Loose Hop Expansion - Cisco CRS Configuration Manual

MPLS Traffic Engineering Interarea Tunneling
• Reduce the SPF duration.
• Decrease the impact of a link or node failure within an area.
Figure 14: Interlevel (IS-IS) TE Network
As shown in the figure, R2, R3, R7, and R4 maintain two databases for routing and TE information. For
example, R3 has TE topology information related to R2, flooded through Level-1 IS-IS LSPs plus the TE
topology information related to R4, R9, and R7, flooded as Level 2 IS-IS Link State PDUs (LSPs) (plus, its
own IS-IS LSP).
Note You can configure multiple areas within an IS-IS Level 1. This is transparent to TE. TE has topology
information about the IS-IS level, but not the area ID.

Loose Hop Expansion

Loose hop optimization allows the reoptimization of tunnels spanning multiple areas and solves the problem
which occurs when an MPLS-TE LSP traverses hops that are not in the LSP's headend's OSPF area and IS-IS
level.
Interarea MPLS-TE allows you to configure an interarea traffic engineering (TE) label switched path (LSP)
by specifying a loose source route of ABRs along the path. It is the then the responsibility of the ABR (having
a complete view of both areas) to find a path obeying the TE LSP constraints within the next area to reach
the next hop ABR (as specified on the headend). The same operation is performed by the last ABR connected
to the tailend area to reach the tailend LSR.
For P2MP-TE tunnels, ABRs support loose hop ERO expansion to find path to the next ABR until it reaches
to the tail-end LSR, without introducing remerge.
You must be aware of these considerations when using loose hop optimization:
• You must specify the router ID of the ABR node (as opposed to a link address on the ABR).
• When multiarea is deployed in a network that contains subareas, you must enable MPLS-TE in the
subarea for TE to find a path when loose hop is specified.
• You must specify the reachable explicit path for the interarea tunnel.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
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Implementing MPLS Traffic Engineering