© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-1 BGP Overview Monitoring and Troubleshooting BGP

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-2 Outline Overview Monitoring Overall BGP Routing Monitoring BGP Neighbors Monitoring the BGP Table Debugging BGP BGP Session Startup Problems BGP Neighbor Not Reachable BGP Neighbor Not Configured BGP AS Number Mismatch Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-3 Monitoring Overall BGP Routing show ip bgp summary router> Displays BGP memory use, and displays BGP neighbors and the state of communication with them

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-4 Monitoring BGP Neighbors show ip bgp neighbors ip-address router> Displays detailed neighbor information

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-5 Monitoring the BGP Table show ip bgp router> Displays all routes in the BGP table in summary format

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-6 Monitoring the BGP Table (Cont.) show ip bgp ip-prefix [mask subnet-mask] router> Displays detailed information about all paths for a single prefix

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-7 Debugging BGP debug ip tcp transactions router# Displays all TCP transactions (start of session, session errors, etc.) debug ip bgp events router# Displays significant BGP events (neighbor state transitions, update runs)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-8 Debugging BGP (Cont.) debug ip bgp keepalives router# Debugs BGP keepalive packets debug ip bgp updates router# Displays all incoming or outgoing BGP updates Use with caution

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-9 Debugging BGP (Cont.) debug ip bgp updates acl router# Displays all incoming or outgoing BGP updates for routes matching an IP access-list debug ip bgp ip-address updates [acl] router# Displays all BGP updates received from or sent to a BGP neighbor (optionally matching an IP access-list)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-10 BGP Session Startup Problems Common BGP session startup symptoms: BGP neighbors do not become active. BGP neighbor is active, but the session is never established. BGP neighbor oscillates between idle and active.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-11 BGP Neighbor Not Reachable Symptom: BGP neighbors do not become active. –show ip bgp neighbors displays the neighbor state as Idle for several minutes. Diagnosis: Neighbor is not directly connected. Verification: Verify with show ip route.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-12 BGP Neighbor Not Reachable (Cont.) Symptom: BGP neighbor is active; session is not established. –debug ip tcp transactions display shows that the TCP SYN packet is not answered with a SYN-ACK packet. Diagnosis: Neighbor is not reachable. Verification: Verify connectivity with ping. Check for the presence of an access-list.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-13 BGP Neighbor Not Reachable (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-14 BGP Neighbor Not Configured Symptom: BGP neighbor is active; session is not established. –debug ip tcp transactions display shows that the TCP SYN packet is answered with an RST packet. Diagnosis: This router is not configured as the BGP neighbor on the neighboring router. Verification: Check IP addresses of BGP neighbors with show ip bgp summary on the neighboring router.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-15 BGP Neighbor Not Configured (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-16 BGP AS Number Mismatch Symptom: BGP neighbor oscillates between Active and Idle. –debug ip tcp transactions displays the TCP session being established and torn down immediately. Diagnosis: There is an AS number mismatch between BGP neighbors. Verification: Verify the AS numbers configured for neighboring routers using the show ip bgp summary on both routers.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-17 BGP AS Number Mismatch (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-18 Summary The show ip bgp summary command displays the overall status of BGP and shows configured neighbors and their state. You can use the show ip bgp neighbors command to get more in-depth information about a specific BGP neighbor. All entries in the BGP table can be displayed with the show ip bgp command. You can also use show ip bgp to display an extended printout about a specific route in the BGP table. You can use the debug ip tcp transactions command to troubleshoot BGP session establishment problems. The command debug ip bgp events displays significant BGP events, while debug ip bgp updates displays the routing information being exchanged between BGP neighbors.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-19 Summary (Cont.) Three common BGP session startup symptoms are that BGP neighbors never become active, that the BGP neighbor is active but the BGP session is not established, and that the BGP neighbor state oscillates between idle and active. If a router is configured with a BGP neighbor that is in another AS but not directly connected, the session stays in the Idle state. If a BGP neighbor is unreachable, no reply is sent for the TCP SYN packet, causing the session to time out. If the TCP session is established using the three-way handshake (SYN, SYN-ACK, ACK), but the session is dropped after a short packet exchange, BGP parameters are mismatched.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-20