Missing Labels Can Forward The Routes To Somewhere Else

Introduction
A figure given is typical SP network in which MPLS is used and loopbacks are used for label advertisements. By default labels are generated for all the RIB routes and being distributed to al its adjacent peers. But in the current scenario only labels are advertised for loopbacks only and the same loopbacks are being used for MP-BGP. OSPF is used for IGP.

Problem Description
Suddenly in running network one of the PE1 lan interface stop getting response from the remote PE6 & PE7. But the freaky thing is that it is reachable from its area only. The same happened to PE3 but the difference is that loopback of PE3 is reachable from the whole cloud.

Findings
We did trace from PE1 to PE6 lan. Packet came to PE2 -> PE5 -> PE7 -> PE6 ->PE5 -> PE6 -> PE5.........the loop occurred between PE6 to PE5. But the actual path was PE1 -> PE2 -> PE5 -> PE6. The route update was properly receiving by PE6 by stating next hop as PE5. One weird thing we found the labels are originating with in the path for LAN routes also. This gives us a little hint it should not be like the way it was happening. After that we started out to dig the problem with the help of labels. On PE5 a label was generated for 10.10.1.0/24 route that was fine but PE5 router was forwarding the label (5001) to PE6. After that we checked on PE6 label for 10.10.1.0/24 route. The label should be 5001 but instead of 5001 we were receiving label 450. Then we again checked local label(450) on PE2 and that label was used for PE7. From PE7 packet was forwarding to PE6 from PE6 label 450 was used and forwarding to PE5 and on PE5 that label was used for PE7. In this way looped occurred among three. At last we concluded that there was some problem with the label advertisement. Why it was because labels should be generated for all routes but should not be advertised to another routers except loopbacks. This is actually the malfunctioning which is occurring for only few routes not for all.

Workaround
Simply we entered the command “mpls ldp advertise-labels”. After adding this command the labels which were generated for routes were distributed to all adjacent neighbours. Then we checked and found problem solved. The local and remote label findings were same and proper route forwarding was occurring. At last we add the command for mpls ldp advertise-labels for “Acl For Loopback” and checked the LIB and LFIB now the labels were locally generated for lan router but not advertised to remote locations.
It was noting apart from IOS bug.
This was the problem which we faced in the scenario is hardly found in real live network and prior to this I had seen this type of problem only in books.


regards
shivlu jain

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