Example 6-22: debug ip bgp updates Command Output
RouterA#debug ip bgp updates
Mobile router debugging is on for address family: IPv4 Unicast
RouterA#clear ip bgp 10.1.0.2
After the neighbor adjacency is reestablished, Router A creates and sends updates to 10.1.0.2. The first update highlighted in the example, 10.1.1.0/24, next 10.1.0.1, is an update about network 10.1.1.0/24, with a next hop of 10.1.0.1, which is Router A’s address. The second update highlighted in the example, 10.97.97.0/24, next 172.31.11.4, is an update about network 10.97.97.0/24, with a next hop of 172.31.11.4, which is the address of one of Router A’s EBGP neighbors. The EBGP next-hop address is being carried into IBGP.
Router A later receives updates from 10.1.0.2. The update highlighted in the example contains a path to two networks, 10.1.2.0/24 and 10.1.0.0/24.
Understanding and Troubleshooting BGP Neighbor States
After the TCP handshake is complete, the BGP application tries to set up a session with the neighbor. BGP is a state machine that takes a router through the following states with its neighbors:
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Idle— The router is searching the routing table to see whether a route exists to reach the neighbor.
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Connect— The router found a route to the neighbor and has completed the three-way TCP handshake.
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Open sent— An open message was sent, with the parameters for the BGP session.
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Open confirm— The router received agreement on the parameters for establishing a session.
Alternatively, the router goes into active state if there is no response to the open message.
After you enter the neighbor command, BGP starts in the idle state, and the BGP process checks that it has a route to the IP address listed. BGP should be in the idle state for only a few seconds. However, if BGP does not find a route to the neighboring IP address, it stays in the idle state. If it finds a route, it goes to the connect state when the TCP handshaking synchronize acknowledge (SYN ACK) packet returns (when the TCP three-way handshake is complete). After the TCP connection is set up, the BGP process creates a BGP open message and sends it to the neighbor. After BGP dispatches this open message, the BGP peering session changes to the open sent state. If there is no response for 5 seconds, the state changes to the active state. If a response does come back in a timely manner, BGP goes to the open confirm state and starts scanning (evaluating) the routing table for the paths to send to the neighbor. When these paths have been found, BGP then goes to the established state and begins routing between the neighbors.
The BGP state is shown in the last column of the show ip bgp summary command output.
| Note | You can observe the states that two BGP routers are going through to establish a session using debug commands. In Cisco IOS Software Release 12.4, you can use the debug ip bgp ipv4 unicast command (or the debug bgp ipv4 unicast events command) to see this process. In earlier IOS releases, only the debug ip bgp events command was available to provide similar output. |
Idle State Troubleshooting
The idle state indicates that the router does not know how to reach the IP address listed in the neighbor statement. The router is idle for one of the following reasons:
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It is waiting for a static route to that IP address or network to be configured.
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It is waiting for the local routing protocol (IGP) to learn about this network through an advertisement from another router.
The most common reason for the idle state is that the neighbor is not announcing the IP address or network that the neighbor statement of the router is pointing to. Check the following two conditions to troubleshoot this problem:
Active State Troubleshooting
If the router is in the active state, this means that it has found the IP address in the neighbor statement and has created and sent out a BGP open packet but has not received a response (an open confirm packet) back from the neighbor.
One common cause of this is when the neighbor does not have a return route to the source IP address. Ensure that the source IP address or network of the packets is advertised into the local routing protocol (IGP) on the neighboring router.
Another common problem associated with the active state is when a BGP router attempts to peer with another BGP router that does not have a neighbor statement peering back at the first router, or the other router is peering with the wrong IP address on the first router. Check to ensure that the other router has a neighbor statement peering at the correct address of the router that is in the active state.
If the state toggles between idle and active, the autonomous system numbers might be misconfigured. You see the following console message at the router with the wrong autonomous system number configured in the neighbor statement:
At the remote router, you see the following message:
Established State
The desired state for a neighbor relationship is the established state. This state means that both routers agree to exchange BGP updates with one another and routing has begun. As mentioned, if the state column in the show ip bgp summary command output is blank or has a number in it, BGP is in the established state, and the number shown is the number of routes that have been learned from this neighbor.
Use the show ip bgp neighbors command to display information about the BGP connections to neighbors. In Example 6-23, the BGP state is established, which means that the neighbors have established a TCP connection and the two peers have agreed to use BGP to communicate.
Example 6-23: show ip bgp neighbors Command Output
RouterA#show ip bgp neighbors
BGP neighbor is 172.31.1.3, remote AS 64998, external link
BGP version 4, remote router ID 172.31.2.3
BGP state = Established, up for 00:19:10
Last read 00:00:10, last write 00:00:10, hold time is 180, keepalive interval
is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(old & new)
Address family IPv4 Unicast: advertised and received
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 7 7
Notifications: 0 0
Updates: 13 38
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