In my previous post of Untagged Lables Instead Of POP Labels depicts how loopbacks of /24 advertise in OSPF generate the untagged packets. In this post I will only explain how the benefit of command show mpls forwarding which help us to troubleshoot the problem within seconds.
As per scenario given PE is attached to P router and advetising the labels for loopbacks only. At any time by mistake if some deletes the loopback acl what will happen? PE router will receive all the outgoing labels as untagged. So immediately without thinking anything you can run a single command "show mpls forwarding" and confirm there might be some problem with the upstream router.
This troubleshooting is very basic for MPLS but really helps the operational guys to save lot of downtime.
Reasons For Coming Untagged Labels
1. ACL is bind with "mpls ldp advertise label for MPLSVPN" but not created.
2. No acl is created and use the command "no mpls ldp advertise label for MPLSVPN".
3. Improper Authentication between or among the adjacent peers consequence untagged labels.
4. MP-BGP loopbacks advertise in OSPF without /32 subnet and without using ip ospf point-to-point network command.
Real Time Scenario Problems:- I have seen many times when the LDP is working among the adjacent neighbors, at that time we receive the compaints that the customers from some backbone links are working and others are not. This is because of improper configuration where we missed to advertise the labels. The command mentioned above to help us troubleshoot such type of problems in a very ease manner.