Multicast - reading the multicast routing table

The Multicast routing table displays is a data structure that keeps track of multicast group memberships, source and receiver information, and the routing paths to forward multicast traffic efficiently. It helps routers to identify and manage multicast traffic, which is a form of data transmission where a single source sends data to multiple receivers simultaneously.

R1#show ip mroute 239.1.1.1

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, 
       G - Received BGP C-Mroute, g - Sent BGP C-Mroute, 
       N - Received BGP Shared-Tree Prune, n - BGP C-Mroute suppressed, 
       Q - Received BGP S-A Route, q - Sent BGP S-A Route, 
       V - RD & Vector, v - Vector, p - PIM Joins on route, 
       x - VxLAN group
Outgoing interface flags: H - Hardware switched, A - Assert winner, p - PIM Join
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 239.1.1.1), 00:01:16/stopped, RP 0.0.0.0, flags: D
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    GigabitEthernet0/2, Forward/Dense, 00:01:16/stopped
    GigabitEthernet0/1, Forward/Dense, 00:01:16/stopped

(192.168.1.1, 239.1.1.1), 00:01:16/00:01:43, flags: T
  Incoming interface: GigabitEthernet0/3, RPF nbr 0.0.0.0
  Outgoing interface list:
    GigabitEthernet0/1, Forward/Dense, 00:01:16/stopped
    GigabitEthernet0/2, Prune/Dense, 00:01:15/00:01:43

Here is a summary of what we see above:

  • R1 is receiving multicast traffic for 239.1.1.1 from 192.168.1.1 on its Gi0/3 interface and has added this entry in its multicast routing table.
  • R1 has been forwarding this traffic for one minute and 16 seconds on the Gi0/1 interface.
  • If R1 does not receive any packets for this group for another one minute and 43 seconds then this entry will be removed from the multicast routing table. As long as we keep receiving packets, this timer will be resetted to three minutes.
  • The T flags means that we are using the shortest path source tree.
  • The RPF neighbor for R1 is 0.0.0.0, this value is empty since it’s a directly connected interface.
  • R1 is forwarding traffic on its Gi0/1 interface. It has been doing this for one minute and 16 seconds and there is no expiry. It will keep doing this until it gets pruned.
  • R1 has stopped forwarding traffic on its Gi0/2 interface since it is pruned.

For more information about the timers displayed in this output, take a look at Multicast - Routing Table Timers.

Links:

https://networklessons.com/multicast/multicast-routing

https://networklessons.com/multicast/multicast-pim-dense-mode

https://forum.networklessons.com/t/multicast-pim-sparse-mode/1335/34?u=lagapides

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