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[Junos] MPLS LSP flushes when the secondary path is up or down

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Article ID: KB31140 KB Last Updated: 23 Nov 2016Version: 1.0
Summary:

If an MPLS LSP (label-switched path) has a primary path and one or more secondary paths, when one of its secondary path is up or down, the LSP will be flushed even when its primary path is always up.

Symptoms:

The age timer of an LSP restarts from 00:00:00 when its secondary path is up or down. All routes (such as L3VPN) that traverse through this LSP will be flushed too, and their age timer of these routes will restart from 00:00:00.

For example, LSP “toR2” is up about 30 minutes and uses the primary path, which traverses through interface ge-1/3/6.0:

user@router# run show route table inet.3 logical-system R1

inet.3: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.10.12.2/32 *[RSVP/7/1] 00:30:51, metric 1
               > to 10.0.31.1 via ge-1/3/6.0, label-switched-path toR2
               > to 10.0.33.2 via ge-1/0/8.0, label-switched-path toR2

Then shutdown interface ge-1/0/8 to let the secondary path down:

user@router# set interfaces ge-1/0/8 disable
user@router# commit

After several seconds, this LSP route will be flushed and its up time will restart from 00:00:00.

user@router# run show route table inet.3 logical-system R1

inet.3: 1 destinations, 1 routes (1 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.10.12.2/32 *[RSVP/7/1] 00:00:16, metric 2
               > to 10.0.31.1 via ge-1/3/6.0, label-switched-path toR2

The same thing happens when the secondary path of this LSP is up again.

Cause:

When an LSP has a primary path and one or more secondary paths, its corresponding route in table inet.3 will have matched nexthops. When one of its secondary path is up or down, the nexthops of the route in table inet.3 will be updated. Therefore, the age timer of this LSP will restart from 00:00:00.

Solution:

This is a cosmetic issue. There is no traffic impact.

The age timer of LSP restarting from 00:00:00 does not mean this route is deleted or added again. This can be confirmed with the command, rtsockmon since there's no route delete or add happening when one of its secondary path is up or down.

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