R1, R2, and R3 are interconnected through IS-IS. R3 is a level-1 router in area 1 (non-backbone area). R1 and R2 are both level-1-2 routers. R4 is interconnected with R1 and R2. R4 and R5 are both level-1 routers that run IS-IS and are deployed in area 0 (backbone area). To enable R3 to select the optimal path, route leakage is configured for level-2 routes on R1 and R2. R3 then has the routing information transmitted outside the area over two level-1-2 routers (R1 and R2).
1. Assume that the loopback interface address (loopback0: x.x.5.32) on R5 is advertised in IS-IS. If route leakage is not configured on level-1-2 routers (R1 and R2), R3 receives a level-1 LSP transmitted by the level-1-2 routers (R1 and R2) in the level-1 area. The ATT in the LSP is set, and R3 generates two default routes. The access to the IP address x.x.5.5 is over the two default routes.
2. To address the sub-optimal path issue, route leakage is configured on the level-1-2 routers (R1 and R2). In this case, the level-1 router (R3) generates two routes (184.108.40.206) transmitted to outside the area over the level-1-2 routers (R1 and R2).
3. Assume that R3 transmits the route (220.127.116.11) learned from R5 to R2. R2 then transmits this route to R4. In this case, a loop is formed. If R4 attempts to access 18.104.22.168, the loop direction is R4-R2-R3-R1-R4.
To address such an issue, a loop prevention mechanism is designed for IS-IS. Specifically, the U/D flag in TLV is set to Down.
After the level-1-2 router (R1) leaks the route destined for x.x.5.5 from level-2 to level-1, the U/D flag in TLV in the routing information of the level-1 LSP is set to Down. The other level-1-2 router (R2) in the level-1 area receives but does not calculate the route or transmit the route to the level-2 area (R4).
In summary, IS-IS route does not cause a loop. Loop prevention is implemented by setting the U/D flag in TLV to Down.