BGP States: Troubleshoot Active Neighbor

BGP (Border Gateway Protocol) is the exterior gateway routing protocol which is used on the Internet to exchange routing information between different autonomous systems or within same autonomous system.

That helps internet routers to make routing decisions about the bgp best path selection for data traffic received on these routers to reach its destination. BGP establishes neighbor relationships between routers, and these relationships go through several states as they are being established and maintained.

BGP States

Before moving on the details of each BGP states, I recommend you to go through the basics of what is BGP and how it works, it is important to understand these concepts to proceed further. 

1. BGP Idle State:

In this state, the router has not yet established a TCP connection with its BGP neighbor. The router is not actively attempting to establish a connection.

2. BGP Connect State: 

After the Idle state, the router transitions to the Connect state. In this state, the router initiates a TCP 3-way handshake connection with its neighbor by sending an initial TCP SYN packet and other end responds to it with a TCP ACK packet and send its own TCP SYN packet.

In the last initiator responds back with a TCP ACK packet and TCP connections establishes. Once the TCP 3-way handshake connection is established, BGP router moves to the next state.

3. BGP Active State: 

If the TCP connection fails to establish within a certain timeout period, the router enters the Active state. In this state, the router repeatedly attempts to establish a TCP connection with its neighbor. This state indicates that the router is actively trying to connect but has not yet succeeded.

4. BGP OpenSent State:

When the TCP connection is successfully established, the router transitions to the OpenSent state.

In this state, the router sends an Open message to its neighbor, which includes its BGP capabilities and other information. The router waits for an Open message from its neighbor to proceed to the next state.

5. BGP OpenConfirm State: 

After receiving the Open message from its neighbor, the router enters the OpenConfirm state. In this state, the router waits for a Keepalive message from its neighbor to confirm that the neighbor has also reached the OpenConfirm state. Once the Keepalive message is received, the router moves to the next state.

6. BGP Established State:

The router enters the Established state when it has received a Keepalive message from its neighbor. In this state, the routers can exchange BGP updates, such as routing information and path attributes. In BGP established state, the router is now fully operational and actively exchanging routing information with its neighbor.

If there is a disruption or issue in the BGP session, the router may transition back to the Idle state and repeat the process of establishing the neighbor relationship.

BGP Neighbor States

It's worth noting that BGP states may vary slightly depending on the BGP implementation or router vendor. The above states are the common ones defined in the BGP standard (RFC 4271).

When troubleshooting BGP neighbor state issues, it's important to identify the specific state in which the problem occurs. Here are a few scenarios with examples and steps you can take to troubleshoot BGP neighbor state issues:

1. BGP Neighbor Stuck in Idle State:

✓ Verify the BGP configuration on both routers, ensuring the correct neighbor IP address, AS number, and BGP timers.

✓ Check the connectivity between the routers. Ensure that there are no network connectivity issues, such as routing or firewall problems.

✓ Verify that the BGP process is running correctly on both routers.

✓ Check the BGP log files for any error messages or warnings that could indicate the cause of the problem.

✓ If the issue persists, enable BGP debugging on both routers to capture more detailed information about the BGP session establishment process.

2. BGP Neighbor Stuck in Connect State:

✓ Check the TCP connectivity between the routers using tools like ping or traceroute to ensure there are no network connectivity problems.

✓ Verify that there are no access control lists or firewall rules blocking the BGP traffic between the routers.

Check the BGP configuration for any misconfigured BGP timers or wrong neighbor IP address.

✓ Verify that the BGP routers have the correct routing information and can reach each other's IP addresses.

✓ If the issue persists, analyze network device logs or capture network traffic to identify any underlying network issues.

3. BGP Neighbor Stuck in Active State:

✓ Check for network connectivity problems between the routers, such as high latency or packet loss.

✓ Verify that the BGP timers are appropriately configured on both routers and are not too aggressive.

✓ Check for any firewall rules or access control lists that might be blocking the BGP traffic.

✓ Ensure that the AS number is correctly configured on both routers and that they are using the same AS number.

✓ If the issue persists, check with your upstream service provider or network administrator to determine if there are any network-wide issues affecting BGP connectivity.

4. BGP Neighbor Stuck in OpenSent or OpenConfirm State:

✓ Verify that the BGP version configured on both routers is compatible. Ensure that both routers support the same BGP version.

✓ Check for any misconfigured BGP capabilities or optional parameters that might be causing the issue.

✓ Verify that the BGP routers are using the correct authentication settings, if authentication is enabled.

✓ If the issue persists, capture BGP debug information or analyze BGP messages exchanged between the routers to identify any specific errors or discrepancies.

In general, when troubleshooting BGP neighbor state issues, it's important to examine the BGP configurations, verify network connectivity, check for any misconfigurations or compatibility issues, and analyze logs and debug output for any error messages or warnings.

Additionally, consulting with network administrators, service providers, or referring to vendor-specific documentation can provide further guidance in troubleshooting BGP  states problem.