speed / duplex settings - strange behaviour

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Jul 27th, 2011
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Hi all,

I know this topic has been discussed many times with supported documents like




But I am sitting with an issue where I need to try and understand why this is happening.

I have two sites Active / standby

At each site one of the switches connecting 4 routers each per site. The configuration between the switches / routers are all forced to 100/full

Now with this setup I can ping between the routers 100% without packet loss.

However between 2 routers let’s call them router A and router B when they ping one another I we see packet loss. Just when these 2 are communicating with one another we see packet loss

Any other device can ping both of them 100%. The cables have been replaced both ends.

Finally we got a way of letting them talk fine, having the router A forced 100 / full and the switch at auto.

Now having the setup duplicated, I find it strange that the same kit / make / model / IOS works with 100 full and on the other side we had to put on the switch auto.

Is there any explanation, or something else we can look at to try and explain this one?

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paulstone80 Wed, 07/27/2011 - 04:00
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Hi Martin,

Just to clarify, you have 4 routers at each site (R1,R2,R3,and R4), and all the routers are connected to one switch (SW1). The issue ocurrs locally between 2 routers that are both attached to SW1?

Do you experience the same issue when all interfaces (switch/router) are set to Auto/Auto?


Martin Bosch Wed, 07/27/2011 - 04:05
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Hi Paul,

Its (R1,R2,R3,and R4) on Switch A fiber link to Switch B then again (R1,R2,R3,and R4)

And this only happens between R1 on Switch A and R1 on Switch B.

The issue has only been picked up on R1 on Switch A.

paulstone80 Wed, 07/27/2011 - 04:21
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Do you see any errors on the interfaces connecting R1 to Switch A, on either the switch or router?

Is the link between SwitchA and SwitchB a Layer 2 link or is it routed? Are all the devices attached to SwitchA and SwitchB on the same subnets?

I would also check the ARP cache on R1 SwitchB when pinging R1 SwitchA to see whether the MAC address is changing. It's possible R1 SwitchB is getting a spurious ARP entry from somewhere, (this wouldn't explain why it works with different duplex settings though!).  

Martin Bosch Wed, 07/27/2011 - 04:26
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No errors on the interface - rememebr packet loss is only seen between Router A on switch A going to Router A on switch B. Router A can ping anything else with out any issues. Same goes for Router A on switch B.

It's a layer 2 link. Same subnet yes. No ARP anomalies have been picked up.

paulstone80 Wed, 07/27/2011 - 04:34
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Do you have HSRP or VRRP runing between any of the routers?

How are routes learned on the network?

Martin Bosch Wed, 07/27/2011 - 04:38
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iBGP is running between router A on switch A and router A on switch B - Where the issue is.

Have a similar setup between router B on switch A and router B on switch B - Works fine.

I thought it might have been something in the IOS of the router, but the IOS is working fine on the other router with the same setup.

paulstone80 Wed, 07/27/2011 - 06:15
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It doesn't sound like you have a Layer1 issue between the router and switch because you don't have connectivity issues to any other devices. That said, the problem appears to go away when the Speed and Duplex settings are changed, which is very strange indeed! I'm not sure if this is a coincidence or a result of the change.

I think the next logical step would be to sniff the network using Wireshark (or similar) and see if you can see any anomilies in the Ethernet Frames and IP packets. I would pay particular attention to the source and destination MAC addresses in the frames/packets and ensure they are addressed correctly. Hopefully with a bit more information you can begin to pin point exactly where the issue lies.


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