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528
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vin.marco
Beginner

LDP label

Hello everybody
I would like to clarify some things that confound me.

The inner label is set to route the packet to our neighbor.

The outer label is set to route the packet to the MP-bgp.

Quite right?
Is the Inner label generated by the PE or by the P?
Thank you

5 REPLIES 5
loris.marcellini
Beginner

Hey there,

The outer label is used by P devices for reachability inside the MPLS
backbone, that is, how to route a packet between PEs throughout the
backbone. Please note P devices usually only have knowledge of internal
loop backs for reachability purpose.

The inner label is used by PEs as a way to allow forwarding of traffic
within a specific VRF. Remember the route target isn't present in the
forwarding plane and therefore whilst RT help routers build their tables
the inner label associated with the packet tells the PE the forwarding
instance the traffic belongs to.

Hope this helps.

Then
in a scenario such as CE - PE - P

PE inserts the INNER and the OUTER.
The INNER to route the packet to P.
The OUTER to route the packet to the remote CE.

ROUTER_AS = PE

ROUTER_ASR#sh ip cef vrf INTERNET xxx.xxx.xxx.xxx/32
xxx.xxx.xxx.xxx/32
nexthop yyy.yyy.yyy.yyy GigabitEthernet0/2/4 label 523633() 146()

523633 -> outer label
146 -> inner label

While in this condition does it mean that my PE has not generated the label or the P has not sent me the laber for the prefix xxx.xxx.xxx?

PDPCPRBEI13001#sh ip cef vrf INTERNET xxx.xxx.xxx.xxx/30
xxx.xxx.xxx.xxx/30
nexthop yyy.yyy.yyy.yyy GigabitEthernet0/2/4 unusable: no label

Thx

I don't have the scenario very clear. Usually the destination for traffic
originated from a CE is another CE. That makes the scenario be as follow:
1) CE-PE-P-PE-CE

or if you collapse Pe and P in one device that would even be

2) CE-PE-PE-CE

or

3) CE-PE-CE


Let's take scenario 1) as a reference:

-Source CE sends IP traffic
-Source PE pushes the INNER (BGP) and the OUTER (LDP) labels.
-The P device swaps the INNER label which is used to route traffic to the
destination PE (1 P == PHP).
-The destination PE inspect the outer label and forwards traffic to the
destination CE.


Your output are a little unclear, could you please post "show ip bgp vpnv4
unicast vrf internet x.x.x.x/x" as as well as "show ip cef vrf internet
x.x.x.x/32" and "show mpls ldp bindings" from both PE and P?

Thanks, L.

Thank you
actually I was unclear with my scenario.
The right scenario is CE-PE-P-PE-CE.

Monday I attach the shows.
Thanks again.

MHM Cisco World
Collaborator

the two Label is use in L2, where outer is the label toward the target PE and inner is VLAN or L2 label.l2vpn.jpg

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