LDP over RSVP Without Area Boundary
Page 548 7450 ESS MPLS Guide
LDP over RSVP Without Area Boundary
The LDP over RSVP capability set includes the ability to stitch LDP-over-RSVP tunnels at
internal (non-ABR) OSPF and IS-IS routers.
Figure 51: LDP over RSVP Without ABR Stitching Point
In Figure 51, assume that the user wants to use LDP over RSVP between router A and destination
“Dest”. The first thing that happens is that either OSPF or IS-IS will perform an SPF calculation
resulting in an SPF tree. This tree specifies the lowest possible cost to the destination. In the
example shown, the destination “Dest” is reachable at the lowest cost through router X. The SPF
tree will have the following path: A>C>E>G>X.
Using this SPF tree, router A will search for the endpoint that is closest (farthest/highest cost from
the origin) to “Dest” that is eligible. Assuming that all LSPs in the above diagram are eligible, LSP
endpoint G will be selected as it terminates on router G while other LSPs only reach routers C and
E, respectively.
IGP and LSP metrics associated with the various LSP are ignores; only tunnel endpoint matters to
IGP. The endpoint that terminates closest to “Dest” (highest IGP path cost) will be selected for
further selection of the LDP over RSVP tunnels to that endpoint. Note that the explicit path the
tunnel takes may not match the IGP path the SPF computes.
al_0214
10
10 10 20
25
25
25
Link & IGP Cost
SPF Path
25
25
LSP3
A>C
LSP2
A>C>E
LSP1
A>B>D>F>G
20 15
HK
E G X Dest.C
A
BDF
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