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
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 (220.127.116.11) transmitted to outside the area over the level-1-2 routers
(R1 and R2).
3. Assume that R3 transmits the route (18.104.22.168) learned from R5 to R2. R2
then transmits this route to R4. In this case, a loop is formed. If R4 attempts
to access 22.214.171.124, the loop direction is R4-R2-R3-R1-R4.
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.