VSS traffic forwarding

[Vignesh M]

Scenario:

I have configured VSS in distribution Layer(Sw1 & Sw2). There is 1 L3 port configured for traffic forwarding from each VSS member Sw1 & Sw2 to core switch. VSS Dist sw1 have the active control plane and data planes for are active for both VSS Sw1&Sw2.

Now I need to access a Loopback interface in core switch from the access switch.

Traffic from access sw will reach vss dist via port channel. Now how will te traffic reaches to core sw?

Queries:

1.Will the traffic be load balanced across VSS Sw1&Sw2 via the uplink connected to core?

2.Will the traffic from VSS to core forwarded only via VSS Sw1 as the active control plane of the VSS is Sw1?

Please clarify.

[Jon Marshall]

edited

[Joseph W. Doherty]

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Posting

Logically, VSS functions as one device, however, when load balancing outbound traffic, whether Etherchannel or L3, VSS doesn't load balance egress the same as a single chassis device would.  VSS avoids using the VSL path by using ingress and egress on the same VSS member.

Your actual egress load balancing will depend on how traffic is sent into the VSS pair.

[Jon Marshall]

Joe

I just remove my post because I wasn't sure about something.

Notice that the links to the core are not an etherchannel but separate L3 links.

As the control plane is responsible for L3 peerings and only the active switch is responsible for the control plane how does the core switch see the VSS system ie. does it see two peerings or only one ?

If it was a L3 etherchannel then yes it would load balance but it isn't.

Any ideas ?

Jon

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

It will "see" non-Etherchannel L3 links as two links to the same logical device.  From the non-VSS device to the VSS device, many routing protocols will do ECMP load-balancing, by default.  From the VSS-device to the non-VSS device, logically the routing table will also show ECMP, but as I described above, VSS won't send flows across the VSL unless it must.

[Jon Marshall]

Joe

Thanks for clearing that up.

Jon

[Vignesh M]

Hi Joe,

Please clarify the below points.

VSS won't send flows across the VSL unless it must.

Vignesh 

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

On a single chassis, or something like a 3750 stack, Etherchannel or L3 links will be load balanced following their "rules".

On a VSS pair, VSS will only use the VSL if there's no direct egress path on the current VSS member.  I.e. "normal" link load balancing is not used.

I suspect this is to avoid saturation of the VSL, as its bandwidth is often a fraction of what a VSS member hosts.

Also, if you have everything dual connected, to the VSS pair, your load balancing should be accomplished on ingress, so egress load balancing shouldn't be needed.