All things related to multicast

[Caren Litvanyi <>]

[...]

On Fri, 30 Oct 2009, Havard Eidnes wrote:

[...]
> If the same is done for MSDP, the routing process on both ends will
> use the directly connected route when determining "which AS does the
> MSDP neighbor's IP address reside in".  This route is of course
> internal to its own AS, not to that of the neighbor.  This wrecks
> havoc when this particular rule which is part of the MSDP peer-RPF
> rules is applied:
>
>   (iv).   N resides in the closest AS in the best path towards R.  If
>           multiple MSDP peers reside in the closest AS, the peer with
>           the highest IP address is the rpf-peer.
>
> (Quoted from RFC 3618, section 10.1.3.)
>
> In this case, N refers to the MSDP neighbor, and because it is on a
> directly connected subnet, it ends up being interpreted as *not* being
> inside the closest AS on the path towards the originator of the MSDP
> message, but instead as being an MSDP neighbor internal to the local
> AS!  If I've understood correctly, this is a certain recipe for peer-
> RPF failures for all SA messages received across the MSDP peering.


Remember that that section of the RFC says to apply the *first* rule
that matches, in order.

see (ii):
(ii).   N is the eBGP NEXT_HOP of the Peer-RPF route for R.
where R is the originator of the SA.

This is why we can and do use the same direct interface addresses
for eBGP peering and MSDP peering.



> We find instances of this sort of configuration in the Los Angeles
> Internet2 router:
>
> Peer address    Local address   State       Last up/down Peer-Group   SA Count
> 137.164.26.145  137.164.26.146  Established   00:08:16 CONNECTOR  
> 374284/375309
>
> Here the "local address" and "peer address" are obviously on the same
> subnet, most probably triggering these MSDP "peer RPF" failures.

The numbers under SA count don't support your thesis.  See above.


> It is my advice that instead of using the identical IP addresses as
> used for the eBGP peering, MSDP should use an IP address which in all
> instances will be interpreted as residing wholly within the given AS
> when looking at the fully resolved routing table of your immediate
> neighbor, for instance by using a Loopback addresses on both sides.

No.

> [...]

--  Caren
------------
Caren Litvanyi, network engineer
Internet2 NOC
Global Research NOC at Indiana University

o:812-961-3790  c:250-896-4369