VSS question

[Andrew White]

Hello,

I'm just reading up about VSS.  I see 2 switches like the 4500/6500 can be connected to each other via a 10G VSS connection so the switches are seen as one and can help with spanning tree loop issue and allow for the creation of MEC's.  I've read that 1 switch is the active and the other is standby, which seems like a waste.  

So does 1 switch remain active only and the other is waiting for the active to fail?

Thanks

[Seb Rupik]

Hi there,

VSS provides a single logical unit with a control plane in Active/Standby between chassis, but crucially an Active/ Active dataplane on both chassis.

In comparison vPC as implemented on the Nexus platform is Active/ Active on both the control and data planes, so could be viewed as the least wasteful! ;)

cheers,

Seb.

[Andrew White]

Thanks, I use a pair of Nexus switches connected via a vPC.

I'm in my mind trying to technically understand the control plane and data plane.

So the VSS has a single control plane to learn paths and routes between the 2 logical switches so one is the master?  But the data place is active across both so forwarding of traffic is enable on both switches?

On a Nexus the control plane is active on both learning paths?

Sorry if that was a bad explanation :P

[Seb Rupik]

Both your summaries are correct.

It is worth pointing out that with VPC because the control plane is active/ active there is a requirement to duplicate protocol process configuration, eg for routing and first hop redundancy.

[Reza Sharifi]

In addition to explanation provided by Sep, here is a link on comparing VSS and vPC.

http://mycciedatacenter.blogspot.com/2013/06/vss-vs-vpc-difference-between-vss-and.html

HTH

[Joseph W. Doherty]

"So the VSS has a single control plane to learn paths and routes between the 2 logical switches so one is the master? But the data place is active across both so forwarding of traffic is enable on both switches?"

In might help to think of VSS like two sups in one 6500 chassis. Normally only one is active, managing the chassis, but data is forwarded on all line cards.

Likewise, with VSS, both chassis forward traffic, in fact, once traffic reaches one, it will prefer using an egress port on its chassis even though logically it may have an MEC or L3 ECMP across the two chassis. I.e. it will not load balance egress from two chassis. For that, you need to load balance traffic to the two chassis.

[balaji.bandi]

You have read main point not detailed here i guess.

VSS : on high level.

2 Physical Switches become 1 Logical Switch,

That meas 1 Control plan and both switch uses Dataplane for high availability.

if primary fails second will take over.