Three devices run in ISIS Level-2. To balance the upstream traffic of NE40, both A and B flood the ISIS default route simultaneously. Observe the route entries at both A and B routers, and it is found that they have learned the default route of each other's, resulting in a loop.
It needs to filter the ISIS default route of the counterpart:
acl number 2003
rule 5 deny source 0.0.0.0 0
rule 10 permit
filter-policy 2003 import
In the stub area of OSPF, two ABRs can flood the default routes simultaneously. On the contrary, when ISIS floods the default route, both sides will calculate the item and put it into FIB. However, the stub area of OSPF does not put it into FIB.
1. For the common area of OSPF, it has two cases for flooding of default route: default-route-advertise and default-route-advertise always. If two ASBRs in area 0 flood the default route simultaneously by default-route-advertise command, both sides will put the route entries flooded by the peer into FIB; if default-route-advertise always command is used, both will not put the route entries into FIB.
2. For ISIS, if two devices in Level-2 area flood the default ISIS route simultaneously, both sides will put the route entries advertised by the peer into FIB.
3. In the stub area of OSPF, there are two ABRs, and their default routes will not take the other as the next hop, protecting from the loop.