Troubleshooting Cisco Router / Switch Interface utilization Overload:

Ethernet interface utilization overload affects performance.  Here are some ways to fix port utilization issues.

1. high cpu / memory utilization will affect the interfaces.
2. Packet drops... this can be normal. But high cpu / mem usage will drop more packets.

3. unreachable destinations will cause overload.

Things to check:

1. Verify router / switch switching mode: 

• Process switching:  This is the original.  
• Everything is process switch ..goes to data plane...then control plane..then passed back to the data plane.  This is very CPU intensive.

•  Fast switching:  Came out in late 80s - 90s.  1st packet goes in the switch and caches the packet information.  The next packet just stays at the data plane and just forwards through.  This is less CPU intensive as only the first packet in that flow punts to the CPU.

•  CEF:  This is on by default: router(config)# ip cef :  This eliminates the packet 1 issue.  When the router boots, it simulates as if every route in the ip table was accessed.  So, it pre-caches.  
• It never goes to the control plane only stays in the data plane.  But not compatible with access control or QOS implementation, it may not be effective with other features you use.

    TIPS: enable CEF will most likely fix high CPU overload,  if you have it disabled.
        router(config)# ip cef
        router(config)# int fa0/0
        router(config-if)# ip route-cache or ip route-cache cef (if supported)


2. verify by cef / arp cache. 
    router# SHOW ADJACENCY

3. verify routing table to the destination is correct.