All things related to multicast

[Zenon Mousmoulas <>]

On 11 Ιουν 2010, at 5:14 ΜΜ, Tyrone Kelly wrote:

> I believe your fix would be resolved if your ISP sends partial  
> routes + a default route.  Where the BGP next hop would then be the  
> next-hop toward the rp/originator, which would satisfy rule  
> condition #2 of the MSDP-rpf peer rules of rfc 3618. This is  
> provided you aren't  doing next-hop self over your ebgp peering  
> toward your provider?

Well, we (GRNET, the provider) can not send to U (the downstream  
network) a default route, i.e. originate and announce to U the  
0.0.0.0/0 prefix. There is a number of reasons behind this, all  
related to routing policy. The way I see it, it all comes down to  
letting the customer pick the default route (with "ip default-network"  
etc.) instead of announcing it to them. This is a long standing  
practice and I'm really not sure there is any reason to change it.

In such a case, the criteria for the EBGP next-hop rule (ii) in RFC  
3618 can not be fulfilled, but if the peer-RPF check includes "RPF- 
RIB" in lookups, as per the RFC 3618 compliant rules[1], then I  
believe the check would pass. I'm not sure what the corresponding,  
matching rule in RFC 3618 would be, however.

We have yet to confirm this theory, since the IOS release on their  
border router is unlikely to support the "ip msdp rpf rfc3618"  
feature. Therefore I was wondering if anyone else had faced a similar  
situation (gateway of last resort configurations should not be very  
uncommon) and had enabled RFC 3618 compliance, so they could confirm  
or deny this approach.

Thanks,
Z.

[1] 
http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gt_msdp.html#wp1048153

> -----Original Message-----
> From: Zenon Mousmoulas 
> [mailto:]
> Sent: Friday, June 11, 2010 2:24 AM
> To: 
>
> Cc: wg-multicast
> Subject: Re: inter-domain MSDP peering without BGP FIRT
>
> On 10 Ιουν 2010, at 9:32 ΜΜ, Bill Owens wrote:
>
> > On Thu, Jun 10, 2010 at 08:57:10PM +0300, Zenon Mousmoulas wrote:
> > > However MSDP peer-RPF check fails: SA
> > > announcements are rejected when no prefix matches the RP/Originator,
> > > so the default network doesn't apply in this case, or so it seems.
> >
> > How is the default route created - static, or supplied by the eBGP
> > peer? I just tried a quick experiment with an internal router that
> > has a static default:
>
> Quite similar to "ip route 0.0.0.0", the default route is installed
> with "ip default-network" (gateway of last resort) pointing to a
> network which is known by eBGP (announced from GRNET).
>
> > c7609#sh config | incl 0.0.0.0
> > ip route 0.0.0.0 0.0.0.0 199.109.35.1
> > c7609#conf t
> > Enter configuration commands, one per line.  End with CNTL/Z.
> > c7609(config)#ip mroute 0.0.0.0 0.0.0.0 199.109.35.1
> > c7609(config)#end
> > c7609#sh ip rpf 2.3.4.5
> > RPF information for ? (2.3.4.5)
> > RPF interface: GigabitEthernet1/1
> > RPF neighbor: nn-3550-ext.nysernet.net (199.109.35.1)
> > RPF route/mask: 0.0.0.0/0
> > RPF type: static
> > RPF recursion count: 0
> > Doing distance-preferred lookups across tables
>
> RPF information for ? (2.3.4.5)
>   RPF interface: <GRNET uplink>
>   RPF neighbor: w.x.y.z
>   RPF route/mask: 194.177.208.0/22
>   RPF type: unicast (bgp <U AS>)
>   RPF recursion count: 0
>   Doing distance-preferred lookups across tables
>
> > c7609#conf t
> > Enter configuration commands, one per line.  End with CNTL/Z.
> > c7609(config)#no ip mroute 0.0.0.0 0.0.0.0 199.109.35.1
> > c7609(config)#end
> > c7609#sh ip rpf 2.3.4.5
> > RPF information for ? (2.3.4.5) failed, no route exists
> >
> > Seems to indicate that the static mroute works. It would be
> > incorrect in this case, since the unicast default route points to
> > our commercial ISP connection, and the R&E side is native BGP/mBGP.
>
> So the difference is you have a static default route pointing to the
> IP next-hop, while they are pointing it to a network learnt via BGP
> from GRNET. However "ip default-network" does not have a multicast
> counterpart; perhaps they could use "ip route" and "ip mroute"  
> instead.
>
> However, the question is: does MSDP work for you in this case? Would
> it accept an SA where the originator would match this default route in
> an RPF lookup?
>
> Thanks,
> Z.