hardMultiple Choice
300-410 Practice Question: A router has CoPP configured with a class-map…
A router has CoPP configured with a class-map that matches BGP traffic (TCP port 179) and polices it to 500 pps. The router has multiple iBGP peers. After applying the policy, some BGP sessions flap, but others remain stable. The flapping peers are those with higher latency. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that CoPP drops packets based on source IP or that BGP uses UDP for keepalives, leading candidates to overlook the impact of TCP retransmissions from high-latency peers on policer thresholds.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
High-latency peers generate more TCP retransmissions, which are more likely to be dropped by the police rate, causing session flaps.
B is correct because high-latency BGP peers experience more TCP retransmissions due to delayed acknowledgments. The CoPP policer drops packets exceeding 500 pps, and these retransmissions increase the packet rate for those sessions, making them more likely to exceed the policer and be dropped. Dropping BGP TCP segments (including keepalives) causes the BGP hold timer to expire, leading to session flaps.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
CoPP drops BGP packets based on source IP, and high-latency peers have different source IPs.
Why it's wrong here
The class-map matches TCP port 179 only, with no source-IP match, so peer addresses are irrelevant to which sessions drop. Source-based classification would be the right approach when selectively protecting specific management hosts, but nothing in this policy distinguishes peers by address.
- ✓
High-latency peers generate more TCP retransmissions, which are more likely to be dropped by the police rate, causing session flaps.
Why this is correct
CoPP polices matched BGP traffic to 500 pps. Higher-latency peers retransmit TCP segments more often, so their packet rate exceeds the policed threshold and drops mount, causing keepalive or update loss and session flaps, while low-latency peers stay under the limit.
- ✗
BGP uses UDP for keepalives, and CoPP only polices TCP.
Why it's wrong here
BGP runs entirely over TCP port 179, including keepalives, so UDP plays no part and this premise is false. It tempts because CoPP class-maps do match transport protocols, and UDP-based services such as SNMP or syslog are policed separately, but BGP is never among them.
- ✗
The CoPP policy is applied to the wrong control plane; it should be applied to the forwarding plane.
Why it's wrong here
CoPP is a control-plane policy; applying it to the forwarding plane would police transit data traffic, not BGP, which terminates on the control plane. Forwarding-plane policing suits interface-level traffic shaping, so this misdirects the fix rather than addressing the 500 pps limit.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.