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Configuring Multicast with HSRP Routers in the Network

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Summary:

This article focuses on configuring Multicast using PIM in an environment that includes a pair of routers configured with HSRP. 

Basic topology:

                                     ------ Router 1
 Receiver ---- NetScreen ---- HSRP IP               ----- RP --- Sender
                                     ------ Router 2              (Server)

Environment:

  • The rendezvous point (RP) is located on the other side of the routers. 
  • Static default route from NetScreen is pointed to the HSRP IP address.
  • The RP is reachable from both the sender and the receiver.
  • The actual sender IP address is unknown, so for that reason, the spt-threshold is set to infinity.
Symptoms:
PIM joins are not successful.  Running debug pim all shows "RPSet not available".  Debug igmp all shows message "igmp packet received on ethernet1 from a.b.c.d not set router alert IP option, dropped"
Solution:

The issue is with PIM and HSRP not working well together.  If a static route is used from the NetScreen to the HSRP cluster, the next hop device needs to match the NetScreen's PIM neighbor.  Generally, the PIM neighbor will be one of the physical IP addresses of the routers in the HSRP cluster, and not the HSRP VIP itself.  If a static route is defined with the next hop as the HSRP IP address, the return packet will appear as one of the physical IP addresses.  When this occurs, the IGMP packet will be dropped.

This limitation is due to an issue with the Cisco router and PIM functionality on the HSRP Virtual IP address.  The workaround is to define a host route on the NetScreen of the RP, with the next hop as the NetScreen's PIM Neighbor.

For example
Assume the following:

  • RP address is 1.1.1.10
  • the HSRP IP address is 10.1.1.1
  • Router 1 IP is 10.1.1.2
  • Router 2 IP is 10.1.1.3
  • NetScreen's ethernet1 IP address is 10.1.1.5
  • NetScreen's PIM neighbor is Router 1.

                                                    10.1.1.2
                                             ------ Router 1
 Receiver ---- NetScreen ---- HSRP IP(10.1.1.1)     ------- RP (1.1.1.10) --- Sender
                    10.1.1.5                 ------ Router 2                 (Server)
                                                    10.1.1.3

The resolution is to add the following static route on the NetScreen:

set route 1.1.1.10/32 interface ethernet1 gateway 10.1.1.2

After this static route is added, PIM joins should succeed, and multicast will be working, even in the HSRP environment.

The ultimate solution is to run a dynamic routing protocol (like OSPF or BGP), and get away from defining static routes from the NetScreen to the next hop.

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