Sup720 multicast

Unanswered Question
Dec 13th, 2008


What would you recommend for a multicast configuration on dual 6500's with Sup720's ? Each of the 720's have about 20 VLAN's/subnets. No routing protocol. All hosts point to the Sup720 as their default gateway and anything that is not connected to the 720 is default routed to a firewall.

I have a multicast source on one VLAN and a receiver can appear on any otherVLAN/subnet. I can get it working using PIM sparse or dense but do I really need this ? PIM seems the only way I can get the 720 to register the IGMP join from the destination and route the multicast traffic appropriately.

Thanks, Stephen.

I have this problem too.
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Giuseppe Larosa Sat, 12/13/2008 - 15:43

Hello Stephen,

you have already found the answer: you need to enable PIM.

PIM is independent of the routing protocol in use so it can be used even if no dynamic routing protocol is in place.

PIM is needed because it is the only way to perform multicast routing and forwarding in a Cisco device: multicast forwarding requires for each multicast group (for PIM SM) for each (source, multicast group) pair (for PIM DM) to build and keep updated a replication tree on each node (the oilist= output interface list). This list is dynamically updated by:

other routers PIM messages (Join or Prune)


by IGMP messages from end users hosts and this is your case.

I would use PIM sparse mode it uses less resources.

There are some specific features that can help a branch/stub router to forward IGMP messages even if they are also L3 devices

see PIM stub

and igmp helper address

But I don't think they fit your scenario because these features move to another deive the duty to build the replication tree.

Hope to help


StevieOliver_2 Sun, 12/21/2008 - 12:39


What would you use as the RP in this type of configuration ? And is there any way to have RP Resillience ? Again, I have this working by simply pointing the RP on both Sup720's to an interface IP address on one of the Supervisors. This seems to work. But I assume if the Sup with the address fails I no longer have an RP so multicast will fail. Bear in mind I have no router in the network apart from the 720's which are not in a redundant configuration themselves. I can see the need for an RP in a multi router network but this network is effectively a router with many interfaces and the multicast traffic only needs to cross from one interface to another. No further.

Thanks, Stephen.

StevieOliver_2 Mon, 12/22/2008 - 13:04


"In an anycast RP configuration, two or more RPs are configured with the same IP address on loopback interfaces"

I can understand this concept in a fully routed network. You have the same address being seen from multiple directions and these are used as the RP. How does this operate in a network where there are only 2 routers. If I configure, say on both routers as a loopback am I effectively telling each router that they are the RP ? They both think is the RP and because that is a connected network on each router they see themselves as the RP ? And how does MSDP come into this if thereis no routing between the Sup 720's ? The 720's just run HSRP for all their connected, shared VLAN's and anything that is not connected to the 720's is default routed to a PIX. This is a layer 2 network at the access layer with a collapsed backbone in the Core where all the VLANs connect to both 720's. The multicast source is on one VLAN and the receivers are on any one of the other VLANs. I simply need the multicast source to send the traffic to it's default gateway, a 720 and the 720 to pass this traffic onto another connected VLAN where it sees a multicast receiver via an IGMP join.

Thanks, Stephen.

Edison Ortiz Tue, 12/23/2008 - 08:54


Anycast RP is just one method of having RP resiliency.

You can also implement any other PIM design with Auto-RP, BSR or Static RP.

Anycast RP along with your current topology, the clients will have the active HSRP router and this router will be the active PIM router. If this router's HSRP function fails, the secondary router will take over and be the PIM router.

MSDP will allow you to sync the information about the active multicast sources.




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