Is it really possible that Interface utilization could cause issue on Cisco gear? Yes!!
•If there are a lot of collision OR late collision in “show interface” command, there might be a duplex mismatch. Especially when you connect a router external interface to a half-duplex modem…
•If there are High CPU utilization, packet drop, unreachable destinations, there could be something loaded on interface.
•Check the switching mode on device:
◦Process Switch: Check header of every single packets received for requesting next hop ARP and routing.
◦Fast Switch: Check header for very first packet and cache the record for rest of the packets. There is still potential issue with handling the first packet.
◦CEF(Cisco Express Forwarding): When router boots up, it pre-caches all the ARP results according to the routing table. So when packets arrive, it just forward according to caches.
•Check the routing table for the routes
•Check CEF / ARP Cache
If you find the above looks familiar, study the CCNP yourself, you might write some fancier ones 🙂