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[Junos] MPLS LSP could not come up due to disabled interface

  [KB34851] Show Article Properties


Summary:

A disabled interface IP address is kept in the routing table and has a rejected nexthop. This may prevent MPLS LSP from establishing due to a loop being detected if the ingress router is using this IP as well.

To avoid this issue, use the command, 'deactivate interface/IP' instead of 'disable' to completely remove the IP from routing table.

Symptoms:

Here is the topology. LSP was created from R1 100.1.1.1 to R3 103.1.1.1. ae2 on R3 has the same IP configured as R1's ae1 due to migration. ae2 was disabled in order to prevent the IP from advertising into IGP.


We found R1 LSP DOWN TOWARDS R3 and showing loop detected on R3 (11.1.1.2).
lab@R1> show mpls lsp name R1_To_R3_LSP-01 extensive
Ingress LSP: 1 sessions

100.1.1.1
From: 103.1.1.1, State: Dn, ActiveRoute: 0, LSPname: R1_To_R3_LSP-01
ActivePath: (none)
Node/Link protection desired
LSPtype: Static Configured, Penultimate hop popping
LoadBalance: Least-fill
Encoding type: Packet, Switching type: Packet, GPID: IPv4
Revert timer: 1800
Primary                    State: Dn
Priorities: 7 0
OptimizeTimer: 900
SmartOptimizeTimer: 180
Include Any: blue
Reoptimization in 683 second(s).
Computed ERO (S [L] denotes strict [loose] hops): (CSPF metric: 20)
10.1.1.2 S 11.1.1.2 S
556417 Jul  3 10:24:11.587 11.1.1.2: Routing loop detected[6 times]
556416 Jul  3 10:23:53.049 Originate Call
556415 Jul  3 10:23:53.048 Clear Call
556414 Jul  3 10:23:53.048 CSPF: computation result accepted 10.1.1.2 11.1.1.2
556413 Jul  3 10:23:39.909 11.1.1.2: Routing loop detected[6 times]
556412 Jul  3 10:23:23.196 Originate Call
556411 Jul  3 10:23:23.196 Clear Call
556410 Jul  3 10:23:23.196 CSPF: computation result accepted 10.1.1.2 11.1.1.2
556409 Jul  3 10:23:05.368 11.1.1.2: Routing loop detected[6 times]
556408 Jul  3 10:22:53.854 Originate Call
Cause:

By design, the disabled interface will have it's IP in the routing table with a Reject nexthop. It will prevent it from being advertised into IGP. But RSVP can still use it for loop detection. Here is the route 10.1.1.1 showing in R3's inet.0 table.

lab@R3> show route 10.1.1.1

inet.0: 30688 destinations, 30696 routes (30686 active, 0 holddown, 2 hidden)
+ = Active Route, - = Last Active, * = Both

10.1.1.1/32    *[Local/0] 00:00:11
                      Reject

When R1 initiated the LSP, it will send out RSPV path message and put it’s output interface IP 10.1.1.1 in RRO for loop detection purposes. When R3 receives this path message, it will compare the RRO list with the local addresses in the inet.0 routing table. It found its local address 10.1.1.1 appeared in RRO, so it returned path error message to R1 indicating that there is a loop. Once R1 receives the path error message, it stopped further LSP calculation and reported loop was detected.

Solution:

Using the command, 'deactivate interface/IP' instead of 'disable' will completely remove the IP from R3's routing table and avoid this type of issue.

Example:

lab@R3# deactivate interfaces ae2

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