4500 VSS throughput confusion

[Jonn cos]

Hi all,

I have confusion regarding total backplane capacity in a VSS environment. Its written in the datasheet that after configuring VSS we get a total of 1.6 Tbps (using SuP 7-E). I have following confusions

1) VSL etherchannel can accomodate a max of 8 ports. If we take 8 x 10G ports that totals 80 Gbps. If we consider it full duplex then its 160 Gbps, how come its calculated to 1.6 tbps ??

2) I was told in an interview that its cisco marketing gimmick. Please tell me is it so ??

[Jonn cos]

someone pls ?

[Reza Sharifi]

1) VSL etherchannel can accomodate a max of 8 ports. If we take 8 x 10G ports that totals 80 Gbps. If we consider it full duplex then its 160 Gbps, how come its calculated to 1.6 tbps ??

The VSL link has nothing to do with backplane of the switch.  The VSL is mostly used for minimum traffic like heartbeat between the 2 chassis, some IGMP traffic and in case the other traffic need to traverse it.  If your VSS is setup correctly, you rarely use the VSL link. The 1.6 number come from the fact that sup 7-E has the backplane capacity of 800 Gbps and when you combine the 2 chassis together you approximately get 1.6.

2) I was told in an interview that its cisco marketing gimmick. Please tell me is it so ??

It not so much of a gimmick.  Every vendor uses this type of numbers to promote their products.  Now, can 1.6 Gbps be reached in case you need that much backplane speed, hard to tell and unless you have device like IXIA to test it, you can never know.

HTH

[Jonn cos]

Sir thats basically my confusion. How 2 chassis are combined to give 1.6 GBps ? i mean they are physically 2 seperate switches so how there backplane is combined ? i thought it was via VSL link but like you said, if VSL has nothing to do with it then how are backplane combined ?

[Reza Sharifi]

When you have VSS, you combine the backplane together and that gives you the forwarding plane of 1.6 Gbps.  VSL is used for communication between primary and stand-by chassis.  For example. if you have an access switch connected to both chasses (VSS) via a Portchannel and if there are 2 hosts connected to the access switche, when one host sends a packet the packet traverse through the primary switch and when the other host sends a packet, it will traverse the stand-by switch.  So, both switches are forwarding, but you only have one control plane, meaning you configure both chassis from the primary only.

It that more clear?

HTH

[Jonn cos]

Sir in interview when i told him that backplane of 2 switches are combined so the interviewer immediately asked me how they were combined ? i told him via VSL, he didnt contradict that but i was all lost down the road ( as i mentioned in my original post). So sir my basic confusion is, how the backplane of 2 switches are combined ?

Sir i would really appreciate if you can help me understand this

[Reza Sharifi]

Try these documents:

http://www.cisco.com/en/US/prod/collateral/switches/ps10902/ps12332/at_a_glance_c45-696645.pdf

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps9336/product_solution_overview0900aecd806fa5d0.html

HTH

[Jonn cos]

Sir i will go through the documents and will get back

[Jonn cos]

Sir let me put it this way

Combining 2 chassis via VSS gives me total of 1.6 Tbps of backplane switching capacity

Now lets say i have 10x 10G host on Switch A and 10x10G host on switch B. Both of them are connected via etherchannel of 4x10G links giving me effectively 40 GBps of inter switch link bandwidth. The max data that can travel cannot exceed 40Gbps so whats the use of having 1.6 tbps when its completely dependent on the physical links between switches ?

Sir this was my confusion. Please guide me

[Jonn cos]

Any guidance pls

[Joseph W. Doherty]

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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.

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Posting

2) I was told in an interview that its cisco marketing gimmick. Please tell me is it so ?? 

It is and it isn't.

In an ideal setup, VSS has at least one connection to every other device on each of its two VSS devices.  Additionally the links to/from each VSS device are set up to load share/balance.  For example, they might be Etherchanneled or L3 links with equal cost routing.

So, as there's at least dual links to each VSS device, effectively you can send twice as much traffic, through the VSS pair as you might with a single device, hence we also have twice the fabric bandwidth to work with.

The VSL links, also ideally, are only used to coordinate the operation of the two VSS devices.  They also, though are used when traffic enters one VSS device but there's no egress link on that device for the ingress's next hop destination (which should only be the case for some kind of failure).

In comparison to dual L3 devices, each also having links to/from adjacent L3 devices, and also with L3 load/balancing, VSS doesn't offer increased bandwidth.  In comparison to dual L2 devices, again also having links to/from adjacent L2 devices, but with STP blocking one of the two paths, VSS does offer twice the bandwidth per VLAN.

[Jonn cos]

Sir thanks alot and its starting to make sense. Sir in my above post like i said

Now lets say i have 10x 10G host on Switch A and 10x10G host on  switch B. Both of them are connected via etherchannel of 4x10G links  giving me effectively 40 GBps of inter switch link bandwidth. The max  data that can travel cannot exceed 40Gbps so whats the use of having 1.6  tbps when its completely dependent on the physical links between  switches ?

Sir pls comment on this scenario also

1) is this design incorrect ?

2) If my devices have only 1 connection to either switches, does it mean i am not able to take advantage of VSS throughput ?

[Joseph W. Doherty · Accepted solution]

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

You might be confusing limitations of host or inter VSS device bandwidths, for other remote devices vs. fabric bandwidth.

Stop for a moment and consider just one switch with say an 800 Gbps fabric.  Assuming your line ports are only gig and if Etherchannel only supports 8 links, why have a fabric of more than 8 Gbps?  The answer is, the fabric supports multiple hosts and the fabric supports the aggregate of all the active ports.

In an ideal VSS set up, no "normal" traffic should cross the VSL.  Any ingress traffic on a VSS box should directly egress to the a next hop for the ingress traffic's destination.  In such an ideal VSS set up, splitting all devices that transit the logical VSS device provides twice as much fabric bandwidth to utilize.

Consider host A has a single link to a switch and host B also single link to the same switch.  Hosts A and B have two potential bottlenecks, the bandwidth of their link to the shared switch and the interior bandwidth shared by all other hosts connected to that same shared switch.  Supposed you wanted to double the bandwidth between hosts A and B by using dual Etherchannel for both.  But also suppose either there are no more free ports and/or doing so will oversubscribe the switch's interior bandwidth.  With VSS, we can split hosts A and B Etherchannel across two VSS switches.  This then potentially allows twice an many hosts ports with twice as much bandwidth between them.

[Jonn cos]

Awesome, meaning my design was wrong.

I was a bit misguided or shall i say i totally mis understood it. Sir you guys are great thanks.