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Knowledge Base

What Are the Functions of the wait for bgp Configuration in the set-overload Command of IS-IS on an NE40E

Publication Date:  2019-05-13  |   Views:  188  |   Downloads:  0  |   Document ID:  EKB1100015028

Contents

Issue Description

The set-overload command of IS-IS is used to configure the overload bit. In which scenarios is the wait for bgp configuration used?

Solution

On a BGP network, if a new device is added or an existing device is restarted, because IS-IS routes are converged faster than BGP routes, the device is used as the forwarding node. This causes traffic loss. If the set-overload command is run and the wait-for-bgp keyword is specified, the device instructs other devices that it is in the overload state and does not consider itself a forwarding node. The overload bit is deleted from LSP only after BGP route convergence is complete, solving the traffic loss issue.

On the BGP network shown in this figure, if service traffic from router A to router D is forwarded over router B and router C and router B is restarted due to a fault, IS-IS routes are converged faster than BGP routes. Router A discovers that the routes destined for router D are recursed to router B. In this case, BGP routes of router B are not converged yet and has not learned information about the routes destined for router D. This causes traffic loss. With the set-overload command, the overload bit of LSP is not deleted until BGP convergence is complete.

Reference topology: