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Remote route not showing in MPLS Forwarding Table

I have the follow GNS3 network which you may recognise from the MPLS Fundamentals CBT Nugget series with Keith Barker:

R1 and R5 are PE routers. R2-R4 are P routers. All CBT and ACME routers are CE routers.

Ignore the blue CBT sites for now. I am trying to investigate the lo0 route on the top left green ACME_1 router (172.16.101.50/32), as seen by R5 - the PE router on the far right side of the MPLS network.

Everything works as described in the nugget but I can't help but notice that when I enter the show mpls forwarding-table vrf 101:ACME there is no label for the 172.16.101.50/32 network in the output:

R5#sh mpls forwarding-table vrf 101:ACME
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
507        No Label   192.168.111.2/32[V]   \
                                       26042         Fa2/0      192.168.202.2
508        No Label   192.168.202.0/30[V]   \
                                       0             aggregate/101:ACME
511        No Label   8.8.8.8/32[V]    1800          Fa2/0      192.168.202.2
R5#


The network is in the bgp vpnv4 label table and all the routing works fine:

R5#show bgp vpnv4 unicast vrf 101:ACME labels
   Network          Next Hop      In label/Out label
Route Distinguisher: 5.5.5.5:1 (101:ACME)
   8.8.8.8/32       192.168.202.2   511/nolabel
   172.16.101.0/30  1.1.1.1         nolabel/107
   172.16.101.50/32 1.1.1.1         nolabel/108 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
   192.168.111.2/32 192.168.202.2   507/nolabel
   192.168.202.0/30 0.0.0.0         508/nolabel(101:ACME)

 

But should I not also see an entry for this route in the show mpls forwarding-table vrf 101:ACME output? I understand that the VPN label will be the bottom label and that the top label (that is actual swapped by the P routers as the packet traverses the network) is going to be based on the next hop (1.1.1.1) and will this be R4's LDP advertised local label, but I would have though that this 172.16.101.50/32 network would show up as well...

 

Again, this is a technical query rather than a problem that needs solving. The whole network is functioning well. I think I'm just misunderstanding a fundamental idea here.

 

Thanks in advance :)

-Steve

 

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SteveWhat you are seeing is

Steve

What you are seeing is what you would expect to see.

On R5 you will see the loopback in the VRF routing table but not the MPLS forwarding table for that VRF.

The reason is that if R5 needs to send traffic to that loopback it does not need an MPLS forwarding table entry, it only needs a route in the VRF ie.

R5 receives traffic with a destination IP of the loopback on the ACME_1 router. It does a lookup in the VRF routing table and finds a next hop IP of R1 because that was the PE that advertised the route.

It then looks in it's MPLS forwarding table (not the MPLS VRF forwarding table) to see how to reach R1.

At no stage does it need to consult the MPLS VRF forwarding table to be able to route traffic to the loopback address.

If you looked on R1 at the MPLS VRF forwarding table you would see the loopback address because it is local to R1 but not local to R5.

Jon

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1 REPLY 1
Highlighted
Hall of Fame Guru

SteveWhat you are seeing is

Steve

What you are seeing is what you would expect to see.

On R5 you will see the loopback in the VRF routing table but not the MPLS forwarding table for that VRF.

The reason is that if R5 needs to send traffic to that loopback it does not need an MPLS forwarding table entry, it only needs a route in the VRF ie.

R5 receives traffic with a destination IP of the loopback on the ACME_1 router. It does a lookup in the VRF routing table and finds a next hop IP of R1 because that was the PE that advertised the route.

It then looks in it's MPLS forwarding table (not the MPLS VRF forwarding table) to see how to reach R1.

At no stage does it need to consult the MPLS VRF forwarding table to be able to route traffic to the loopback address.

If you looked on R1 at the MPLS VRF forwarding table you would see the loopback address because it is local to R1 but not local to R5.

Jon

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