BGP flap is a term used to describe the rapid and frequent switching of a BGP (Border Gateway Protocol) route between active and inactive states. This can occur due to a variety of reasons, including routing errors, network instability, or problems with the BGP configuration. To resolve a BGP flap issue, you will need to identify and address the underlying cause of the problem.
How to Resolve the BGP Flap Issues?
Some common steps to take when troubleshooting BGP flap include:
Step 1: Check if the BGP neighbor relationship is established.
Run “show IP BGP summary” to check whether the BGP neighborship is established or not. If it’s not, troubleshoot this first and make sure that your neighbor can reach each other over the network.
Step 2: Check the TTL values.
Make sure that the TTL value is set high enough on both routers. A high TTL value helps to prevent BGP flap issues by ensuring that messages are not discarded due to timeouts.
Step 3: Check if the network interface or IP address has changed recently.
If there has been any configuration change then this could be the cause of the BGP flap issue. Check that any changes have been propagated to all routers and verify that the correct IP address is being used for router-to-router communication.
Step 4: Make sure that routing policies are configured correctly.
Check whether there is any conflicting route between the two devices. If there is a conflict, then you will need to re-configure your routing policies accordingly.
Step 5: Check for any looping routes.
Looping routes can cause BGP flap issues. Check the routing table and make sure that there are no loops present in the network.
Step 6: Update outdated or buggy router software.
Outdated or buggy router software can cause BGP flap issues so make sure that all of your routers are running the latest version of their software.
Step 7: Check for physical network problems.
Finally, check for any physical network issues such as cable faults or link flapping that could be causing the BGP flap issue. If there are any faulty or broken cables, replace them and make sure that all links are working properly.
Now we are explaining the TTL issue in BGP Flap.
Example of Time To Live TTL Issue
In a BGP (Border Gateway Protocol) flap issue, the TTL (time to live) values of the BGP routes can be used to help identify the source of the problem and potentially resolve it.
For example, consider a network with two BGP routers, A and B, that are connected via a link. Router A has a default route pointing to router B, and router B has a default route pointing to router A. If both routers advertise their default routes to each other, a routing loop will occur, causing the default route to continuously flap between the two routers.
When a BGP route is advertised, it is assigned a TTL value that indicates how many hops it can traverse before it is discarded. In this case, the default route advertised by router A will have a TTL value of 1, indicating that it can only be used for one hop (from router A to router B).
Similarly, the default route advertised by router B will have a TTL value of 1, indicating that it can only be used for one hop (from router B to router A).
However, because of the routing loop, the default route will be continuously forwarded between router A and router B, causing the TTL value to decrement each time it is forwarded.
Eventually, the TTL value will reach 0, and the route will be discarded. This will cause the default route to flap between being up and down, causing instability in the network.
To resolve this issue, one of the routers would need to be configured to not advertise its default route to the other router, breaking the loop and stabilizing the network. The TTL values can be used to identify the source of the problem and ensure that the correct changes are made to the BGP configuration.