periodic keepalives.
4. It creates and maintains connections between peers, using TCP port 179.
5. BGP sends a triggered update when a keepalive, an update, or a notification is not received
6. It has its own routing table, although it is capable of both sharing and inquiring of the interior IP
routing table.
7. BGP uses a very complex metric, and is the source of its strength. The metric, referred to as
attributes, allows great flexibility in path selection.
8. It selects the route based on the AS Path.
9. It selects that route, which provides a network with least AS.
10. Max AS length 65535
11. 1 to 64511 Public
12. 64512 to 65535 Private
13. BGP doesn’t discover neighbour automatically. In BGP we have to define neighbours. They are
called BGP peers.
14. BGP supports only one type of authentication that is MD-5

How to advertise Routes in BGP?
Using two ways
 Using Network Command
 Redistribution
BGP Messages
1. Open- BGP sends Open message using TCP Port no 179.
Content of Open Message
Version – 4 (Right now we are using BGP V4)
MY-AS – Itself AS number
Router ID
Hold Time – 180 sec (Default)
2. Keep alive- BGP sends periodic keep alive. Default time 60 sec.
3. Update- When two routers become BGP neighbour they send update message.
Contents of update
a) Routes
b) Route Attributes- These are those criteria’s which are used to select the best path.
4. Notification- When a neighbour is reset then it sends notification message. It contains the
cause of resetting.
BGP Tables
1. Neighbour
2. BGP Table
3. Routing Table
BGP States
1. Idle- Searching for Neighbour.
2. Connect- TCP three way handshake done.
3. Open sent- Open message has been sent.
4. Open confirm- Open message has been received.

5. Established- Connection Done.
BGP Terminology
1. Next Hop Self- When a BGP edge router learns the external routes then it advertise those
routes with default next hop to IBGP neighbour.
To solve this drawback we have a solution that is called next hop self.
This command tells a router give your own IP address as a next hop to your IBGP neighbor.
(Note: Always applied on edge routers.)
2. Route Reflector Client- Normally an IBGP neighbour never exchanges the routes of one
neighbour with another neighbour.
To solve this problem we have a solution that is called route reflector client.
This command tells a router do exchange the routes of one neighbor with another neighbor.
Figure 3 BGP States

3. EBGP-Multihop- When a BGP router wants to establish EBGP neighbourship then it set TTL
value 1 in open message. If your neighbour is not directly connected, then you have to change
the TTL Value using EBGP Multihop command.
4. Max-Path- By default BGP selects one best path using route attributes. If you want to
implement load balancing then you have to change max path value.
5. BGP- By default, the redistribution of iBGP into IGP isn’t allowed on Cisco IOS.
6. Update Source- When we want to establish neighbourship via loopback then we have to use
update source command.
7. Aggregation- In BGP Summarization is called Aggregation.
BGP Attributes/Rich Metric/Path Attributes
1. Weight- Cisco Proprietary Attributes.
Directly Connected – 32768
Indirectly Connected – 0
Preferred – Directly Connected.
2. Local Preference
Default 100 < 4.2 billion
Preferred – Higher
3. Self-Originate- a router gives preference to self-originate routes.
4. AS-Path- Lowest Path will be Preferred.
5. Origin- ebgp >ibgp >Redistributed
6. MED- Multi Exit Discriminator
By default – 0
Lowest will be preferred.
7. eBGP > iBGP- eBGP will be preferred.
8. IGP Cost- Select Lowest Cost

9. Ebgp Peer- Old neighbour will be preferred.
10. Router ID- Lower Router ID will be preferred.
By default BGP selects single path, for load balancing we use maximum path command.
BGP Path Selection
BGP could possibly receive multiple advertisements for the same route from multiple sources. BGP
selects only one path as the best path. When the path is selected, BGP puts the selected path in the IP
routing table and propagates the path to its neighbours. BGP uses the following criteria, in the order
presented, to select a path for a destination:
1. If the path specifies a next hop that is inaccessible, drop the update.
2. Prefer the path with the largest weight.
3. If the weights are the same, prefer the path with the largest local preference.
4. If the local preferences are the same, prefer the path that was originated by BGP running on this
router.
5. If no route was originated, prefer the route that has the shortest AS_path.
6. If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP
is lower than EGP, and EGP is lower than incomplete).
7. If the origin codes are the same, prefer the path with the lowest MED attribute.
8. If the paths have the same MED, prefer the external path over the internal path.
9. If the paths are still the same, prefer the path through the closest IGP neighbour.
10. Prefer the path with the lowest IP address, as specified by the BGP router ID.
Message Header Format
1. Marker- Included for compatibility, must be set to all ones.
2. Length- Total length of the message in octets, including the header.
3. Type- Type of BGP message. The following values are defined:
a) Open (1)
b) Update (2)
c) Notification (3)
Figure 4 Message Header Format

