blocking certain multicast routes

[CSCO10022991]

Hi All,

I've two routers running "ip multicast-routing" with "ip pim dense-mode". Is there anyway I can block certain multicast routes on the receving router so that when using the command "show ip mroute", I can only see certain interested multicast routes ?

Thanks in advance.

[Peter Paluch]

Hello Felix,

It is not entirely clear what you are trying to accomplish. Do you simply want to limit the show ip mroute output, or do you actually want to prevent the router from forwarding multicast traffic to selected multicast groups?

Best regards,

Peter

[CSCO10022991]

Hi Peter,

I've no control over the mulitcast source router. It's sending multicast routes to my receiving router, some of which are I'm not interested in.

Therefore, I'm trying to limit the output of the show ip mroute so that the receiving router will display only multicat routes that I'm interested. 

Is there anyway which I can block the rest of the unwanted multicast routes from appearing on the receiving router.

Thanks.

[Peter Paluch]

Hi Felix,

It's sending multicast routes to my receiving router, some of which are I'm not interested in. 

No, this is not how multicast routing in PIM-DM works. The PIM-DM initially causes the multicast traffic to be flooded throughout the entire network, and routers not having any subscribers or neighbors wanting to receive that traffic will prune themselves afterwards. There is no exchange of "multicast routes" in PIM-DM. The PIM-DM merely allows routers to tell each other whether they want or do not want to continue receiving a particular multicast traffic after at least one packet already arrived.

Entries in your multicast routing table have been created as a result of receiving a particular multicast traffic. If you want your multicast routing table to not contain selected entries, you actually need to block the corresponding multicast traffic.

Does this make sense?

Best regards,

Peter

[CSCO10022991]

Hi Peter,

Correct me if I'm wrong, if I block certain multicast traffic using the ip multicast boundary in command, then the multicast routing table should not contain the routes of the blocked multicast traffic, right ?

Thanks again.

[Peter Paluch]

Hello Felix,

Yes, that is true. Have you tried to implement it already?

Best regards,

Peter

[CSCO10022991]

Hi Peter,

We tested between two routers in the lab. And noticed that if applying the ip multicast boundary in command on

the receiving router interface, we can still see the unwanted multicast routes.

But if applying the ip multicast boundary out command on the transmitting router interface, the unwanted multicast

routes do not appear.

Does that means that the only way it to apply the ip multicast boundary out command on the transmitting router

interface so that the unwanted routes will not appear on the receiving router ?

Thanks.

[Peter Paluch]

Hello Felix,

Would it be possible to post the configuration of your router including the ip multicast boundary command showing that it either does or does not work? Also, can you post the show ip mroute output and explain to us which entries you do not want to see?

Thank you!

Best regards,

Peter

[CSCO10022991]

[CSCO10022991]

[Peter Paluch]

Hello Felix,

I am currently researching this issue. In the meantime, are you able to use the ip multicast boundary out command on the upstream router?

Best regards,

Peter

[CSCO10022991]

Hi Peter,

Our intention is to block at the receiving router, but since it's not working, in the meantime, we'll use the ip multicast

boundary out command on the upstream router.

Thanks. 

[Peter Paluch · Accepted solution]

Hello Felix,

I have verified your scenario in lab, and I confirm your findings - if the ip multicast boundary command is used in the in direction then the router will still create mroute entries, both for accepted and denied traffic. The reason to create a mroute entry for a denied multicast group is that this entry carries the Pruned state for the respective group so that PIM-DM Prune message can be sent through this interface to stop the unwanted multicast traffic. As you can notice yourself, the (S,G) entries in your outputs for the unwanted groups carry the Pruned state, and the outgoing interface list is set to Null. The net effect is that

• the router will indeed refuse to receive the unwanted multicast traffic, stopping the multicast traffic at the upstream router instead of silently discarding it itself
• the unwanted multicast traffic will not be forwarded further

In other words, this is how the multicast boundary in the inbound direction is implemented in IOS for PIM-DM, it is working as expected and it can not be done differently.

So if you really want to prevent the mroute entries from being created in your downstream router, the only option I currently see is to implement the ip multicast boundary out on the upstream router.

Best regards,

Peter

[CSCO10022991]

Thanks Peter for the confirmation.