PE3#sh mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 Pop tag 18.104.22.168/32 196675 Se0/0 point2point
17 19 22.214.171.124/32 13534 Fa0/1 126.96.36.199
18 Pop tag 188.8.131.52/30 0 Fa0/1 184.108.40.206
19 Pop tag 220.127.116.11/32 0 Fa0/1 18.104.22.168
20 Untagged 22.214.171.124/32 347682 Fa0/0 172.20.13.1
What is mean of Untagged and POP taged in above output ?
POP Tag, removes the top label and forwards the remaining payload.
Untagged, means that the packet was the last label in MPLS stack, the resultant packet does not have an MPLS label hence forwarded accordingly. (something like implicit null)
Actually, I think that "untagged" entry means that you pop all remaining labels even if more than one had left. This would actually indicate that your LSP is broken.
It doesn't necesseraly mean that the label is the last one in the stack but rather that the outbound interface is not mpls enabled and therefore all remaining labels have to be removed before forwarding the packet to the outbound interface.
Hope this helps,
Yeah, That's correct, but what I meant was that the label is "last" to be popped, after that the packet may have one or more labels, in that scenario the packet would be dropped instead of being forwarded...
Is my understanding correct as above?
The untagged state will result in all the remaining labels in the stack being removed and the packet to be forwarded to the next hop, where it will be looked up against the FIB (assuming that it is an IP packet and not L2 stuff) and forwarded accordingly, which is probably not the desired behavior. "untagged" is definitely not the state you want to see when you carry a label stack.
Hope this helps,
Thanks for explaining further, so now I think I have understood the same.
In simple words, a packet going out of an interface not enabled for mpls in a router enabled with tag-switching would have its outgoing label in the LFIB as 'untagged' ie there is no tag for this prefix and should be routed.
For a connected prefix, a MPLS neighbor will advertise an 'implicit-null' to its LDP neighbor. The neighbor will have a pop tag for the prefix so that there is no MPLS lookup in the final hop. Pop tag is a special and valid mpls label, untagged means just that no tag.
Hope this helps. let me know if it does
gautham,u want to say like.....
Suppose RtrA and RtrB are connected directly and RtrA is advertising route 'X' as implicit null to RtrB than RtrB will put in LFIB for route 'x' as pop tag?
This post is a bit old but I found an answer elsewhere and figured I would post it. Hope this helps.
The label operations are push, pop, swap, untagged and aggregate.
Push - impose a label. Can be a new label on an new packet or an additional label on an already labeled packet.
Pop - Remove the top label. If it was the last label remove and send as untagged. If it had multiple labels remove the top and continue to send.
Swap - swap the top label.
Untagged - all labels removed and forward the packet unlabeled.
Aggregate - all labels removed and an IP lookup is done on the packet.
This information comes from a great Cisco Press book by the way, MPLS Fundamentals by Luc De Ghein