Learning Multicast Part 6 Troubleshooting

David Hedley’s
Tuesday Tech Talks
Multicast Part 6 Troubleshooting
Turning networking on it’s head
© 2018-2020 David M. Hedley All Rights Reserved.

David Hedley's Tuesday Tech Talks – Multicast Pt. 6
• Cisco Certified Network Professional: Enterprise, Routing & Switching
• Cisco Certified Design Professional
• Cisco Certified Specialist: Enterprise Core, Enterprise Design, Wireless
Design, Wireless Implementation, Advanced Infrastructure
Implementation.
• Cisco Certified Network Associate: Routing & Switching, Wireless, Data
Center, Security
• Cisco Certified Design Associate
• ITIL v4 Foundations
• CompTIA A+
• https://www.youracclaim.com/users/david-hedley/badges

• Specific brands and models are for illustration purposes only.
• They do not imply any endorsement by the vendor, in any way.
• This talk does not represent the business process of any employer or
client, past or present, this is based on my own work and study.
• I only include equipment that I have experience with.
• At the time of writing, I have not received any compensation, or
inducement from any vendor.

• Definition
• In computer networking, multicast is group communication[1] where data
transmission is addressed to a group of destination computers
simultaneously. Wikipedia https://en.wikipedia.org/wiki/Multicast retrieved July 5,2018
• Originally defined in RFC 966 (1985)

• Purpose: How to troubleshoot your multicast network.
• Question: How do I test and troubleshoot what I’ve built (have)?

• Triple Constraints
• Time
• Quality or Scope
• Money or Budget

• Unicast vs. Multicast

• Multicast Advantages
• Eliminates Traffic Redundancy = Less Bandwidth
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Unicast vs Multicast
Mulicast Unicast

• Multicast Addressing (IPv4)
• Source (Class A, B, C) 1.0.0.0 – 223.255.255.255
• Destination (Class D) 224.0.0.0 – 239.255.255.255
• 232.0.0.0 – 232.255.255.255 Source Specific Range
• 239.0.0.0 – 239.255.255.255 Administratively Scoped Addresses
• SOURCE CAN NEVER BE CLASS D GROUP ADDRESS!!!!

• Host-Router – Internet Group Management Protocol (IGMP)
• Hosts tells routers about group membership.
• Routers solicit group membership from hosts.
• IGMP v. 1 RFC 1112 (1989) https://www.rfc-editor.org/rfc/rfc1112.txt
• For IPv6 Multicast Listener Discovery (MLD) v.1 is equivalent to IGMP v. 2
• MLD v. 2 is the IPv6 equivalent for IGMP v. 3.

• Multicast Global Commands
• Ip multicast-routing [distributed]
• Ip multicast multipath

• Multicast Boundaries
• ip multicast boundary access-list [filter-autorp]
• Placed at the boundary interfaces of a multicast region.
• Do NOT use the filter-autorp option on an RP.
• Filter-autorp will filter Auto-RP messages which for ranges denied by the
boundary ACL.
• Example
• Ip access-list standard MCAST_BOUNDARY
• Permit 239.200.0.0 0.0.255.255
• Permit 224.0.0.0 7.255.255.255
• Deny 224.0.0.0 15.255.255.255

• Hot Standby Router Protocol (HSRP) and PIM
• When there are 2 or more routers running PIM on a single segment, one will
be elected the Designated Router (DR)
• DR is responsible for PIM Joins upstream
• Chosen by highest IP Address.
• Highest IP device might not be optimal path.
• DR can be manually set with ip pim dr-priority command.
• Default priority is 1.
• Highest priority is the DR..

