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

BGP Transit Autonomous Systems

Monitoring and Troubleshooting IBGP in a Transit AS

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

Outline

• Overview

• Monitoring IBGP

• Common IBGP Problems

• Troubleshooting IBGP Session Startup Issues

• Troubleshooting IBGP Route Selection Issues

• Troubleshooting IBGP Synchronization Issues

• Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-3

Monitoring IBGP

show ip bgp neighborsshow ip bgp neighbors

router>

• Displays whether a neighbor is an IBGP neighbor

show ip bgpshow ip bgp

router>

• Uses a special marker (i) for IBGP routes

show ip bgp prefixshow ip bgp prefix

router>

• Displays whether the prefix is an IBGP route

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-4

Monitoring IBGP (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-5

Monitoring IBGP (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-6

Common IBGP Problems

• IBGP sessions will not start.

• IBGP route is in the BGP table but is not selected.

• IBGP route is selected but is not entered in the routing table.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-7

Troubleshooting IBGP Session Startup Issues

Symptom:• IBGP session does not start.

Diagnosis:• IBGP session is run between loopbacks, and update-source

keyword is missing.

Verification:• Use debug ip tcp transactions. You should see BGP sessions

coming from unexpected IP addresses.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-8

Troubleshooting IBGP Session Startup Issues (Cont.)

Symptom:• IBGP session does not start.

Diagnosis:• Loopback interfaces are not reachable.

Verification:• Do extended ping between loopback addresses to verify

reachability.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-9

Troubleshooting IBGP Session Startup Issues (Cont.)

Symptom:• IBGP session does not start.

Diagnosis:• Packet filters prevent establishment of BGP sessions.

Verification:• Use debug ip tcp transactions and debug ip icmp to see whether

the initial TCP SYN packets are rejected.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-10

Troubleshooting IBGP Route Selection Issues

Symptom:• An IBGP route is in the BGP table but is never selected as

the best route.

Diagnosis:• The BGP next hop is not reachable.

Verification:• Use show ip bgp prefix to find the BGP next hop.

• Use show ip route to verify next-hop reachability.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-11

Troubleshooting IBGP Synchronization Issues

Symptom:• An IBGP route is selected as the best route but not entered

into the IP routing table.

Diagnosis:• BGP synchronization is not disabled.

Verification:• Disable BGP synchronization, clear the BGP sessions, and

re-examine the IP routing table after the BGP table becomes stable.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-12

Summary

• You can use the show ip bgp neighbors and show ip bgp prefix commands to monitor IBGP operation.

• Common IBGP configuration problems include IBGP sessions that do not reach the Established state, routing information that is received via IBGP that is never selected, and the best BGP route never being installed in the routing table.

• There are a number of problems that can occur during IGBP session startup. You can use debug ip tcp transactions to see BGP sessions coming from unexpected IP addresses, use an extended ping and explicitly refer to the IP address of the loopback interface to ensure that packets are sourced from the loopback interface, or use debug ip tcp transactions and debug ip icmp to see whether the initial TCP SYN packets are rejected.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-13

Summary (Cont.)

• It is important to include the subnet linking the transit AS to an external AS in the IGP to prevent the BGP next hop from being unreachable. To verify next-hop reachability, check the BGP route in the BGP table by using the show ip bgp prefix command.

• Routers with BGP synchronization enabled will not be able to install IBGP routes in the routing table or propagate them to other EBGP neighbors. Configure no synchronization in the router BGP configuration to solve this problem.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—2-14