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BGP Route Flapping, Causes & Troubleshooting

March 06, 2024 , ,

BGP Route Flapping, Causes & Troubleshooting

BGP is used to exchange routing information between multiple autonomous systems (AS) on the Internet. An autonomous system is a group of networks that share a common administrative area. 

BGP is used to route traffic across multiple independent systems and is the protocol used by Internet Service Providers (ISPs) to communicate routing information with one another.

⭐Related : BGP Cheat Sheet for Network Engineers
⭐Related : 51 facts about BGP routing Protocol for Network Engineers 

 What is BGP route flapping ?

BGP route flapping is the quick and constant changing of BGP route advertisements, in which routes appear and disappear from the routing table. This occurrence can be caused by a number of situations, including network misconfigurations, intermittent connectivity issues, or topological instability.

What is BGP route flapping ?
Fig 1.1- BGP Route Flapping

 Root Causes of BGP Route Flapping

⭐ MTU Mismatch: MTU mismatch can cause the BGP flapping where two BGP routers does not match MTU size on their side.

⭐ Path MTU Discovery Issues: Incorrect Path Maximum Transmission Unit Discovery (PMTUD) might cause BGP route flapping. If PMTUD determines an inaccurate maximum data size for the TCP BGP session, it may create instability.

⭐ Interface Flapping: Changes in interface status, such as link flapping or hardware failures, can cause BGP route flapping since the BGP session frequently comes up and down.

⭐ Routing Policy Misconfigurations: Incorrectly set routing rules, such as route filters or route redistribution, can cause BGP route instability by rejecting valid routes or creating routing loops.

⭐ BGP Peer Flapping: Uncertainty in BGP peer connections can result from configuration conflicts, authentication problems, or route refresh issues, causing the BGP session to reset again.

⭐ Route Reflectors and Route Reflector Clients: When route reflectors are set up, configuration errors or overloading conditions in the reflectors or their clients can increase route flapping by interfering with the propagation of BGP updates.

⭐Hold Timer Expiry: BGP sessions use hold timers to detect whether a peer is still alive. If the hold timer runs out owing to connectivity issues or misconfigurations, the BGP session may fail.

 Troubleshooting BGP Route Flapping

⭐ Interface Stability Checks: First, ensure that the physical and logical interfaces participating in BGP sessions are stable. Check for hardware flaws, cable difficulties, or setup errors that could cause interface flapping.

⭐ MTU Size Checks: Verify that the Maximum Transmission Unit (MTU) settings match between BGP peers. MTU mismatches can lead to flapping. Use commands like ping with specific packet sizes and DF-bit settings to test MTU

⭐ BGP Neighbor checks: Perform an in-depth review of BGP neighbor connections to detect any discrepancies or irregularities. Check BGP peering setups, including AS numbers, IP addresses, and authentication settings, for compatibility and consistency.

⭐ Routing Policy Validation: Examine how routing policies, such as route filters, prefix lists, or route maps, are applied to BGP sessions. To avoid unintentional route filtering or redistribution concerns, ensure that routing policies are correctly configured and matched with the expected routing behavior.

⭐ Route Dampening: Use route dampening technologies to decrease unstable routes and reduce the impact of flapping on routing infrastructure. Based on observed route flapping patterns and network dynamics, adjust route dampening parameters such as half-life and suppress limits accordingly.

⭐ Topology Stability Checks: Assess the stability of the underlying network topology to find any repeating patterns or abnormalities that could lead to route flapping. Address any underlying issues, such as link failures or routing loops, to increase the network's overall resilience.

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