[c-nsp] MSDP and my limited knowledge question

[Phil Mayers]

On 04/09/12 12:00, Adam Vitkovsky wrote:

> What I meant when I said IGMP register messages was actually IGMP membership
> report message (commonly known as IGMP joins)

Yeah, I guessed that ;o)

> Also to clarify
> IGMP querier is the router elected by PIM(igmp ver2/3) as a router that
> sends IGMP queries for the local subnet
> PIM designated forwarder is elected by PIM as a router responsible for
> forwarding m-cast traffic down/from the local subnet

> Since IGMPver2 PIM process is responsible for election of both function -so
> they always reside on the same router

Are you sure about this? In our environment, which is dual-routed HSRP, 
we usually see the PIM DF be the *opposite* router to the IGMP querier, 
because the election rules are different (PIM DF - higher IP; IGMP 
querier - lower IP) and we're too lazy to add yet another line to the 
SVI configs to set PIM/IGMP priority. For example:

# sh ip igmp vrf PROD int vl76
Vlan76 is up, line protocol is up
   Internet address is x.x.x.254/24
   IGMP is enabled on interface
   Multicast Routing table PROD
   Current IGMP host version is 2
   Current IGMP router version is 2
   ...
   Multicast routing is enabled on interface
   Multicast TTL threshold is 0
   Multicast designated router (DR) is x.x.x.254 (this system)
   IGMP querying router is x.x.x.253

# sh ip pim vrf PROD int vl76 det
Vlan76 is up, line protocol is up
   Internet address is x.x.x.254/24
   Multicast switching: fast
   Multicast packets in/out: 0/15871
   Multicast TTL threshold: 0
   PIM: enabled
     PIM version: 2, mode: sparse
     PIM DR: x.x.x.2541 (this system)

It doesn't seem to cause any problems, so we don't worry about it.