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New Member

Link flapping on gigabit port

All,

Network topology of our network is as follows:-

.28 (WS-C2950G-24-EI) .6 (WS-C3524-XL-EN ).3 (WS-C3508G-XL-EN)

We have Fiber MMF between .6 and .3 connected using GBICs 1000BASESX on both ends. Always we are loosing connection to .6 switch because link is flapping in every two hours, and every time shutdown and un-shutdown port on .3 bringing up the link?

We tried changing duplex setting on .3 to full & half, but no go.

When I type show log on .3 switch I am seeing link flapping in log where as on .6 switch I do not see anything?it means something is wrong on .3 switch?

The following is the interfaces configuration from switches?

.6 WS-C3524-XL-EN

interface GigabitEthernet0/1

description **** 8102 ****

.3 WS-C3508G-XL-EN all IDFs are connected to this switch aggregator swtich also root bridge for all switches

interface GigabitEthernet0/3

duplex half

.28 WS-C2950G-24-EI

interface GigabitEthernet0/2

description **** SW24-26-uplink ****

Note:- .3 is our main aggregator switch in which all IDFs room switches are connected and there is no other port free on it?.

Please need your valuable suggestions.

Regards,

Khan

15 REPLIES
New Member

Re: Link flapping on gigabit port

Again it went down and we did the followed the same steps to bring up that port...

Your assistance will be highly appreciated.

Purple

Re: Link flapping on gigabit port

sounds like it is going err-disabled . Usually it will give a reason why it err-disabled the port . When it goes down do a show interface status" and see if it says it is err-disabled for the port . For gig port between switches we never hardcode anything just leave it as auto on "both" ends , have never had a problem .

New Member

Re: Link flapping on gigabit port

It was auto, when we faced this problem we changed it to full...but again it was repeating then we changed it to half....but no go...

It is not going to err-disabled....jst link is going down when we shutdown and no shut its coming up...

Regards,

Khan

Purple

Re: Link flapping on gigabit port

Don't know these specific switches but if they are using gbics you might want to change those out .

New Member

Re: Link flapping on gigabit port

How would Identify whether GBIC is faulty or not...because i have to make sure before changing anything on this switch...Please advice

New Member

Re: Link flapping on gigabit port

Is there any way we can isolate this problem..If i say to management just replace the GBIC and even after replace not solve this problem.then its probably not good for me...So, you help on the same really appreciated...

New Member

Re: Link flapping on gigabit port

please anybody assits me on this issue..

Re: Link flapping on gigabit port

This problem could be due to bad cable, bad GBIC on either end, faulty switchport at either end, speed/duplex mismatch.

you need to isolate this issue by taking care of all these things one by one.

New Member

Re: Link flapping on gigabit port

Hi,

This is sometimes possible that the other end will not record the flapping which happens in .3 switch. I am suspecting that it either has a GBIC problem or the other pair of the Fiber upink has some problem.

To trace that its a link issue, configure and enable UDLD on both the erring uplinks gigabit interfaces. Their respective interfaces will be in error-disable state once it verifies a link unidirectional link.

Normally, the gigabit port will sense that the connection is up if its receives a traffic from the other end, which means the link of its received is good. But I also suspect that its transmit is having issues, because only the other end who received it encounters the flapping which means connection from transmit of .6 might has some link problem to the received of .3. That activity possibly caused some flapping only in .3.

With the help of UDLD you will therefore identify that if there is a link failure the port in .3 will be in error disable state and .6 will sense it as well and will notify that the link is down.

I hope this information can help. Thanks.

New Member

Re: Link flapping on gigabit port

Thank you very much for great explanation on the same...It would be really nice, if you could more detail information about the commands which needs to implement on both swtiches...

New Member

Re: Link flapping on gigabit port

Before you do any config changes you need to rule out the fact of faulty gbics or bad fiber it isn't rare to see fiber or gbics go bad. Swap the gbics out with a known good one as well as the cables.

This needs to be ruled out before you make any config changes cause this really does sound like a faulty hardware problem. Hope this helps tuff guy. Have a great day!!!

Purple

Re: Link flapping on gigabit port

Some times it does come down to part swapping when everything else is ruled out . :-)

New Member

Re: Link flapping on gigabit port

Hi Azmath,

For the UDLD to work, you have to configure on both peer interface. Below is the configuration's command line you may issue in both interfaces of .6 and .3:

configure terminal

interface gigabitethernet0/0

udld port aggressive

Please check this link for the use of UDLD for more information:

http://www.cisco.com/en/US/products/hw/switches/ps5528/products_configuration_guide_chapter09186a00801e85e7.html#99485

Do a "show udld gig0/0" to verify if udld is already up. The UDLD status of the interface should be in "Biderectional", Otherwise UDLD is not yet working.

The behavior of UDLD, when it detects a failure in uplink connection in either (or detects a unidirectional traffic); the port will be in err-disabled state. If this occurred in your network, you have to verify your fiberlink and/or your GBIC module on both end.

Thanks,

Denwardson Estrada

New Member

Re: Link flapping on gigabit port

Thank you very much Denwardson Estrada!!! I'm configuring the same on both switches, incase if i need your help, I'll let you know...

New Member

Re: Link flapping on gigabit port

Hi,

I think you can chek if the fibre is not too long. 1000SX on MMF does not support more than 500metres.

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