Interprovider MPLS VPN - "drop -- rewrite null"

Unanswered Question
Feb 8th, 2008


i have an interprovider VPN where a remote route is received on ASBR and forwarded to my AS PE but traffic coing from my PE is dropped because label forwarding is not installed:

c2851-ASBR#sh ip bgp vpnv4 all la

Network Next Hop In label/Out label

Route Distinguisher: 3302:141141 20/100192 22/18 21/16

c2851-ASBR#sh mpls forwarding-table

Local Outgoing Prefix Bytes tag Outgoing Next Hop

tag tag or VC or Tunnel Id switched interface

16 Pop tag 0 Gi0/1

21 16 3302:141141: \

0 Gi0/1

22 18 3302:141141: \

0 Gi0/1

As you can see the label 100192 is not present in the mpls forwarding.

If i create the vrf on the ASBR everithing works correctly:

c2851-ASBR#sh mpls forwarding-table

Local Outgoing Prefix Bytes tag Outgoing Next Hop

tag tag or VC or Tunnel Id switched interface

16 Pop tag 0 Gi0/1

20 100192[V] 488 Gi0/0.124

21 16[V] 472 Gi0/1

22 18[V] \

0 Gi0/1



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attrgautam Fri, 02/08/2008 - 10:11

Have you configured no bgp default route-target filter on the ASBR? I am assuming you are doing an Option-B NNI in this case. Because creating a VRF on the ASBR helps, I think you might be missing this command

sb-txr-cco Mon, 02/11/2008 - 00:15


yes the command is there (otherwise the route are not sent to the external peer).

We are doing option B and the route are received form external peer and propagated to internal peer (the PE see and install the route).

The ASBR doesn't install in MPLS forwarding the label for the external route.


arun kumar Mon, 02/11/2008 - 07:03


Probably you might check the below thing. The interconnect interface between providers been enabled with Label switching:

IP labeling not enabled

LSP Tunnel labeling not enabled

BGP tagging enabled

MPLS operational

Optimum Switching Vectors:

IP to MPLS Feature Vector

MPLS Feature Vector

Fast Switching Vectors:

IP to MPLS Fast Feature Switching Vector

MPLS Feature Vector

MTU = 4470


sb-txr-cco Mon, 02/11/2008 - 07:19


another bit can be usefull, the flow from the other ASBR reach the internal PE so the MPLS part of the configuration is right.

Traffic coming from externalAS with label 16 and 18 are forwarded the the internal PE after the correct label swap (22/18 an 21/16) the traffic from internal to external is not forwaded from the ASBR.

from another point of view if enabling the vrf on the ASBR makes MPLS forwarding functional i think that MPLS is correctly configured.


olorunloba Wed, 02/13/2008 - 08:02

I think you need to look at your eBGP configuration. You have a /32 host route for the BGP next-hop, but the BGP next-hop for the problematic route is Verify the reason why the next-hops are different, as I assume you are pairing with loopbacks across two interfaces (G0/0 and G0/1).

Outputs from your PE would also be helpful, for us to see the VPN label, BGP next-hop sent to the PE.

Also labels 22 get swapped to 18 and not the other way.

sb-txr-cco Wed, 02/13/2008 - 08:36

the lab setup is quite simple:

AS1 PE --ibgp-- AS1 ASBR ( ----ebgp---- AS2 ASBR ( --ibgp-- AS2 PE (lo0

From the AS2 ASBR the next hop for the internal route is the lo0 of the AS2 PE and the next hop for the AS1 route is the ip address of the btb interface (the ebgp peer is built on the directed connected interface). So no problem on the ebgp next hop.

The label swap from 22 to 18 is unidirectional and is used for the traffic coming from AS1 PE directed to the AS2 PE for destination network annouced by AS2PE.

From the AS2PE point of view there is no problem on the traffic forwarding (label 20 is imposed to the packet and this is the right behaviour) but AS2ASBR doesn't swap it with label 100192 because it is not in the mpls forwarding table. so i think that the problem is not on the PE but in the behaviour of the ASBR.


olorunloba Wed, 02/13/2008 - 09:30


Can you give the output of the show mpls forwarding on AS2 PE. I will like to know the label stack it uses for sending traffic to AS1 via AS2 ASBR. Kind of confirming that it is using label 20, and not that ASBR is not swapping.

Use show mpls forwarding vrf details command.


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