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vss failover

Hi, we have 2 no of 4500x as lan gateway switches, 2- 10g bundled links are connected between them as VSL link.

n all access layer 2960 switches are connected as MEC (Multi chassi ethechannel) to 4500 by 2 cables.


1)when vsl link is down, both 4500 switches becomes active and that affects access layer switches are confused bcz they found 2 active switches with same unreachable..

2) when vsl link gets restored, standby again didnt get active...n it automatically shutdown the portchannels of access layer switches...

vsl link doesnt support pagp n lacp, only support on protocol...

Is there any solution, does access layer switch get to know that vsl link betn 2 core is down n so both core made active n will do some action to avoid it..


Re: vss failover


See this:

Dual-Active Detection Using Enhanced PAgP

Port aggregation protocol (PAgP) is a Cisco-proprietary protocol for managing EtherChannels. If a VSS MEC terminates to a Cisco switch, you can run PAgP protocol on the MEC. If PAgP is running on the MECs between the VSS and an upstream or downstream switch, the VSS can use PAgP to detect a dual-active scenario. The MEC must have at least one port on each switch of the VSS.

In virtual switch mode, PAgP messages include a new type length value (TLV) which contains the ID of the VSS active switch. Only switches in virtual switch mode send the new TLV.

For dual-active detection to operate successfully, one or more of the connected switches must be able to process the new TLV. Catalyst 4500, Catalyst 4500-X, and Catalyst 49xx series switches have this capability. For a list of other Cisco products that support enhanced PAgP, refer to Release Notes for Cisco IOS Release at this URL:

When the VSS standby switch detects VSL failure, it initiates SSO and becomes VSS active. Subsequent PAgP messages to the connected switch from the newly VSS active switch contain the new VSS active ID. The connected switch sends PAgP messages with the new VSS active ID to both VSS switches.

If the formerly VSS active switch is still operational, it detects the dual-active scenario because the VSS active ID in the PAgP messages changes. This switch initiates recovery actions as described in the "Recovery Actions" section.

Recovery Actions

An VSS active switch that detects a dual-active condition shuts down (by err-disabling) all of its non-VSL interfaces to remove itself from the network, and waits in recovery mode until the VSL links have recovered. You might need to intervene directly to fix the VSL failure. When the shut down switch detects that VSL is operational again, the switch reloads and returns to service as the VSS standby switch.

Loopback interfaces are also shut down in recovery mode. The loopback interfaces are operationally down and not err-disabled.

Community Member

vss failover

Hi Leo

thanx...very much..

after going through above mentioned link, found:

You must configure trust mode on the port channel that will detect PAgp dual-active detection.

i am confused,  need to create turst mode, exact for which etherchannel group (vsl link one or MEC link one or need to crate another different etherchannel between 2 vss switches)..

n after that how down stream access switch will communicate in dual - active mode...

n after getting up the vsl link, how every thing will become normal..

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