06-15-2014 05:06 PM
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
Solved! Go to Solution.
06-17-2014 09:06 AM
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
06-16-2014 09:29 PM
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,
06-17-2014 09:06 AM
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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