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The explanation is here: http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_44_se/configuration/guide/swethchl.html#wp1195432 “You should exercise care when using the on mode. This is a manual configuration, and ports on both ends of the EtherChannel must have a similar configuration. If the group is misconfigured, packet loss or spanning-tree loops can occur.”
Regards,
Antonio Soares, CCIE #18473 (R&S) amsoares@netcabo.pt

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From: Kim teu [mailto:kim.teu@gmail.com] Sent: quinta-feira, 20 de Novembro de 2008 22:31 To: Antonio Soares Cc: Cisco certification Subject: Re: Layer 2 loop breaks OSPF adjacency over Layer 3 links
THANKS. You are right. The loop brought down the entire data center because of the loss of OSPF adjacency. I had to go there personally to break the loop…
I did found the problem. I removed the “channel-group 12 mode on” command on one switch, but forgot to remove it from the partner switch…
Regards, Kim

On Thu, Nov 20, 2008 at 3:42 PM, Antonio Soares wrote:
A STP Loop can stop everything in your network. I saw an entire data center going down in less than 1 second! When this is happening, the best to do is to break every existent L2 redundant link quickly. In some situations, if you don’t have physical access to the equipments, this is just not possible!
Then when the storm is over, you can try to trace the source of the problem. But i can tell you it’s very difficult or even not possible to find out what failed.
Cisco has a good document about this:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080136673.shtml

Regards,
Antonio Soares, CCIE #18473 (R&S) amsoares@netcabo.pt
—–Original Message—– From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Kim teu Sent: quinta-feira, 20 de Novembro de 2008 14:55 To: Cisco certification Subject: Layer 2 loop breaks OSPF adjacency over Layer 3 links
Hello all,
*Has anyone experienced a layer 2 loop event that caused OSPF adjacency to be dropped for layer 3 links? Any idea why?*
**
*What’s the best way to diagnose the cause of the layer 2 after the outage? The following is what I see in the log.** VLAN 56 is looping…*
*The switch is seeing packet sourced from itself::*
>>>472825: .Nov 19 15:38:56 CST: %STANDBY-3-DUPADDR: Duplicate address 192.168.97.1 on Vlan56, sourced by 0017.df1c.6800

*The standby state change to active because the switch lost it connection to the active switch because of SPT loop prevention::*
>>>472826: .Nov 19 15:39:00 CST: %STANDBY-6-STATECHANGE: Vlan72 Group 72 state Standby -> Active
>>>472827: .Nov 19 15:39:03 CST: %STANDBY-6-STATECHANGE: Vlan116 Group >>>116 state Standby -> Active
>>>472828: .Nov 19 15:39:26 CST: %STANDBY-3-DUPADDR: Duplicate address 192.168.97.1 on Vlan56, sourced by 0017.df1c.6800
>>>472829: .Nov 19 15:39:38 CST: %STANDBY-6-STATECHANGE: Vlan152 Group >>>152 state Active -> Speak
>>>472830: .Nov 19 15:39:47 CST: %OSPF-5-ADJCHG: Process 10, Nbr 192.168.200.5 on Vlan704 from EXSTART to DOWN, Neighbor Down: Dead timer expired
>>>472831: .Nov 19 15:39:54 CST: %STANDBY-6-STATECHANGE: Vlan134 Group >>>134 state Active -> Speak
>>>472832: .Nov 19 15:39:56 CST: %STANDBY-3-DUPADDR: Duplicate address 192.168.97.1 on Vlan56, sourced by 0017.df1c.6800
>>>472833: .Nov 19 15:39:59 CST: %STANDBY-6-STATECHANGE: Vlan152 Group >>>152 state Standby -> Active >>>472834: .Nov 19 15:40:14 CST: %STANDBY-6-STATECHANGE: Vlan134 Group >>>134 state Standby -> Active
Kim

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