• Troubleshooting
• Rendezvous Point
• Show ip pim rp mapping (any PIM enabled device)
show ip pim rp mapping
PIM Group-to-RP Mappings
This system is an RP (Auto-RP)
This system is an RP-mapping agent (Vlan108)
Group(s) 239.200.0.0/16
RP 192.168.254.4 (?), v2v1
Info source: 192.168.254.4 (?), elected via Auto-RP
Uptime: 5w0d, expires: 00:02:23

• Troubleshooting
• Show ip pim autorp (any PIM enabled device)
show ip pim autorp
AutoRP Information:
AutoRP is enabled.
RP Discovery packet MTU is 1500.
224.0.1.40 is joined on Vlan108.
AutoRP groups over sparse mode interface is enabled
PIM AutoRP Statistics: Sent/Received
RP Announce: 51565/51564, RP Discovery: 51949/0

• Troubleshooting
• show ip pim bsr-router (any PIM enabled device)
show ip pim bsr-router
PIMv2 Bootstrap information
This system is the Bootstrap Router (BSR)
BSR address: 172.31.255.255 (?)
Uptime: 20:42:25, BSR Priority: 192, Hash mask length: 30
Next bootstrap message in 00:00:47
Candidate RP: 172.31.255.239(Loopback239)
Holdtime 150 seconds
Advertisement interval 60 seconds
Next advertisement in 00:00:32
Group acl: MCAST_GROUP_ANNOUNCE
Candidate RP priority : 224

• Troubleshooting
• PIM
• Show ip pim interface (any PIM enabled device)
show ip pim int
Address Interface Ver/ Nbr Query DR DR
Mode Count Intvl Prior
192.168.254.4 Vlan108 v2/S 1 30 1 192.168.254.4

• Troubleshooting
• PIM
• Show ip pim interface interface detail (any PIM enabled device)
show ip pim int vlan 108 detail
Vlan108 is up, line protocol is up
Internet address is 192.168.254.4/24
Multicast switching: fast
Multicast packets in/out: 0/0
Multicast TTL threshold: 0
PIM: enabled
PIM version: 2, mode: sparse
PIM DR: 192.168.254.4 (this system)
PIM neighbor count: 1
PIM Hello/Query interval: 30 seconds
PIM Hello packets in/out: 103995/104800
PIM State-Refresh processing: enabled
PIM State-Refresh origination: disabled
PIM State-Refresh: 1 non-capable neighbor
PIM NBMA mode: disabled
PIM ATM multipoint signalling: disabled
PIM domain border: disabled
PIM neighbors rpf proxy capable: FALSE
Multicast Tagswitching: disabled

• Troubleshooting
• PIM
• Show ip pim neighbor interface (any PIM enabled device)
show ip pim neighbor
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
P - Proxy Capable, S - State Refresh Capable, G - GenID Capable
Neighbor Interface Uptime/Expires Ver DR
Address Prio/Mode
192.168.254.1 Vlan108 5w0d/00:01:31 v2 1 / G
• What if no neighbor?
• Missing PIM statements
• Unicast addresses not on same subnet
• Link down

• Troubleshooting
• PIM
• Show ip mroute [ group-address ] [ count ]
show ip mroute verbose
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry, E - Extranet,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report,
Z - Multicast Tunnel, z - MDT-data group sender,
Y - Joined MDT-data group, y - Sending to MDT-data group,
V - RD & Vector, v - Vector
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 224.0.1.39), 5w1d/stopped, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Vlan108, Forward/Sparse, 5w1d/stopped
(192.168.254.4, 224.0.1.39), 5w1d/00:02:27, flags: PLT
Incoming interface: Vlan108, RPF nbr 0.0.0.0
Outgoing interface list: Null

• Troubleshooting
• PIM
• Show ip mroute [ group-address ] [ count ]
show ip mroute count
IP Multicast Statistics
6 routes using 2736 bytes of memory
4 groups, 0.50 average sources per group
Forwarding Counts: Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second
Other counts: Total/RPF failed/Other drops(OIF-null, rate-limit etc)
Group: 239.255.255.250, Source count: 0, Packets forwarded: 0, Packets received: 13350
RP-tree: Forwarding: 0/0/0/0, Other: 13350/0/13350
Source: 192.168.254.4/32, Forwarding: 0/0/0/0, Other: 0/0/0
Source: 192.168.254.4/32, Forwarding: 0/0/0/0, Other: 0/0/0

