Question

After applying a new CoPP policy on a core router, users report that large file transfers through the router intermittently stall, while small pings and normal browsing work fine. Investigation shows the router must generate ICMP 'fragmentation-needed' (Type 3, Code 4) messages for a path with a smaller MTU, but Path MTU Discovery is failing end-to-end. The CoPP policy includes a class matching ICMP that is police-limited to a very low rate with 'exceed-action drop'. Which action best resolves the issue?

Accepted Answer

Create a dedicated CoPP class that matches ICMP unreachable/fragmentation-needed traffic and allow it at an adequate rate rather than lumping it into a tightly policed generic ICMP class

Answer

Remove the ICMP class entirely so all ICMP is placed in class-default and forwarded without policing

Answer

Increase the interface MTU on the transit link so fragmentation is never required

Answer

Change the exceed-action from drop to transmit on the existing generic ICMP class only during peak hours