Orhan Ergun No Comments

IS-IS, a link state routing protocol, requires careful attention during network design in order to avoid traffic blackholing. In the topology below, IS-IS routing protocol is used.

is-is routing protocol

The primary path is the blue link. Using IS-IS, the overload bit signals the BGP. If the overload bit is set on Router B, Router A does not use router B as a transit node; rather, it uses Router B backup path (as depicted with yellow link in the topology above). Overload bit is used to avoid blackholing during transient-network events.

Once the overload bit is set, other nodes do not use the router as a transit node.

A case of classical design is IS-IS and BGP. If BGP runs on this network between only the edge devices, Router B and Router C will not know the prefixes of BGP. Of course, we all know that by default IGP (OSPF, IS-IS, and EIGRP) converges faster than BGP does.

Thus, in case of failure, when the failed links return, Router B and Router C’s IGP process will converge even though BGP is not ready. In sum, they will drop the traffic.

IS-IS Overload bit can be set on the router so that the router does not advertise the failed link unless BGP converges. This behavior is similar to IGP-LDP synchronization, as explained here.

Also, similar behavior in OSPF is achieved with OSPF max-metric router ISA feature. With that feature, OSPF node floods its link with maximum metric so that it is not used as a transit node.

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