• Troubleshooting
• IGMP
• show ip igmp [ vrf vrf-name ] groups [ group-name | group-address | interface-type
interface-number ] [detail]
show ip igmp groups
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter Group Accounted
239.255.255.250 Vlan50 11:17:30 00:01:15 192.168.254.72
239.200.129.4 Vlan50 20:43:24 00:01:11 192.168.254.66
224.0.1.40 Loopback0 20:46:09 00:02:00 172.31.255.255

• Troubleshooting
• IGMP
• how ip igmp snooping [ groups [ count | vlan vlan-id [ ip-address | count ] ] | mrouter [ [ vlan vlan-id ] | [ bd bd-id ] ] | querier
| vlan vlan-id | bd bd-id ]
246_Core#show ip igmp snooping
Global IGMP Snooping configuration:
-------------------------------------------
IGMP snooping : Enabled
IGMPv3 snooping (minimal) : Enabled
Report suppression : Enabled
TCN solicit query : Disabled
TCN flood PortFast : Disabled
TCN flood query count : 2
Robustness variable : 2
Last member query count : 2
Last member query interval : 1000
Vlan 1:
--------
IGMP snooping : Enabled
IGMPv2 immediate leave : Disabled
Multicast router learning mode : pim-dvmrp
CGMP interoperability mode : IGMP_ONLY
Robustness variable : 2
Last member query count : 2
Last member query interval : 1000

• Troubleshooting
• General Concepts
• For ASM, all layer 3 devices need to be using the same RP or a set of RP’s joined with
MSDP, for any group.
• PIM needs to be enabled on all the links between, source and RP (if needed), receiver
and RP (if needed), and source and receiver.
• Multipath command is essential where there are equal cost paths anywhere along the
route.
• Boundaries can block traffic, so be on the lookout for boundary statements, and ACL’s on
interfaces.
• The unicast routing must be working for multicast to work.

• Troubleshooting
• Sample Troubleshooting.
• On last hop router, does the (*,G) exist?
• Is the application working?
• If I add ip igmp join-group group-address on to the local Layer 3 interface, does the (*.G) show up?
• (*,G) exists but (S,G) doesn’t
• Show ip mroute group-address, where does the incoming interface point for the (*.G)?
• If the incoming interface is NULL, then this devices thinks it’s the RP. Is that correct?
• Show ip pim rp mapping, you can find out which device is the RP, and login to that device. Look at
show ip msdp sa-cache, if running MSDP, and look for sender. Else look at show ip mroute and see
if the sender is registered with the RP.
• Check first hop and last hop routers using show ip pim rp mapping and make sure they’re using the
same RP.
• T flag not setting.
• Check path with ping and traceroute, is the unicast route workable?
• Use mtrace to check the multicast path.
• Look for multipath statements where equal cost paths exist.

• Troubleshooting
• Other tools
• Take a look at https://www.cisco.com/c/en/us/support/docs/ip/ip-multicast/13726-
57.html
• Mtrace

• Components

• Tech Talk #2
• To do list!
• List of multicast apps, with technical requirements, and current configs
• Decide and/or document where these multicast apps need to operate, by application
• Look at the needs for multicast on your wireless network.
• Decide and/or document where your RP’s should be(are) placed, and the ranges they
advertise.
• Should there be additional redundancy for any (all) of your RP’s (if needed.)
• Does first line staff know enough to do basic troubleshooting? Remedy as needed.
• Decide and/or document how you will use the administratively scoped range.
• Document all group addresses and ranges in use on your network, and include what
group is responsible for the application and note any additional support requirements.

• Thanks for watching!
• You can subscribe to my YouTube Channel
https://www.youtube.com/channel/UCZ3pcIh5Zmbp3rdjhfR7BOg
• Or connect with me on Linkedin https://www.linkedin.com/in/david-
hedley-541985/
• You can suggest topics in the comments!

Learning Multicast Part 6 Troubleshooting

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