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7.4 | ![]() |
IP Routing with Integrated IS-IS | |
7.4.2 | ![]() |
Suboptimal IS-IS routing |
An L1 router knows the topology only of its own area and has L1 or L1L2 neighbors within this area. An L1 router has an L1 link-state database with all the information for intra-area routing. It uses the closest L2 capable router in its own area to send packets out of the area, a scenario that may result in suboptimal routing. An L1L2 router that is attached to another area will set the attached bit in its L1 LSP. All the L1 ISs in an area will get a copy of this LSP and know where to forward packets to destinations outside the area. If the routers are running Integrated IS-IS, a default IP route will automatically be installed in the L1 routers pointing toward the nearest L1L2 router that set the attached bit in its L1 LSP. An L1L2 router that is not attached to another area can also detect that an L2 only neighbor is attached to another area and can set the attached bit on behalf of this L2 only neighbor. If there is more than one point to exit the area, the closest L1L2 router is selected based on the cost. If there are two equal cost paths then the traffic may load balance over the two paths.
Suboptimal routing
As a second example of suboptimal routing, Router R1 routes packets
destined for Router R2 to its L1L2 router.
A feature available since IOS release 12.0 allows L2 routes to be
leaked in a controlled manner into the L1 area to help avoid this
situation. All IS-IS areas are ’stub’ areas. However, with the
route-leaking feature, leaking L2 routes into L1, a sort of IS-IS
not-so-stubby area is created. Route leaking helps reduce suboptimal
routing by providing a mechanism for leaking, or redistributing, L2
information into L1 areas. By having more detail about inter-area
routes, an L1 router is able to make a better choice with regard to
which L1L2 router to forward the packet. Route leaking is defined in
RFC 2966 for use with the narrow metric TLV types 128 and 130. IS-IS
extensions for Traffic Engineering, IETF Internet draft
document draft-ietf-isis-traffic-04.txt, defines route leaking for use
with the wide metric TLV type 135.
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