d) KeepAlive (4)
e) Route-Refresh (5)
BGP Example
R1 (config) #int fa0/0
R1 (config-if) #ip add 172.168.101.1 255.255.255.0
R1 (config-if) #no shut
R1 (config-if) #int s0/0
R1 (config-if) #ip add 192.168.1.1 255.255.255.0
R1 (config-if) #ip add 192.168.4.2 255.255.255.0
R1 (config-if) #do sh ip int br
Figure 5 BGP Example Topology

R2 (config-if) #ip add 172.168.102.1 255.255.255.0
R2 (config-if) #ip add 192.168.1.2 255.255.255.0
R2 (config-if) #ip add 192.168.2.1 255.255.255.0
R3 (config-if) #int 172.168.103.1 255.255.2550
R3 (config-if) #ip add 192.168.2.2 255.255.255.0
R3 (config-if) #ip add 192.168.3.1 255.255.255.0
On Router 3, we will create some loopback also
R3 (config-if) #int lo 1
R3 (config-if) #ip add 172.30.1.1 255.255.255.0
R3 (config-if) #ip add 172.30.2.1 255.255.255.0
R3 (config-if) #ip add 172.30.3.1 255.255.255.0
R3 (config-if) #ip add 172.30.4.1 255.255.255.0
R3 (config-if) #ip add 172.30.5.1 255.255.255.0
R4 (config-if) #ip add 172.168.104.1 255.255.255.0
R4 (config-if) #ip add 192.168.3.2 255.255.255.0

R4 (config-if) #ip add 192.168.4.1 255.255.255.0
Here we will Perform BGP Routing
R1 (config) #router bgp 100
R1 (config-router) #network 192.168.1.0
R1 (config-router) #network 172.168.101.0 mask 255.255.255.0
R1 (config-router) #neighbour 192.168.1.2 remote-as 100
BGP Creates Three Tables
 Neighbour Table

R1#sh ip bgp neighbours
R1#sh ip bgp neighbours 192.68.1.2
R1#sh ip bgp neighbours 192.68.4.1
 BGP Table
R1#sh ip bgp
 Routing Table
R1#sh ip route bgp
Now if we talk about the network 172.30.0.0, we can see this network on R1 via 2.2. Because it
advertises routes with default next hop which is 2.2. Now on edge router we run a command next hop
self.
Here edge router is R2 & R4.
R2 (config-router) #neighbour 192.168.1.1 next-hop-self
R4 (config-router) #neighbour 192.168.4.2 next-hop-self
Here we tell router, provide your own IP add as a next hop.
In BGP a router will share the information with the directly connected router only. Router 1 would not
share the information with the Router 3. For that here we will run route reflector client command.
R2 #sh ip route bgp
We won’t see any routes of 104 networks.
R4 #sh ip route bgp
We won’t see any routes of 102 networks.
R1 (config-router) #neighbour 192.168.1.2 rout-reflector-client
R1 (config-router) #neighbour 192.168.4.1 rout-reflector-client
R4#sh ip route bgp
Now we can see here 102 routes.
R2#sh ip route bgp

We can see 104 routes here.
R1#sh ip bgp
First rich metric is weight
For indirectly connected 0
Next local preference 100. Always prefer higher
Next AS path 200
Next self-originate
MED is by default 0
We have two ID 4.1 & 1.2. 1.2 will prefer (lower will prefer)
R1#sh ip route bgp
Here we can see 172.30.0.0 route learn via 192.168.1.2
R1#traceroute 172.30.1.1 via 1.2
R1#sh ip protocols
By default it selects only one path.
If we want to implement load balance then we need to change max path value.
R1 (Config) #router bgp 100
R1 (config-router) #maximum-paths?
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BGP (border gateway routing protocol)

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BGP (border gateway routing protocol)