Port channel & error

Unanswered Question
Mar 22nd, 2009


I have 2 Switches A& B connected to each other over Portchannel interfaces each with 2 physical interface in it.My query is related to physical errors that occur on one of these physical interfaces.

A is connected to B via PoCh with interfaces gig 2/3 & 2/4.B is similarly connected to A via gig 2/3 & 2/4.

1)Now it appears gig 2/4 on both A & B are bouncing for couple of days.Also i can see large amount of runts,giants,frames,input errors & CRC on these two interfaces & correspondingly the PoCh shows CRC as well, which should be the accumulated aggregrate of individual ports.

There are some performance issues on this link.However when extended ping is used to test the connectivity from A thru B to reach a certain destination which lies beyond B,the response is gud with no drops.

I was wondering..wont the icmp response indicate anything problematic if the errors are there on the interfaces? Or is that this would only come into picture when there is huge utilisation on the link.When checked the usage was around 10%.

2.Also, any idea what does lost carrier on an interface indicate..although this is not related to this specific case!

Any comments/suggestion on this would be highly helpful.

Thanks in advance!!

I have this problem too.
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vaughancahill Sun, 03/22/2009 - 03:12

Typically runts/errors indicate a duplex mismatch on the link. This is the first thing I would check and also test physical cable.

In regards to your ICMP testing remember that data placed across the port channel is load-balanced using a hashing algorithm, matching the source and destination mac address by default, so it is likely that your data is being placed over the non affected link.

As you have assumed errors on the affected link will be amplified with increased traffic as some of the frames that are currently being retransmitted may be successful, but other frames may fail, obviously as traffic goes up the likelihood of frames having errors increases.

Lost carrier is when there has been no carrier signal, this could be a sign of a faulty interface, or cabling.

suthomas1 Sun, 03/22/2009 - 04:11

Duplex has been verified to be correct on either of the interfaces.i will check to see if i get an idea on how the data is being placed over the link by tracing the mac.Would increased errors mean the interface will flap due to high traffic supplemented by errors.


vaughancahill Sun, 03/22/2009 - 04:43

You can work out manually which link the traffic will be sent across using an XOR calculation for example you have two links (1 bit), so the XOR is performed on the last bit in each address for example:

MAC Address one:



5 = 101

6 = 110

XOR = 1

Note: If it was 1 - 1 the XOR would be 0.

So traffic between these two addresses would be carried on the link with index 1.

The lowest port interface configured in the channel would be 0, and the next increasing port would be 1, and so on…

The interface flap and errors are a symptom of a physical or duplex mismatch error, the flaps will increase because the error condition is increased with the increased traffic. This does not mean the errors cause the link to flap rather they are related to the cause.


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