VSL Link

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Ganesh Hariharan Tue, 03/09/2010 - 21:34


1)That's the cable used for VSL in order to  connect 2 core 6509-E?

2)am thinking to connected THE 2 10 gig on the VSS Supervisor and two 10gig links from the line card?

3)is the supervisor VSS-720 comes with two 10 GIG moudule built-in or shoud i add it to the supervisor?

4)what is the best Practice to VSL Connection



The two chassis periodically exchange special Layer 2 dual-active hello messages containing information about the switch state. VSL is used to carry on proprietary encapsulated frames also control plane messages to form a single device, one supervisor the active one controls both chassis with second supervisor acting as hot standby as in a single chassis with two supervisors.

Check out the below link for more info on VSL Links


Hope to Help !!


Giuseppe Larosa Tue, 03/09/2010 - 23:26

Hello Alsayed,

this should be possible configuration guide requires a dedicated direct link but it does not say that it must be a tengiga.

To be noted it would be a waste of bandwidth to simply exchange IP BFD messages over a tengiga (it is alread over a GE to be honest)

And configuration example uses GE ports:

Router (config)# interface gigabitethernet 1/9/48
Router (config-if)# no switchport
Router (config-if)# ip address
Router (config-if)# bfd interval 100 min_rx 100 multiplier 3
Router (config-if)# no shutdown
Router (config-if)# interface gigabitethernet 2/1/48
Router (config-if)# no switchport
Router (config-if)# ip address
Router (config-if)# bfd interval 100 min_rx 100 multiplier 3

Ganesh Hariharan Wed, 03/10/2010 - 07:23


so these ports takes over when the VSL goes down and the VSS uses PFD in order re-form the VSS?is that right?



If for some rare reason all VSL connections are lost between the virtual switch members leaving both the virtual switch members up, the VSS will transition to thedual active recovery mode.

In the dual active recovery mode, all interfaces except the VSL interfaces are in an operationally shut down state in the formerly active virtual switch member. The new active virtual switch continues to forward traffic on all links.

Hope to Help !!


Giuseppe Larosa Wed, 03/10/2010 - 10:44

Hello Alsayed,

actually no:  IP BFD and the other possible methods are used to avoid the worst case that is when both supervisors want to be active because they think the other one is dead.

the result would be two devices pretending to be the same device using the same ip addresses in the same vlans (the SVIs) and creating confusion in all network.

So special care is needed to provide "out of band" heartbeat between the two supervisors and IP BFD, fast hellos and Enhanced PAGP have this objective.

Deployment of more then one method of dual active detection is recommended:

so one GE link can be used for IP BFD

one GE link for fast hellos

Enhanced PAGP is different because the out of band heartbeat channel is not built with a direct connection between the two chassis, but by adding informations to PAGP frames that travel over etherchannel bundles:

an access layer switch can use a bundle of  two links one to chassis 1 and one to chassis 2 of VSS

if the access layer switch supports E PAGP some information flow from chassis1 to chassis 2 and viceversa via this third party device.

so IP BFD and the others cannot be an alternate path for VSL but are safety measures to allow the two supervisors to be aware of each other also by  other means.

Hope to help



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