[c-nsp] MSDP and my limited knowledge question

[Adam Vitkovsky]

Oh yes I'm sorry for confusion thank you Phil for pointing that out

What I meant when I said IGMP register messages was actually IGMP membership
report message (commonly known as IGMP joins)
IGMP membership report messages from local subnet are translated by the
designated forwarder into PIM joins and sent up the shared or source-tree
depending on whether the source for the particular m-cast group is known or
not


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


> PIM registers are sent for multicast *sources*. They're sent by the PIM
designated forwarder towards the RP and flooded down the (*,g) tree so that
(s,g) joins can be triggered. They are re-sent periodically to refresh state
on the RP.

For more details
During the PIM register process the PIM designated forwarder (first-hop
router -router that has the source on the directly connected subnet)
encapsulates the m-cast packet sent by source host into a unicast register
message that is sent as unicast to the RP
RP de-capsulates the register msg and learns the multicast group and the
particular source and can build the shared tree towards the first-hop router
as designated by the (*,g) state in all the routers  
After the shared-tree is built from the RP all the way to the first-hop
router -the first hop router will than start sending the m-cast traffic up
the shared tree
Now the RP receives the m-cast traffic twice  -via the PIM register messages
and via the shared tree
RP than sends register stop messages towards the first hop router -informing
it that it no longer needs to send the register messages
This whole process can span over a network consisting of many m-cast routers




adam

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Phil Mayers
Sent: Tuesday, September 04, 2012 11:01 AM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] MSDP and my limited knowledge question

On 09/04/2012 09:12 AM, Adam Vitkovsky wrote:
> As Paul said IGMP register messages are supposed to be handled by the 
> router elected as designated querier for the local subnet Though AFAIK 
> IGMP register process is only used by the m-cast receivers not the 
> sources

For clarity, because I think you typo-ed a couple of things there:

PIM registers are sent for multicast *sources*. They're sent by the PIM
designated forwarder towards the RP and flooded down the (*,g) tree so that
(s,g) joins can be triggered. They are re-sent periodically to refresh state
on the RP.

IGMP joins are sent by multicast *receivers*. They trigger (*,g) joins [or
(s,g) joins for IGMPv3 SSM] towards the RP. When a new source appears on the
(*,g) tree, the routers are triggered to do an (s,g) join.

MSDP essentially "echos" PIM register state between PIM RPs, allowing
separate PIM networks to be connected without (*,g) flooding on the
interconnect link, which wouldn't scale well.

The OPs problem is that he's trying to source multicast traffic from an IP
on his Linux box that isn't a "connected" route on the c4900. Thus, the
c4900 isn't the PIM DF, so doesn't send the PIM register or MSDP
advertisments. That doesn't (can't) work, I believe.

AFAIK there's no "PIM DF proxy" IOS feature, though I can see situations in
which it would be useful.
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