In environments that require redundancy towards clients, HSRP will normally be running. HSRP is a proven protocol and it works but how do we handle when we have clients that need multicast? What triggers multicast to converge when the Active Router (AR) goes down? The following topology is used:

PIM1

One thing to notice here is that R3 is the PIM DR even though R2 is the HSRP AR. The network has been setup with OSPF, PIM and R1 is the RP. Both R2 and R3 will receive IGMP reports but only R3 will send PIM Join, due to it being the PIM DR. R3 builds the (*,G) towards the RP:

We then ping 239.0.0.1 from the multicast source to build the (S,G):

The (S,G) has been built:

The unicast and multicast topology is not currently congruent, this may or may not be important. What happens when R3 fails?

No replies to the pings coming in until PIM on R2 detects that R3 is gone and takes over the DR role, this will take between 60 to 90 seconds with the default timers in use.

We can increase the DR priority on R2 to make it become the DR.

HSRP aware PIM is a feature that started appearing in IOS 15.3(1)T and makes the HSRP AR become the PIM DR. It will also send PIM messages from the virtual IP which is useful in situations where you have a router with a static route towards an Virtual IP (VIP). This is how Cisco describes the feature:

HSRP Aware PIM enables multicast traffic to be forwarded through the HSRP active router (AR), allowing PIM to leverage HSRP redundancy, avoid potential duplicate traffic, and enable failover, depending on the HSRP states in the device. The PIM designated router (DR) runs on the same gateway as the HSRP AR and maintains mroute states.

In my topology, I am running HSRP towards the clients, so even though this feature sounds as a perfect fit it will not help me in converging my multicast. Let’s configure this feature on R2:

R2 is now the PIM DR, R3 will now see two PIM neighbors on interface E0/2:

R2 now has the (S,G) and we can see that it was the Assert winner because R3 was previously sending multicasts to the LAN segment.

What happens when R2’s LAN interface goes down? Will R3 become the DR? And how fast will it converge?

HSRP changes to active on R3 but the PIM DR role does not converge until the PIM query interval has expired (3x hellos).

We lose a lot of packets while waiting for PIM to converge:

HSRP aware PIM didn’t really help us here… So when is it useful? If we use the following topology instead:

PIM2

The router R5 has been added and the receiver sits between R5 instead. R5 does not run routing with R2 and R3, only static routes pointing at the RP and the multicast source:

Without HSRP aware PIM, the RPF check would fail because PIM would peer with the physical address but R5 sees three neighbors on the segment, where one is the VIP:

R2 will be the one forwarding multicast during normal conditions due to it being the PIM DR via HSRP state of active router:

Let’s try a ping from the source:

The ping works and R2 has the (S,G):

What happens when R2 fails?

The pings time out because when the PIM Join from R5 comes in, R3 does not realize that it should process the Join.

As it turns out, the PIM redundancy command must be configured on the secondary router as well for it to process PIM Joins to the VIP.

After this has configured, the incoming Join will be processed. R3 triggers R5 to send a new Join because the GenID is set in the PIM hello to a new value.

After configuring this, the PIM DR role converges as fast as HSRP allows. I’m using BFD in this scenario.

The key concept for understanding HSRP aware PIM here is that:

  • Initially configuring PIM redundancy on the AR will make it the DR
  • PIM redundancy must be configured on the secondary router as well, otherwise it will not process PIM Joins to the VIP
  • The PIM DR role does not converge until PIM hellos have timed out, the secondary router will process the Joins though so the multicast will converge

This feature is not very well documented so I hope you have learned a bit from this post how this feature really works. This feature does not work when you have receiver on a HSRP LAN, because the DR role is NOT moved until PIM adjacency expires.

HSRP AWARE PIM
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