Cisco 9400 Stackwise Virtual

[Ex-Engineer1968]

In a Cisco 9400 Stackwise Virtual configuration, I understand that one switch is active and the other is in standby mode. Can someone break that down a bit more...

Is one switch active and the other in standby only in terms of control plane processing - managing RIBs, FIBs, etc, but BOTH switches actually forward traffic (data plane)? 

I think this statement is true and that BOTH switches do in fact forward traffic. Otherwise, the crosslink between the two switches would be ridiculously overuitilized and traffic flows would be inefficient.

[MHM Cisco World · Accepted solution]

Is one switch active and the other in standby only in terms of control plane processing - managing RIBs, FIBs, etc, but BOTH switches actually forward traffic (data plane)?  Totally Correct

the Active have one active control plane and one active data plane 
the standby have standby control plane and one active data plane 

MHM

[Joseph W. Doherty · Accepted solution]

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@Ex-Engineer1968 wrote:

In a Cisco 9400 Stackwise Virtual configuration, I understand that one switch is active and the other is in standby mode. Can someone break that down a bit more...

Is one switch active and the other in standby only in terms of control plane processing - managing RIBs, FIBs, etc, but BOTH switches actually forward traffic (data plane)? 

I think this statement is true and that BOTH switches do in fact forward traffic.

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Correct, as also confirmed by @MHM Cisco World 

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@Ex-Engineer1968 wrote:

Otherwise, the crosslink between the two switches would be ridiculously overuitilized and traffic flows would be inefficient.

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Also correct, which is why I believe current design guides, still emphasis everything should be dual connected (connection to each switch) to avoid using the cross link except for handling traffic due to a path failure on either one of the pair devices.  (In fact, I believe certain rules, like flow load balancing across an Etherchannel, hosted across both devices are also ignored.  I.e. the cross path is only used when it's the only possible path.)

Unfortunately, I believe renaming VSS technology "Stackwise Virtual" makes it easier to confuse this technology's pros and cons versus Stackwise (physical), where bandwidth between physical switch devices generally is of much more capacity.

[Ex-Engineer1968]

Joe, I used to be on this board everyday answering questions with you and Edwin Ortiz, Jon Marshall, and an Italian guy... Giuseppe? MANY years ago lol

[Leo Laohoo]

LOL

Small world.

[Joseph W. Doherty]

Yep, recall all 3.  I think (?) Jon still occasionally posts.  Another name going back to the beginning of these forums is @Rick Burts, who still occasionally posts too.

@Leo Laohoo, who I see just added a reply, goes way back too, although early on, his posts were often questions more so than answers.  Probably when all networking was still new and shiny before he became a jaded expert.  ; )

[MHM Cisco World · Accepted solution]

Correct, if we have server and client and both single leg connect to one SW 
then the traffic will pass over SVL, we dont want that
so we config PO connect Server and client to both SW 
and make sure the etherchannel hash is same, i.e. reduce the chance that the traffic pass over SVL. 
for the MAC for data plane and ARP it Sync so both SW have ideentical MAC and ARP table 

[Joseph W. Doherty]

Just to add, it's not just edge hosts, ideally any device, including other switches and routers, should be connected to the each member switch.

Again, my understanding, since VSS, LB on Etherchannel does not apply across the "stack" pair, but does apply to links on same switch.  For example, if you had a 4 link Etherchannel, 2 links on each stack member, LB would be used for traffic on the 2 links on the same switch.  Again, SVL is only used when only possible egress.  Also, as the usual case, ingress LB depends on other side device.

BTW, although I believe VSS and this Stackwise does not LB across the pair (Etherchannel or routing), if everything is connected to each member, LB across the pair is achieved by LB to the pair.

[Ex-Engineer1968]

 gents. I agree. IHAC who swears up and down that when he asked Cisco this question last year, he was told that one switch is active and forwarding traffic but that the second switch is in complete standby mode - no forwarding. That just didn't seem right to me at all. Even in regular stackwise on 1U boxes, there is 1 switch that is the master in terms of the control plane, BUT all switches in the stack are forwarding. 

[Joseph W. Doherty]

Well, is one unit was just standby, it would be a rather expensive standby, especially for the larger chassis deployments.