[c-nsp] Bizarre MPLS label problem, hex value?

[Nathan]

On Feb 18, 2008 7:51 AM, Oliver Boehmer (oboehmer) <oboehmer at cisco.com> wrote:
> Nathan <> wrote on Monday, February 18, 2008 12:59 AM:
> > I have four routers in a row, A B C D, I want packets to go inside a
> > VRF from A to D. Packets travel from B to D without any problem, but
> > not from A to D.
[...]
> >     A#sh ip cef vrf ${vrf} 0.0.0.0 0.0.0.0
> >     0.0.0.0/0, version 109, epoch 0, cached adjacency ${B as seen
> >     from A} 0 packets, 0 bytes
> >       tag information set
> >         local tag: VPN-route-head
> >         fast tag rewrite with
> >             Recursive rewrite via ${D loopback} 0x20, tags imposed
> >       {413} via ${D loopback}, 0 dependencies, recursive
> >         next hop ${B as seen from A}, GigabitEthernet0/1.7 via ${D
> >         loopback}/32 valid cached adjacency
> >         tag rewrite with
> >             Recursive rewrite via ${D loopback} 0x20, tags imposed
> > {413}
> >
> > What does that 0x20 mean??
>
> I don't know offhand, but in order to proceed, you need to follow the
> recursion to find out the outer (IGP/LDP) label of the packet. The "413"
> shown above is the vpn label received via MBGP.
> A seems to have multiple paths to D, so the final label stack will be
> determined at "run-time".

Yes, there is a secondary route through a router Z that is connected :

         directly to B
         directly to A on the same subnet as B (subnet A,B,Z)
         directly to A on another subnet


However the only routes to C and D go through B, and the OSPF cost of
A-Z-B is bigger than A-B.

There's also another router connected to the Z-B subnet and to the
A-B-Z subnet, but I don't see any reason for packets to go through
there unless an Ethernet cable becomes unplugged.

> Please do "show ip cef ${D loopback}" on A to find out.

A#show ip cef ${D loopback}
${D loopback}/32, version 1788, epoch 0, cached adjacency ${B as seen from A}
0 packets, 0 bytes
  tag information set, shared
    local tag: 40
  via ${B as seen from A}, GigabitEthernet0/1.7, 267 dependencies
    next hop ${B as seen from A}, GigabitEthernet0/1.7
    valid cached adjacency
    tag rewrite with Gi0/1.7, ${B as seen from A}, tags imposed: {}

Am I correct in supposing that there should be a tag imposed there?
When I do the same on B I do see a tag imposed, the same 396 as B
prepends to the vpn label when I do the sh ip cef for the VPN route,
that seems normal.

Could this possibly be some kind of redistribution problem between BGP
and OSPF? I redistribute some internal routes between BGP and OSPF,
but the why and the "how to avoid" of that is a story in itself.

Thanks,
-- 
Nathan