Discussion

14.11 Protecting an LSP’s Path
Problem
You want to protect an LSP’s traffic in the event of a link or router failure to ensure
that it always arrives at the egress end of the LSP.
Solution
When you create an LSP, RSVP establishes a single path between the ingress and
egress routers. One way to protect the LSP’s path is to establish an end-to-end secondary
path for the LSP. First, explicitly set up the primary LSP:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R5 to 10.0.0.5
aviva@R1# set label-switched-path R1-to-R5 bandwidth 50m
aviva@R1# set label-switched-path R1-to-R5 primary primary-path-R1-to-R5
aviva@R1# set path primary-path-R1-to-R5
Then, configure a secondary path to the same egress router:
[edit protocols mpls]
aviva@R1# set label-switched-path R1-to-R5 secondary secondary-path-R1-to-R5 standby
aviva@R1# set path secondary-path-R1-to-R5
Discussion
When you create a basic LSP on the ingress router, one route is set up to reach the
egress router and all the LSP’s traffic is forwarded along this route. If a failure occurs
along the path—for instance, if a router’s interface goes offline, if an entire router
goes down, or if the physical link between two routers is cut—the ingress router
recalculates the LSP’s path and re-establishes the LSP if possible. However, until the
ingress route learns of the LSP failure and recalculates a new LSP, all traffic going
into the LSP is dropped and never reaches the egress. Depending on the length of the
LSP, the speeds of the interfaces, and other factors, it can take some seconds for the
new LSP to become operational. One way to provide a redundant path is to set up a
secondary path in advance and have it always be on call in case the primary path fails
so that it can immediately take over forwarding the LSP’s traffic. An optimal secondary
LSP takes a completely different path through the network so that there are no
common links or routers shared by the two LSPs. For this recipe, the network topology
is extended to six routers (see Figure 14-7).
The first part of this recipe sets up the primary path on the ingress router, R1. The
first command names the LSP and specifies the address of the egress router, R5. The
second command reserves bandwidth for the LSP, here requesting 50 Mbps. The third
command creates the primary LSP and names it primary-path-R1-to-R5. The final
command, the set path command, tells MPLS about the name of the path.
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