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Bronze

LFIB look up for " POP" operation

Hi everybody

 

 I have a quick question .

R1#show mpls forwarding-table

 


Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface

18     Pop tag     23.23.23.0/24     0          Fa0/1      12.12.12.2

 

 

Let say R1 receives a labelled packet with label 18.  What will R1 do next ?

 

 

1)  Will R1 simply pop the tag and forward the packet out of Fa0/1 i.e no CEF look up is performed ( except for adjacency table to fnd the next hop mac )

 

2) Will R1 pop the tag and then perform IP look up in cef ?

 

 

 

Thanks

 

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Sarah,If the outgoing label

Sarah,

If the outgoing label is " no label" or "aggregate" or "unlabeled", the action is to remove the label and perform IP lookup.

 

In this case, the action is POP. It is not required that the label 18 is the bottom of stack. The forwarding semantci per this table is to just pop label 18. So the received packet with top label as 18 will perform a "CEF" lookup to realize that it needs to pop the label and send over interface F0/1 (it can be a labelled packet or IP packet). So there wont be any IP lookup (except the IP header processing incase it is the last label - like copying the lowest TTL etc).

 

-Nagendra

2 REPLIES
Bronze

 I found the answer ( after

 

I found the answer ( after much efforts ;)

 

1)  Will R1 simply pop the tag and forward the packet out of Fa0/1 i.e no CEF look up is performed ( except for adjacency table to fnd the next hop mac )

 

Yes,

 

 

Cisco Employee

Sarah,If the outgoing label

Sarah,

If the outgoing label is " no label" or "aggregate" or "unlabeled", the action is to remove the label and perform IP lookup.

 

In this case, the action is POP. It is not required that the label 18 is the bottom of stack. The forwarding semantci per this table is to just pop label 18. So the received packet with top label as 18 will perform a "CEF" lookup to realize that it needs to pop the label and send over interface F0/1 (it can be a labelled packet or IP packet). So there wont be any IP lookup (except the IP header processing incase it is the last label - like copying the lowest TTL etc).

 

-Nagendra

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