HSRP aware PIM

[concico9982]

Dears,

i would like to allow multicast traffic to flow between different vlan in an HSRP environment.

Here attached the network drawing.

In the attached example multicast traffic is required between hosts in vlan 10 and vlan 20.

Is HSRP aware PIM what i may need?

If so, is the the configuration correct?

Thanks

[Reza Sharifi]

Hi,

Your configs on the routers are correct since you already have ip pim enabled for each SVI.  Just make sure IGMP is enables for vlan 10 and 20 on your access switches:

verify with show command below:

sh ip igmp snooping vlan 10

HTH

[concico9982]

In my previous drawing I wrote in red the PIM configuration. Is the configuration of the rendezvous point with loopback0 on core A correct?

Moreover In blue the PIM commands related to HSRP. Are they mandatory in such environment?

Thanks

[conor.maton]

Hi Reza,

I appreciate this topic is over 4 years old but I have a question regarding this subject.

I notice in the OP topology that only one loopback address is configured which is also the RP address. What happens if the normally active HSRP router fails to the standby HSRP router? Does the loopback/RP address failover to the new router and ensures multicast services remain?

If so what would happen if the standby HSRP router has a loopback address configured for other purposes (e.g. management)? Perhaps it's a logical failover?

Thanks in advance.

[Richard Burts]

In an IOS router there is not anything involved in HSRP failover that would affect or change the loopback interface address. If the HSRP active router fails and the standby takes over any loopback on the first router remains the same as it was and any loopback on the second router remains as it was. If the loopback of the first router is configured as RP then after HSRP failover that same router remains as RP.

There is a solution for this situation called Anycast RP in which two routers each have a loopback interface and use the same IP for the loopback. These routers then use MSDP to exchange information and keep each other in sync. It provides redundancy and failover capability for PIM RP. You will find additional details about Anycast RP in this link

http://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/ip_multicast/White_papers/anycast.html

HTH

Rick 

[conor.maton]

Thanks for your reply Richard.

Just to confirm: with the RP configuration, only one RP address can be configured. Therefore should the active HSRP router fail which has the RP address you have lost all multicast services. is that correct?

[Richard Burts]

There are several things to address here:

- the original post was addressing one aspect of multicast PIM. You are really asking about a different aspect of PIM.

- the original post is talking about the DR in PIM that does multicast forwarding of packets. For the DR function there is an alternative that keeps the PIM DR function on the router that is currently the HSRP active router. And this provides failover for the DR so that if HSRP fails over to the standby router then PIM DR moves to the formerly standby/now active HSRP router. This failover for DR has nothing to do with RP.

- you are asking about PIM RP which manages multicast group membership. This is quite different from DR in PIM. While there is failover for DR associated with HSRP there is no kind of failover that is associated with HSRP for RP.

- you seem to believe that only a single RP can be configured. That is not the case. It is possible to have more than one RP configured. You might have router A configured as RP with one IP address and have router B configured as RP with a different IP address. In this case router A might function as RP for certain multicast groups while router B functions as RP for other groups. And this could provide redundancy so that if one of the routers fails then there is still an RP. It is also possible to have two routers configured as RP using exactly the same IP address (typically a loopback address). This is the Anycast RP solution that I mentioned. It certainly provides redundancy so that if one RP fails there is no impact on PIM functionality.

- you seem to assume that if there is an HSRP failover that the configured RP becomes inaccessible. That is true only in a limited case (the case where the RP is configured with the IP address of the Ethernet interface where HSRP is running) and is not true for most cases. Consider the example of the original post where the RP is a loopback interface. Even if there is an HSRP failover that only involves the interface in HSRP and has no impact on reachability to the RP on the loopback. And consider the case where the RP is the Ethernet interface where HSRP is running. And consider the possibility that HSRP is configured to track some interface and the tracked interface goes down. This generates an HSRP failover but the original HSRP/RP interface is still active and reachable and there is no impact on PIM functionality.

