hardMultiple ChoiceObjective-mapped
CoPP BGP Police Rate Too Low
A large enterprise network is experiencing intermittent BGP session resets between R1 and R2. R1 has the following relevant configuration: ! R1 control-plane service-policy input CoPP !
access-list 100 permit tcp any any eq bgp
class-map match-all BGP-CLASS match access-group 100 ! policy-map CoPP
class BGP-CLASS
police 8000 conform-action transmit exceed-action drop
class class-default
police 1000000 conform-action transmit exceed-action drop ! R2 shows:
R2#show ip bgp summary
BGP router identifier 2.2.2.2, local AS number 65002 BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 1.1.1.1 4 65001 12345 12345 0 0 0 00:02:34 0
What is the root cause?
Quick Answer
The answer is a CoPP BGP police rate that is too low, causing intermittent BGP session resets. When the Control Plane Policing (CoPP) policy limits BGP traffic to only 8000 bps, as shown in the configuration, the combined rate of BGP updates and keepalive packets can easily exceed this threshold. The router then drops the excess packets—including critical keepalives—triggering the BGP hold timer to expire and forcing the session to reset, which explains the brief uptime of just over two minutes on R2. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how CoPP interacts with routing protocol stability; a common trap is to overlook that policing applies to all matched traffic, not just updates, so keepalives get dropped too. Remember the memory tip: “Police the updates, not the pulse”—always ensure your BGP police rate accommodates both routing table bursts and steady-state keepalives to avoid flapping.
⚠ Common exam trap
Cisco often tests the misconception that CoPP misconfiguration is always due to ACL or direction errors, but here the trap is that a seemingly reasonable police rate (8000 bps) is actually too low for BGP control-plane traffic, causing intermittent drops that mimic a flapping session.
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
✓
The BGP session is flapping because the CoPP policy drops BGP keepalive packets when the police rate is exceeded.
The BGP session resets because the CoPP policy applies a police rate of 8000 bps to BGP packets matched by access-list 100. When BGP keepalive or other packets exceed this rate, they are dropped, causing the hold timer to expire and the session to reset. The short uptime of 2 minutes 34 seconds on R2 confirms the session is flapping due to packet loss.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The BGP session is flapping because the CoPP policy drops BGP keepalive packets when the police rate is exceeded.
Why this is correct
BGP keepalives are small, but if the overall BGP traffic (including updates) exceeds 8000 bps, the policer drops packets, causing keepalive loss and hold timer expiry.
- ✗
The access-list 100 is misconfigured; it should match TCP port 179 specifically.
Why it's wrong here
The ACL matches any TCP traffic, which is too broad, but the issue is primarily rate-limiting, not misclassification.
- ✗
The class-default police rate is too low, causing all traffic to be dropped, including BGP.
Why it's wrong here
The class-default rate is 1 Mbps, which is likely sufficient for other traffic; the problem is specifically the BGP class.
- ✗
The CoPP policy should be applied to the control-plane input direction, but the service-policy is missing the 'control-plane' keyword.
Why it's wrong here
The configuration shows 'control-plane' and 'service-policy input CoPP', which is correct.
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Same concept, more angles
1 more way this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer notices that BGP sessions between two directly connected routers are flapping every few minutes. The routers are running IOS-XE 17.3 and have CoPP enabled. The engineer checks the CoPP policy and sees a class-map matching BGP packets with a police rate of 8000 bps. The BGP session uses MD5 authentication and the routers exchange a full BGP table with 500,000 prefixes. What is the most likely cause of the BGP session flapping?
hard- A.The BGP MD5 authentication is causing excessive CPU utilization, triggering CoPP drops.
- ✓ B.The CoPP police rate of 8000 bps is too low for the BGP keepalive and update traffic, causing packet drops.
- C.The CoPP class-map is not matching BGP packets correctly because it uses a wrong access-list.
- D.The BGP hold timer is set too low, causing the session to reset before CoPP drops are noticed.
Why B: The CoPP police rate of 8000 bps is insufficient for the BGP traffic generated by exchanging a full BGP table of 500,000 prefixes. BGP updates for such a large table can easily exceed 8000 bps, especially when combined with keepalive messages. When the police rate is exceeded, CoPP drops BGP packets, causing the session to flap as the hold timer expires due to missing keepalives or updates.
JA
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.