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input overrun errors on Ethernet but not Serial WAN connection

        We recently ran into a situation where we upgraded a WAN connection from a T1 circuit to a 5mb Ethernet circuit at one of our sites.  As soon as we started using the new circuit, we started seeing input overrun errors on the LAN connection to the router from the switch.  The uplink was 1 Gb/full duplex.  The overrun errors seemed to be caused by too much traffic coming from the LAN side for the router to process out the 5 Mb WAN interface.  We set the uplink speed to 100 mb/full duplex and it solved the issue.  Is this normal for this type of a WAN connection?  How come we didn't have the same issue when using the T1 circuit which is 1.5 Mb?   Is there some difference between the way a serial interface and Ethernet interface handle traffic???  The router model is a 2901.  Any input would be appreciated.

1 Reply 1

Vasilii Mikhailovskii
Rising star
Rising star

Hello, Arnold.

The overruns you saw is because interface packet input buffer was overflowed. It could be caused by a short traffic burst (for any reason).

After you downgraded to 100M, you made your switch to queue packets instead of router, so either switch is dropping packets, or just queuing them much better, than your router did.

Per my understanding it has nothing common with your WAN speed, as whenever your WAN link is saturated, you observe packet drops on the WAN interface (not LAN).

I guess you WAN link upgrade could make WAN traffic more burstful, then it was before.

PS: if you WAN bandwidth is no equal to LAN bandwidth then you need some sort of QoS, especially when you have sub-rate limitation implied by you ISP.

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