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Multicast Mroute behavior for local VLAN multicast (with overlap)

We have a multicast address that exists at various locations but we want to isolate them at each location to a single VLAN.  We run PIM Sparse with anycast RP throughout our Enterprise.  Our original thought was to just turn off PIM on the SVI/VLAN at a campus and isolate that multicast to the VLAN it is in.

We are still learning the Multicast address via PIM from the other sites we have not turned PIM off yet (since it is an overlapping address).  The question came up that if you disable PIM for that VLAN/SVI would the multicast stay local if the default gateway for that VLAN still has a multicast route it learned via PIM from another site?  Or would that VLAN without PIM still use the mroute table to connect to the Multicast address?

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Hall of Fame Super Blue

Re: Multicast Mroute behavior for local VLAN multicast (with ove

The question came up that if you disable PIM for that VLAN/SVI would the multicast stay local if the default gateway for that VLAN still has a multicast route it learned via PIM from another site?  Or would that VLAN without PIM still use the mroute table to connect to the Multicast address?

If you disable PIM on the SVI then that SVI should not be able to route multicast traffic either to clients in the vlan or from clients in the vlan.

Jon

5 REPLIES
Hall of Fame Super Blue

Re: Multicast Mroute behavior for local VLAN multicast (with ove

The question came up that if you disable PIM for that VLAN/SVI would the multicast stay local if the default gateway for that VLAN still has a multicast route it learned via PIM from another site?  Or would that VLAN without PIM still use the mroute table to connect to the Multicast address?

If you disable PIM on the SVI then that SVI should not be able to route multicast traffic either to clients in the vlan or from clients in the vlan.

Jon

New Member

Multicast Mroute behavior for local VLAN multicast (with overlap

Thank you for the quick reply

Hall of Fame Super Blue

Re: Multicast Mroute behavior for local VLAN multicast (with ove

No problem.

One thing i forgot to mention is that if you disable PIM then nothing is making IGMP queries for that vlan anymore. This means that IGMP snooping within that vlan will not work properly as it listens to responses to IGMP queries.

You could look at using the IGMP snooping querier function if supported by the switch. This generates the IGMP queries needed by IGMP snooping to map the port to multicast mac address at L2.

Enabling it would still mean you could not route the multicast stream to and from the vlan as you need PIM on the L3 interface to do that.

Jon

Re: Multicast Mroute behavior for local VLAN multicast (with ove

Hello

.

@Jon

Not sure this would be applicable to SVI's as I assume it would be like disabling pim, but I was wondering about prohibiting the RP announcements on the host interfaces then they I suppose wont be able to join the MC group - what do think ?

access-list 10 deny 224.0.1.40 ( RP announce messages)

acccess-list 10 permit any

int xx

ip multicast boundary 10

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

Please don't forget to rate any posts that have been helpful. Thanks.
Hall of Fame Super Blue

Re: Multicast Mroute behavior for local VLAN multicast (with ove

Paul

Sorry, i didn't realise this was addressed to me.

To be honest i have never used this command but reading up on it it would seem you certainly could block the flow of multicast packets between the interface for the group(s) you wanted to so it would be a way of isolating traffic between vlans.

In terms of the RP announcement messages there is a keyword you can add to the command "filter-autorp" which filters discovery and announcement messages as well. But i wouldn't like to say for sure exactly how it would or wouldn't work without testing ie. do you need to filter the RP announcements before they reach the SVI or would it work applied to the SVI.

I could see a use for this if you had a number of SVIs that needed to route the multicast stream between themselves but not to other L3 subnets. But the topology would be a little difficult on a L3 switch ie. if half the SVIs need to route the multicast stream but the other half didn't where do you apply the command. It would have to be on each SVI you didn't want to receive the stream.

It would work better if, for example, you had a L3 switch connected to a router with L3 routed ports and all the SVIs should route the multicast stream but it should stay local to the L3 switch so it is blocked on the L3 uplink (and other potential streams from the router to the L3 switch blocked as well).

Hope that made some sense. It's sometimes difficult to explain things without a diagram

Jon

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