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Webcast-Catalyst9k
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Multicast problem in an HSRP environment.

I have a Cat-3560 connected to two 6509's that are running HSRP. IP PIM is config'd in my environment using Anycast RP with MSDP. IP multicast works great everywhere except the segment that has HSRP on it:

6509-A

int vlan 20

ip-address 164.x.x.3/25

ip pim-sparse-mode

standby 1 priority 110

standby 1 preempt

standby 1 ip 164.x.x.1

6509-B

int vlan 20

ip-address 164.x.x.2/25

ip pim-sparse-mode

standby 1 preempt

standby 1 ip 164.x.x.1

A client on the 164.x.x.0 segment is trying to join a mcast group with unpredicatable results, but when I remove HSRP off one of the 6509s, the client has no problem joining and getting the multicast stream.

Is there anything special to run multicast in a design where clients are connected to a switch which in turn is

connected to two routers via HSRP?

5 REPLIES

Re: Multicast problem in an HSRP environment.

I have noticed that you used preempt on both Routers .. this is not correct you should use preempt only on one HSRP ( the one with higher priority ) otherwise both might try to become the HSRP active and who knows perhaps this is causing your issue .. anyway .. I have set up a customer which has similar configuration and this is the config I had applied to the HSRPs and they are working OK.

int vlanX

ip address 172.23.28.253 255.255.255.0

standby 28 ip 172.23.28.1

standby 28 priority 200

standby 28 preempt

ip helper-address 172.24.1.17

ip helper-address 172.24.1.38

ip pim dense-mode

ip cgmp

I hope ot helps .. please rate it if it does !!!

Re: Multicast problem in an HSRP environment.

Actually the correct HSRP configuration should be setting preempt in both routers.

The Router with higher priority will become active if you set preempt in both routers.

you could add a delay in this command to prevent that a fast flapping situation cause the change in the HSRP.

standby 1 preempt delay minimum 60

http://www.cisco.com/warp/public/619/6.html

Vlad

Hall of Fame Super Gold

Re: Multicast problem in an HSRP environment.

It is certainly not true that you should not use preempt on both routers or that having preempt on both routers will cause both of them to become active.

There are multiple scenarios in which you need preempt on both routers. Perhaps the most common is this: if you configure priority so that one router has higher priority and you want it to be the active router in HSRP you need preempt on that router. If you do not configure preempt on the higher priority router if it ever goes out of service and loses its active state, it will not become the active router when it comes back in service. Then if you configure the track feature and do not configure preempt on the standby router it can not become the active router if the tracked interface goes down.

There are other scenarios but I believe this is the most common set of reasons to configure preempt on both routers.

HTH

Rick

Re: Multicast problem in an HSRP environment.

Having preempt on both routers will not cause the two routers to become active, as others would say. the cause of two routers being active is the lost of hellos from either of the router. for that you need to check the connectivity between the two routers. Have you checked the HSRP's status on each router when you were having mcast issues? Do you see a client join on show ip igmp group? what does the show ip mroute look like on each router when issue is present? you should take a snapshot of both outputs when issue is there and when issue is not there.

Re: Multicast problem in an HSRP environment.

Yes .. indeed I had reviewed the HSRP election process and in fact you guys are correct !! .. it is possible to have preempt on more than one router ...

Cheers for you comments !!

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