Multicast - Routing Table Timers

The multicast routing table in a Cisco IOS router displays various pieces of information concerning the Multicast topology. Among the information shown are a series of timers. These timers will be somewhat different depending upon the type of multicast being used.

PIM Dense mode scenario

Take a look at the following routing table output in a PIM Dense Mode scenario:

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

There are essentially six timers that are indicated for the (192.168.1.1, 239.1.1.1) entry, and these are highlighted in red. Below is an explanation of each of them:

(192.168.1.1, 239.1.1.1), 00:01:16/00:01:43,

  • This line displays information that is pertinent to the entry itself, independent of any interfaces
  • 00:01:16: This is the "uptime" of the multicast routing entry, indicating how long this particular multicast route has been active. This value continues to increase indefinitely until the route expires.
  • 00:01:43: This is the "expiry time" of the multicast routing entry. It indicates how much longer this route will remain active before timing out, assuming no new multicast traffic is seen. Every time new multicast traffic arrives, this value is reset to 180 seconds.

GigabitEthernet0/1, Forward/Dense, 00:01:16/stopped

  • 00:01:16: This timer is associated specifically with the GigabitEthernet0/1 interface and indicates the uptime of the multicast route on this interface.
  • stopped: In the context of Dense Mode multicast routing (indicated by "Dense"), this means that there is no expiry time. The expiry timer is stopped, which means that this interface will continue to forward until it is pruned. This is characteristic of Dense Mode..

GigabitEthernet0/2, Prune/Dense, 00:01:15/00:01:43

  • 00:01:15: This is the uptime of the multicast route on the GigabitEthernet0/2 interface.
  • 00:01:43: This timer is specific to PIM Dense mode. It is the prune timer for the GigabitEthernet0/2 interface. The interface is pruned and is thus not forwarding multicast traffic. This timer indicates how much longer the interface will remain in the pruned state before it will again be eligible to receive multicast traffic. If no prune message is received within this period, the interface will transition to the forwarding state and will resume receiving multicast traffic.

PIM Sparse mode scenario

Take a look at this output of the show mroute command in a PIM sparse mode scenario:

R1#show ip mroute 239.3.3.3

(*, 239.3.3.3), 00:05:01/stopped, RP 2.2.2.2, flags: SPF
  Incoming interface: GigabitEthernet0/1, RPF nbr 192.168.12.2
  Outgoing interface list: Null

(192.168.1.1, 239.3.3.3), 00:05:01/00:02:59, flags: FT
  Incoming interface: GigabitEthernet0/3, RPF nbr 0.0.0.0
  Outgoing interface list:
    GigabitEthernet0/1, Forward/Sparse, 00:05:01/00:02:36

Here we have four timers that are indicated for the (192.168.1.1, 239.3.3.3) entry, and these are highlighted in red. For the most part, the timers are the same as those used for Dense Mode, except for the second timer in the interface entry. Below is an explanation of each of them:

192.168.1.1, 239.3.3.3), 00:05:01/00:02:59

  • As before, this line displays information that is pertinent to the entry itself, independent of any interfaces.
  • 00:05:01: This is the "uptime" of the multicast routing entry, same as in Dense Mode.
  • 00:02:59: This is the "expiry time" for the multicast routing entry, same as in Dense Mode

GigabitEthernet0/1, Forward/Sparse, 00:05:01/00:02:36

  • 00:05:01: This timer, associated with the GigabitEthernet0/1 interface, indicates the uptime of the multicast route on this specific interface.
  • 00:02:36: This is a unique timer for Sparse Mode multicast routing. In Sparse Mode, this timer is related to the Join/Prune mechanism. It indicates the amount of time left before the router expects to receive another Join message for this group on this interface. If the timer expires without receiving a Join message, the interface will be pruned from the multicast distribution tree for this group.

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

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