- your question does raise a very valid point. If there is a single RP configured then multicast has a significant single point of failure. But this aspect has nothing to do with HSRP.

HTH

Rick 

[conor.maton]

Hi Richard,

Many thanks for your help here, been a great learning curve for me as multicasting is a topic I am a little deficient in.

I like the anycast RP solution. I have two HSRP routers and I am using Loopback0 for my routers for management, however there is no reason why I cannot use Loopback1 with a shared IP address for the RP and use the Loopback0 address for the MDSP peers.

Do I need to combine the above with HSRP aware PIM? Also, what are the 'hello' timers between MSDP infromation sharing and can these be changed?

Thanks again.

[Richard Burts]

You are welcome. It has been an interesting discussion and I have learned a new thing in the process (which is one of the reasons why I like these forums so much - they show me things that I did not know before).

The anycast is a good solution for providing redundancy for the RP in multicast PIM. And using loopback1 on both routers s RP address and loopback 0 for MSDP seems appropriate.

You ask whether you should combine anycast with HSRP aware PIM and it sounds like you believe that there is a relationship between anycast and HSRP aware PIM. But remember they are different things and accomplish different things for PIM. So using (or not using) one feature has no impact on whether you should use the other feature. In your situation I believe that both features have value and I would suggest that you implement both. But do not try to link them together.

HTH

Rick

[conor.maton]

I need to use the forums more often, cannot beat first hand experience from enginners/architects!

I think I will continue with the HSRP configuration and use the Anycast PIM solution. Conigure additional loopback addresses on the routers that will share the same IP address and kept in sync via MSDP. If the active HSRP router fails over to the standby HSRP router at least the RP address will always be active, never change and there will be no loss in service.

Many thanks for your help!

[Richard Burts]

I am glad that this discussion has been helpful. And I hope that you will continue to use these forums. They are excellent places for you to explore various topics and to learn from the experience of others. And as you gain in experience I hope that you will also be interested in sharing what you have learned by responding to questions in the forum.

HTH

Rick

[conor.maton]

Hi Richard,

As you have been helpful to me the last few days I am wondering if you can help me again please.

See attached myquery on this thread: https://supportforums.cisco.com/discussion/13260121/multicasting-ip-base-license

I have also attached the topology.

As disucssed my routers will be running HSRP and these routers connect to stacked Cisco 3850 switches with IP Base licenses.

I have learned that only PIM-Stub is supported on these Switches and not PIM-SM so this is a problem I need to tackle.

With regards to my config, there are a number of VLANs with the VLANs SVIs residing on the switches and act as the VLANs's default gateway. I have a HSRP VLAN configured; these connect to access ports on the switches and there is a static route from the switches with a next hop of the HSRP virtual IP address.

It looks like the HSRP Aware PIM will work with this setup as this seems to mitigate any requirement to have a routed uplink to the HSRP routers. However I am still unclear as to wether the RP fails over to the standby router if the active fails.

A method to mitigate this would be to combine HSRP aware PIM with Anycast RP. However you have recommended not to combine both technologies.

Can you help me with my scenario please? Let me know if you require any further infromation.

[Richard Burts]

Sorry for a delayed response. I have been on travel and was not able to participate in the forum. I am not clear I am puzzled how you believe that I recommend not to combine both technologies, especially since in a previous post in this thread I did suggest to use both HSRP aware PIM and Anycast.

HTH

Rick

[edit] I hit enter and then realized that I wanted to comment about your asking if the RP would fail over. Perhaps part of the question is semantics. If we understand fail over in the traditional way that RP is on one router, then the router fails and RP moves to a different router then the answer is that no the RP does not fail over. With Anycast running there are two RP functioning. And if one RP fails then it does not move to the other router. But if we think of fail over in a different way and think of providing redundancy and the ability to continue to function when an active RP fails then perhaps we can say that yes there is fail over. With Anycast some devices may use one RP while other devices may use the other RP. And if one RP fails then all devices will use the surviving RP.