[c-nsp] MPLS VPN aggregate label / route lookup

[Peter Rathlev]

On Tue, 2008-06-17 at 17:04 +0200, Oliver Boehmer (oboehmer) wrote:
> Peter Rathlev <> wrote on Tuesday, June 17, 2008 4:33 PM:
> > - Add "ip route vrf A 10.0.0.0 255.255.255.0 Null0 250"
> > - Add "network 10.0.0.0 mask 255.255.255.0" to MP-BGP
> > - Wait for the new prefix to be visible all over the network
> > - Change Vlan123 to 10.0.0.1/24
> > - Clean up
> 
> did you test this in the lab? A simple "redistribute connected" and a
> change of the vlan netmask works just as well.. but as you're using
> network statements, the above sounds ok.

Well, we "don't like" redistribute connected and are not yet large
enough for network statements to be a problem. :-)

What we're trying to avoid is that the old /25 prefix is withdrawn
before the new /24 is announced everywhere. Maybe BGP always bundles
theses messages and the RIB is updated "instantaneously"?

> It doesn't matter in your case (see below), but a Null0 static creates
> an aggregate vpn label, so the PE will pop it and perform another
> lookup, using the connected prefix in your scenario.

Ok, nice to know.

> > On another note: We have other PEs that build "full" mpls
> > forwarding-tables, including all local prefixes, but the one in
> > question only has aggregate labels for the VRFs. It's neighbor at the
> > POP has ~3800 label assigned, this one only has ~50, and they hold
> > the same VRFs and the same prefixes. They should be configured the
> > same way. Can anybody point at how I can find out what makes this one
> > use aggregates? 
> 
> Can I take a look at the config (unicast, if you prefer)?

I've unicasted the config, but I think I found a clue. All the prefixes
that have labels on R2 are from eBGP neighbors, and even though R1
receives similar prefixes from parallel neighbors, a local preference
setting on R2 stops those from being installed in the RIB on R1. So all
RIB next hops on R1 for these prefixes (those that miss labels) are
towards R2, using "Default-IP-Routing-Table", and thus just the Loopback
label, since it's all L3VPN.

I guess R1 never assigns labels for the same reason that it doesn't send
these prefixes to other iBGP neighbors -- the locally know route isn't
the best.

Regards,
Peter