This document provides troubleshooting information for common problems with Border Gateway Protocol (BGP). To see more information, or to go to the next flowchart, click the command boxes in red.
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There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware versions.
The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.
Note: *Sample log messages to be checked when neighbor is not coming up:
BGP_SESSION-5-ADJCHANGE: neighbor[ip address] IPv4 Unicast topology base removed from session Peer closed the session BGP_SESSION-5-ADJCHANGE: neighbor[ip address] IPv4 Unicast topology base removed from session Unknown path error
Note: **Example of ping with packet size and enable does not fragment bit in IP header:
Router#ping 10.10.10.2 size 1400 df-bit Type escape sequence to abort. Sending 5, 1400-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds: Packet sent with the DF bit set !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/37/84 ms
Note: **Also, check the duplex settings of the interface.
Note: If the reset has occurred due to interface flapping, disable the eBGP's fast failover by issuing the no bgp fast-external-fallover command. By default, BGP resets the neighborship if the link used in order to reach the neighbor goes down. Disable this feature under BGP Configuration in order to keep BGP stable and prevent the interface from flapping.
If flapping occurs due to high CPU, refer to Troubleshooting High CPU Utilization on Cisco Routers.
Note: If the BGP routes are not in the routing table, verify if the network statement under the BGP configuration is correct.
Note: In the debug ip bgp x.x.x.x updates command, x.x.x.x is the neighbor to which the route should be advertised.
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Refer to Cisco Technical Tips Conventions for information on conventions used in this document.