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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
Assume the cost on all links is ten (10). Router A, an L1, in Area X will send all traffic destined for outside Area X to Router B, an L1L2. This is because Router B is the closest L1L2 neighbor. Router B is directly connected to Area Y. Router C, also L1L2, is in Area X and is directly connected to Area Z. Router A will send packets destined for Area Z to Router B. Because Router B, Router E, and Router C are backbone routers, Router B will send this packet to Router C through Router E for delivery into Area Z. The more optimal path would be for Router A to send the packet directly to Router C through Router D.

As a second example of suboptimal routing, Router R1 routes packets destined for Router R2 to its L1L2 router. This router looks at the destination area and routes directly into Area 2. Once in Area 2, the packets are routed as L1 to Router R2. Even though the initial next hop is another L1L2 router, the routing is still L1. Return packets from Router R2 to Router R1 are routed by R2 to its nearest L1L2 router. This router happens to see the best route to Area 1 as being by way of Area 4 and routes the return packets by a different route to the incoming packets. The path taken is not actually the least cost path from R2 to R1. Asymmetric routing, packets in different directions taking different paths, is not necessarily detrimental to the network but can make troubleshooting difficult.

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. Both drafts define an up/down bit to indicate whether or not the route defined in the TLV has been leaked. If the up/down bit is set to zero (0) the route was originated within that L1 area. If the up/down bit is set to one (1), the route has been redistributed into the area from L2. The up/down bit is used to prevent routing loops. An L1L2 router does not re-advertise into L2 any L1 routes that have the up/down bit set. Route leaking is configured with the IS-IS router configuration mode command redistribute isis ip level-2 into level-1 distribute-list <100-199>.

| Interactive Media Activity Drag and Drop: Identifying IS-IS Routers Upon completion of this activity, the student will be able to identify the different types of IS-IS routers in a network.