Multicast Notes

The amount of multicast on the outline is a bit sparse, so to speak. Sorry, I just couldn’t resist. Anyway, with this in mind, there are just a few things to note with respect to multicast design.

  1. Don’t use Dense mode. Only use Sparse.
  2. When using Anycast-RP, or any design where you’re using multiple loopbacks, it’s a best practice to hardcode the router id in your IGP.
  3. If you can use Anycast-RP and AutoRP together, do it.
  4. A new alternative to the RP of last resort is no ip pim dm-fallback

Sparse mode requires a Rendezvous Point (RP) to hold the shared tree information. There are several ways of assigning RPs. You can use AutoRP, BSR, or Anycast-RP.

AutoRP requires candidate RPs and mapping agents, which can be set up with the commands:

ip pim send-rp-announce scope #
ip pim send-rp-discovery scope #
ip pim autorp listener (required since you’ll be using sparse mode)
no ip pim dm-fallback (so it won’t fall back to dense mode)

Also note that the interface or IP address you use must be reachable via IGP.

Some outputs:

R101 is the RP and Mapping Agent:

R101#sho ip pim rp map
PIM Group-to-RP Mappings
This system is an RP (Auto-RP)
This system is an RP-mapping agent (Loopback0)

RP (?), v2v1
Info source: (?), elected via Auto-RP
Uptime: 01:04:06, expires: 00:02:13

BSR is another RP mechanism – BSR stands for BootStrap Router. You can configure it like so:

ip pim bsr-candidate
ip pim rp-candidate
no ip pim dm-fallback

Anycast RP is an RP mechanism where you configure the same RP address on multiple routers. You then need to ensure all routers in your topology have this IP address configured statically as their RP.

On the RP’s:
ip pim rp-address
interface loopback10
ip address
interface loopback0
ip address
ip msdp peer connect-source loopback0
ip msdp originator-id loopback0

You can also combine AutoRP and Anycast RP to avoid the configuration of an RP address on every router.


Cisco IOS 12.4T IP Multicast Configuration Guide

Verifying IP Multicast Operation