Questions about backplane bandwidth

[weiler]

Greetings all-

As a networking neophyte, I'm having a bit of trouble understanding some (probably) basic concepts on switching backplanes... I was hoping someone could enlighten me on a couple topics...

1: What's the difference between "forwarding rate" and "backplane bandwidth"? Are they connected?

2: What exactly do "fabric enabled" line cards get you in a 6500 series switch? I noticed that the fabric enabled line cards are twice the price of non-fabric enabled line cards... Does this mean that the line card shares the aggregate backplane bandwidth available on the SUP, thus enabling, say, a 48 port card (with GigE ports) to forward packets at "line speed" (i.e. all 48 ports spewing a gigabit of traffic simultaneously?

3: I notice that other switches out there say they have a higher forwarding rate than similar Cisco switches. For example, the HP Procurve 2810-48G claims to forward packets at 71.4 Mpps at 64 bytes while the 3750 with 48 10/100/1000 ports only forwards at 38.7 Mpps. The 3750 is like $15,000 or something, the HP is like $3,000. I know the 3750 has a few more features, but how can HP claim to forward faster than such an expensive Cisco equivalent (while sort-of equivalent)?

Thanks for any insight!

-erich

[Joseph W. Doherty]

With the 20 Gbps dual channel fabric cards, you have a total of 80 Gbps (duplex), but yes 48 gig ports could congest against the fabric channels.

In practice, very unlikely you're going to generate that much substained traffic, but if it's a concern, only use 40 ports. (I believe the channels are divided into banks of 24, so you could uses 20 ports of the first 24 and 20 ports of the second 24.)

If you look at the specs of the 8 port 10 gig card, it's oversubscribed 2:1. There's actually a new command to deactive every other port to conform with the fabric bandwidth.

If you really want to be able to push 5 such 48 gig ports cards at nearly wire rate, and expecially depending on your packets sizes, you likely need DFC on the cards. This would be such as the 6748 with DFC.

More info on the 6500 10/100/1000 cards found here: http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet0900aecd8017376e.html

[lamav]

Erich:

You're quite welcome...good luck!