352-001 · Question #532
A BGP route reflector in the network is talking longer than expected to converge during network changes. Troubleshooting has shown that the router cannot handle all the TCP acknowledgements during rou
The correct answer is D. Increase the size of the hold queue. Increasing the hold queue on BGP-facing interfaces gives the router a larger buffer for incoming TCP segments and acknowledgements, preventing drops when the processor is temporarily overloaded during large route updates.
Question
A BGP route reflector in the network is talking longer than expected to converge during network changes. Troubleshooting has shown that the router cannot handle all the TCP acknowledgements during route updates. Which action can be performed to tune device performance?
Options
- AIncrease the size of the large buffers
- BDecrease the size of the small buffers
- CIncrease the keepalive timers for each BGP neighbor
- DIncrease the size of the hold queue
How the community answered
(45 responses)- A4% (2)
- B13% (6)
- C11% (5)
- D71% (32)
Why each option
Increasing the hold queue on BGP-facing interfaces gives the router a larger buffer for incoming TCP segments and acknowledgements, preventing drops when the processor is temporarily overloaded during large route updates.
Large buffers in IOS buffer pools are allocated for large-payload packets and do not directly address the interface-level queuing problem caused by high TCP ACK rates during BGP updates.
Decreasing small buffers would reduce the number of available buffers for small packets such as TCP ACKs, making the TCP acknowledgement processing problem worse rather than better.
Increasing BGP keepalive timers only extends the interval between keepalive messages; it does not address the inability to process TCP acknowledgements arriving during active route update sessions.
The hold queue is the interface-level input or output queue that buffers packets while the router CPU is busy processing. When a BGP route reflector receives a flood of TCP ACKs during a large update, a small hold queue causes drops, forcing TCP retransmits and slowing convergence. Increasing the hold queue depth allows those ACKs to be buffered rather than dropped, giving the CPU time to process them and improving BGP update throughput.
Concept tested: BGP convergence tuning via interface hold queue
Source: https://www.cisco.com/c/en/us/support/docs/routers/7500-series-routers/23001-hold-queue.html
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