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How to address BGP problems on Cisco devices

Cisco How-To Tutorials
Border Gateway Protocol (BGP) is an interdomain routing protocol used to exchange network reachability information. Problems with BGP could stem from factors such as these:

Errors in configuration.
Problems with Neighbor Establishment.
Transit traffic saturating resources in a multihomed network.
High volume of routing information, which requires a large amount of memory.
Routes missing from the BGP or routing table due to issues with advertising or redistributing routes.

For basic BGP operation problems, ensure that peering between BGP routers is successful. These are potential sources of peering problems:

Configuration errors.
Maximum Transmission Unit (MTU) mismatch issues.
Reachability issues when interfaces other than directly connected interfaces are used while peering.

If certain BGP routes are missing from the routing table, determine if they are learned through BGP and exist in the BGP table. Routes existing in the BGP table might be missing from the routing table due to the BGP synchronization rule or due to the non-availability of a valid route to the next hop. If the routes are missing from the BGP table altogether, determine if the peering has been successful, and check for the existence of route filters. If BGP routes are not being advertised, check if the conditions for advertising BGP routes are being satisfied. An internal Border Gateway Protocol (iBGP) learned route is not advertised to other iBGP peers, and configuring a route reflector might be necessary. Aggregate routes are not advertised unless at least one component route exists in the BGP table. Similarly, routes advertised by issuing the bgp network command are also subject to the existence of corresponding routes in the routing table. Another factor that could contribute to errors in BGP is the availability of sufficient resources. Ensure that BGP routers are able to cope with the high amount of routing information that is typically exchanged between routers configured for BGP. If resources are an issue, routing information learned through BGP can be controlled by configuring route filters. Multihomed networks could also run into problems such as becoming a transit Autonomous System (AS) or not being able to load balance traffic. In such cases, additional configuration might be required. For more information on BGP issues refer to: Troubleshooting BGP

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