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FAQ-Does a Loop Occur Due to IS-IS Route Leakage

Publication Date:  2019-04-19 Views:  41 Downloads:  0
Issue Description

Network topology:

Networking description:


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: 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 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 ( transmitted to outside the area over the level-1-2 routers (R1 and R2).

3. Assume that R3 transmits the route ( learned from R5 to R2. R2 then transmits this route to R4. In this case, a loop is formed. If R4 attempts to access, 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 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 leakage does not cause a loop. Loop prevention is implemented by setting the U/D flag in TLV to Down.