IS-IS Overload Bit – Why IS-IS Overload bit is used ? What are the use cases ? In this post, I will explain the Overload bit which is an important feature of IS-IS routing protocol.
When a router which runs an IS-IS routing protocol have resource issue (CPU, Memory), device shouldn’t receive network traffic. It shouldn’t be a transit router between any two other nodes in the network.
Routing protocols have a capability to signal other routers for this. Router which runs routing protocol, can send a warning to other routers to stop sending transit network traffic through them.
In OSPF, ‘ Max-metric router lsa ‘ feature is used and works in very similar way with IS-IS Overload bit.
When router which runs IS-IS Routing protocol sets Overload bit in it LSP (Link State PDU), routers detect that Overload bit is set thus they don’t not use this router for transit traffic but they use it for packets destined to the overloaded router’s directly connected networks and IP prefixes.
What happens when router has system resource problem doesn’t set IS-IS Overload Bit ?
Other routers would still send the traffic to this router, but since there is a resource issue, there will not be place to incoming LSPs into Link State Database which would create database inconsistency. As you might know, ,in IS-IS , every routers should have exact same database in a particular IS-IS level.
Otherwise, this inconsistency creates routing loops. Thus, when a router runs out of system resources, it is important to alert the other routers with Overload bit.
IS-IS and BGP Interaction to avoid traffic blackholing , How IS-IS Overload bit helps ?
Let’s look at below topology.
Figure – IS-IS and BGP Interaction – Overload Bit helps to avoid traffic blackholing
In the above topology, Router B will be taken down for a maintenance purpose. Before maintenance window starts, Router B should alert other routers that they shouldn’t send traffic anymore through it.
This is done with the IS-IS Overload bit. But what is BGP and IS-IS Interaction ? How Overload bit will help to avoid traffic blackholing ?
In the above topology, let’s imagine that BGP is running between each and every router (Full Mesh IBGP).
When Router B sets Overload Bit, it doesn’t receive any transit traffic anymore. That’s why everything is normal.
But when Router B comes up after the maintenance, if Overload bit is cleared before the BGP convergence, traffic is blackholed at the Router B.
Let me explain in this way. When BGP destination traffic, comes from Router A to Router B, Router B needs to do IP destination based lookup. Who is the destination ? BGP prefix. Did BGP convergence ? No !
That’s; why, until BGP converge, Router B should stay away from the networking traffic. Overload Bit stay as set until BGP finishes its job, when BGP converge (With End of Rib Marker message or Keepalive it is understood that BGP is converged) IS-IS Overload Bit is cleared and Router B start receiving traffic again.