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Dual 6800 switches in VSS. What interface speeds should we use for the VSS?

We are upgrading our University Network. We want two 6807 switches in VSS on the Core. All our sites and main buildings on the Campus will have 4500-X switches in distribution/head of cabinet dual linked at layer 2 to each 6800 Chassis at 10Gig (so 20G uplinks)

Our Data Center switches (two Dell S4810's) will be connected to each 6800 chassis at 40G.

User traffic rarely if ever moves between access layers.

A few questions I hope you can help me with:

1. As we will have dual links in LACP from each site/building what interface speeds should we be using for inter-chassis for the VSS?

2. What traffic will cross the VSS links in this setup?

3. Am I correct in assuming not much traffic because each 4500-X will always take the shortest path to it's destination, usually the Data Center so will always go between client/server using closest 6800 chassis?

4. Would user traffic ever go between chassis across the VSS links except control information and routes and so on?

Thank you.

Everyone's tags (3)
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VIP Advisor

LCAP/Etherchannel running at

LCAP/Etherchannel running at layer 2, in short, uses a hash to determine what member of the channel it uses.  It does not consider the shortest path to the destination.

Are the links to the two Dell switches layer 2 or layer 3?  If layer 2, do the two Dell switches appear as one virtual switch?  Aka, does each Dell switch have an Etherchannel connection to the 6800s (using a pair of circuits each) or does the Dell switches look like one switch so there is a single pair of Etherchannel ports and cables connecting to the 6800's?

VIP Advisor

I've reconsidered.

I've reconsidered.

(1) Personally I would make the VSS links as big as the largest inbound link, which in this case is 40Gb/s.  Then if you have a path failure the network wont get crippled.

(2) As long as the Dells, weather they appear as a single switch or not, are dual connected by a channel then no data should flow across the VSS link except for things like (as long as there are no path failures):

  • Unknown unicast and unknown multicast traffic.
  • Broadcast traffic
  • Control traffic and state synchronisation.

(3) Yes.

(4). Pretty much answered by (2).

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