400-007 Question #86: Real Exam Question with Answer & Explanation

The correct answer is A: Increase the size of the hold queue.. When a BGP router is overwhelmed by TCP acknowledgements during large route updates, increasing the input hold queue size allows more packets to be buffered before being dropped, giving the CPU time to process the backlog.

Question

A BGP route reflector in the network is taking longer than expected to coverage during large network changes. Troubleshooting shows that the router cannot handle all the TCP acknowledgements during route updates. Which action can be performed to tune the device performance?

Options

AIncrease the size of the hold queue.
BIncrease the size of the large buffers.
CDecrease the size of the small buffers.
DIncrease the keepalive timers for each BGP neighbor.

Explanation

When a BGP router is overwhelmed by TCP acknowledgements during large route updates, increasing the input hold queue size allows more packets to be buffered before being dropped, giving the CPU time to process the backlog.

Common mistakes.

B. Large buffers are used to accommodate larger-sized packets in memory, but the symptom is insufficient queuing capacity for incoming TCP ACK packets, not a buffer size issue for oversized frames.
C. Decreasing small buffer size would reduce the router's capacity to hold small packets such as TCP ACKs, which directly worsens the problem described.
D. BGP keepalive timers control session liveness detection intervals and have no effect on the router's ability to process incoming TCP acknowledgements during route updates.

Concept tested. IOS hold queue tuning for BGP TCP performance

Reference. https://www.cisco.com/c/en/us/support/docs/routers/7500-series-routers/28219-hold-queue.html

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