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BGP Route Flapping on PE Router – Lagos PoP

Intermittent BGP session resets on PE-LAG-01 every ~4 minutes. Hold-timer expire events in MRT logs.

EO

Emeka Okafor

CISCO CCIE #62341· MTN Nigeria· Jun 5, 2026

Problem

Intermittent BGP session resets on PE-LAG-01 every ~4 minutes. Hold-timer expire events in MRT logs.

Handling Procedure

Captured BGP debug during reset. Verified interface counters. Ran MTU path discovery with DF-bit set.

Solution

Set MTU to 9000 bytes on transit link. Enabled IP MTU 1490 on BGP TCP session. Cleared session.

Root Cause

Provider enabled 802.1q on their PE adding 4 bytes overhead, causing BGP TCP fragmentation.