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I understand the benefits of the Cisco 6500 VSS with the obvious selling points of single management, single routing instance, STP elimination, port-channels across chassis, etc. With two standalone Cisco 6500s that may have L3 and L2 port-channels between them, they at least have no operational dependency on one another through the control-plane.

In a VSS world -- and I have no direct experience with this -- we now have software and other protocols that control both switches. In my designs that expect control-plane software to have bugs, does the VSS lower the MTBF as I suspect and is a trade-off with the capababilities gained or am I missing how MTBF is improved?

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    I'd really love to see raw data on this, but I doubt even Cisco has it. Casual look on c-nsp and bugtool shows many VSS-specific failure modes, but obviously it does not prove anything. In our network top 3 reasons for outage each separated by huge margin are 1. Operator mistake 2. Broken SW 3. Broken HW/Infra. But usually designs focus on making 3. better, while at same time making 1-2 worse. – ytti Jul 13 '13 at 8:13
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    Give your #2 Broken SW, in your experience, does this happen more or less often with VSS? – generalnetworkerror Jul 13 '13 at 8:15
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    We don't run VSS, so no data. I'm very hesitant to be early-adopter on any solution which adds fate-sharing signalling between nodes, I'm much more focused on stateless local-repair solutions. But I fully acknowledge that for some applications VSS can be extremely appealing. – ytti Jul 13 '13 at 8:21
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    Are you asking about service outages or actual linecard failures? MTBF is generally used to describe the rate of hardware failures. In that case VSS does not impact the hardware either way. – smoothbSE Jul 13 '13 at 19:37
  • @smoothbSE, good point on MTBF being for h/w failures. I used the term loosely to indicate any type of failure. – generalnetworkerror Jul 16 '13 at 2:44
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Short version of the answer: a little bit of both, but it's not meant to be a technology to directly improve availability

Long version of the answer: As others have pointed out, manufacturer's traditional definitions of MTBF and availability focus on hardware failures. Other factors -- human error, buggy software, planned maintenance, etc. -- are considerations in developing an architecture but are made at the individual user level.

For a hardware-only perspective, VSS doesn't impact availability. It's the same hardware being used, so the same MTBF/MTTR numbers are utilized and the end availability equations are the same.

For a more holistic perspective, it's really a toss-up and will depend largely on your individual wants and needs. On one hand, you could consider it less reliable since it's a complex piece of technology and a single "virtual point of failure" (i.e., the VSS control plane) will impact both pieces of redundant gear. On the other hand, it can be viewed to increase availability since a single virtual device renders the network much simpler, making it less likely for other things to go wrong (fewer devices to manage, no HSRP/VRRP, non-looped STP domain, simpler L3 topology, etc.).

The market has pretty much shown that most network engineers view VSS and similar technologies as an improvement over a traditional L2 distro/access topology, but there are other technologies you could go with. For example, a routed L3 access layer could achieve most of the benefits of VSS, but VLANs would be unable to span multiple access layer devices, making the solution potentially useless in some scenarios (e.g., virtualized data centers).

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From a functional point of view VSS basically takes two chassis and runs them on a single control plane. If you want to create an 18-slot 6500 then it is the ideal technology. If the goal is greater availability then it's a lot harder to justify. They key point is that in establishing a VSS pair you've created a single functional chassis. Any failure mode on the control plane - from software defect to configuration error - has an immediate effect upon the entire complex.

For what it's worth I haven't really seen many new deployments of VSS over the past five years but I have seen a fair number where the feature has been removed in favor of running independent 6K pairs.

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I my experience VSS lengthens MTBF through reduced operational complexity, (i.e. no HSRP/VRRP, less STP tuning, simpler routing, etc.) especially for shops with less-experienced engineers. Reconvergance after link failures is generally faster as the rest of network sees the pair as one device from a L2 and L3 perspective. My guess would be there are fewer outages related to VSS software bugs, than outages attributed to the interactions and failure modes of the various protocols typically run at that layer.

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