Understanding Cat 6500 architecture - forwarding of L2 packets

xavier.walker · ‎03-30-2009

Hi all.

Having read the white paper http://www.cisco.com/en/US/partner/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html

I'm still unsure how the following scenario works:

Imagine a scenario with two 6500 chassis - switch A and switch B, both with Sup720 engines, both with line cards with no DFC boards (for example WS-X6748-GE-TX). There is a trunk between the two switches so devices on one switch can communicate with devices on the other.

Both switches / Sup720 modules have synchronised L2 VLAN databases. On top of this, both switches also have a L3 VLAN interface declared. HSRP is used to define switch A as the priority switch, with the Sup720 of switch B becoming the active routing engine should switch A fail.

From the white paper, I see how packets which enter a port which has no DFC need to be managed by the Sup720.

Questions are: for traffic entering switch B, does the supervisor of switch B manage the forwarding or does the Sup of switch A have to do it (because it is the running L3 router)?

Question for both L2 traffic on the same subnet and L3 traffic where routing is required (presumably via Sup720 of switch A).

Does the location of the destination port have any impact; is the behaviour different if the destination port is on switch A or B ?

Many thanks.

Jon Marshall · ‎03-30-2009

Xavier

Can't read the link because i don't have partner access but answers to your questions as follows -

"for traffic entering switch B, does the supervisor of switch B manage the forwarding or does the Sup of switch A have to do it (because it is the running L3 router)? "

Switch B would manage the forwarding as it is active as well as supervisor in switch A. Notice also that switch B is also acting as a L3 router so it can also forward traffic at L3.

Specifically -

L2 traffic on same subnet - Switch B will forward. The destination port could be either on the same switch or on Switch A in which case the traffic is forwarded across the trunk link.

L3 traffic - depends. In the first example lets say we have a client on Switch B who's HSRP default-gateway is active on Switch A. Regardless of the where the destination port is the packet will be L2 switched from Switch B to switch A across the trunk to the clients L3 vlan interface. That vlan interface will then route it onto the destination vlan. If the destination port is on Switch A is it just forwarded out, if it is on Switch B it is simply L2 switched back across the link to Switch B and then forwarded out the port.

It is important to understand that HSRP dictates this traffic flow but Switch B supervisor is still capable of routing in it's own right ie. it can peer with other routers and L3 forward traffic.

Jon

xavier.walker · ‎03-30-2009

Thanks for your answer Jon, it makes perfect sense. Much appreciated!

I can't believe you need partner access for the white paper?!? Still, there are quite a few typos in it so it is confusing in places...

Edison Ortiz · ‎03-31-2009

Can't read the link because i don't have partner access

Public link (You know it pretty well)

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper0900aecd80673385.html

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Edison.