problem with catalyst 6500 line card switching capacity

[szhongling]

1.When I install only one CEF720 line card (e.g. WS-X6748-GE-TX) in 6500 chassis, I want to know the switching capacity between Port ASIC(1) and Port ASIC(2). Do they communicate each other via 20Gb Fabric Channel? or other?

Also between Port ASIC(1) and Port ASIC(3), do they communicate with each other via 20Gb Fabric Channel? or via 16G Local Switching Bus? or other?

The same above questiones when I install the DFC in CEF720 line card.

2.Same case with dCEF720 line card (e.g. WS-X6708-10G-3C/3C XL), I want to know how do they communicate with each other between Port ASCI 1 and 2, 1 and 3, 1 and 4

Could anyone explain this to me?

Thank you for any input

[deyadav · Accepted solution]

Hi Shi,

Ans 1) For 6748-GE-TX, we have 4 port ASIC's which are aligned to 2 Fabric ASIC's. Port ASIC 1 and 2 are over Fabric 1,  3 and 4 are over Fabric 2.

Communication between Port Asic 1 and 2, does not involve Fabric, and they are locally switched. The only difference is with the use of CFC, packet header still traverses over Shared Bus towards PFC/Supervisor for forwarding decision. With DFC, packet never moves out of Line card (exception control traffic, broadcast, multicast, unicast flood) and is switched locally between Port Asic 1 and 2, or Asic 3 and 4.

Communication between Port ASIC 1 and 3, or 2 and 4 would certainly traverse over Fabric, no difference if we are using CFC or DFC, except packets moving towards forwarding engine.

Ans 2)  WS-X6708-10G-3C/3C XL, Communication between mentioned Port ASIC 1, 2 and 3 would happen locally and no traffic would leave over Fabric. However for Port ASIC 4, traffic from Port ASIC 1, 2 and 3 would traverse over Fabric.

HTH.

Regards,

Deepak

[szhongling]

Hi Deepak,

Much appreciated, thanks a million.

[szhongling]

Hi Deepak,

Then the "locally switched" means switched through the 16G local switching bus via Bus ASIC? or switched through Port ASICs via Fabric Replication Engine?

And, what is the function of "Bus ASIC" and "Fabric Replication Engine"?

[deyadav · Accepted solution]

Locally switched, in this context would mean packets which never leave over Fabric and were switched by DFC through Port ASIC using Fabric Replication Engine.

CFC has a connection to the DBUS which is a 32Gbps bi-directional Shared bus. CEF720 Line cards have a 16Gb connection to this shared bus.

With CFC, packet header would always leave over that 16Gb connection via Bus ASIC towards PFC (Centralized forwarding engine). Bus ASIC is the ASIC which provides access to the shared Bus.

With DFC installed, we no longer have any connection towards the Shared Bus, as the DFC is now the forwarding engine. Lets take some examples:

1) Packet movement between Port ASIC 1 and Port ASIC 3 for WS-X6748 Line card, with CFC installed.

Packet ingress on Port ASIC 1, is further sent to Fabric ASIC. Fabric ASIC then either sends a Compact or Truncated header towards the Shared Bus via Bus ASIC. Once the result is sent over Rbus, now the Fabric ASIC picks up the result from the Bus ASIC, and switches this packet over Fabric towards Port ASIC 3, as it is not mapped to it.

2) Packet movement between Port ASIC 1 and Port ASIC 2 for WS-X6748 Line card, with CFC installed.

Same as above. The only difference is that once result is received by Fabric ASIC, it sends the packet out directly over Port ASIC 2 without using the crossbar fabric.

3) Packet movement between Port ASIC 1 and Port ASIC X for WS-X6748 Line card, with DFC installed.

With DFC installed we do not send any traffic over Shared Bus, as we no longer maintain a connection to it as it's now not required. DFC is true sense does a local switching, however packets may still traverse over Fabric if they need to reach out to a destination on other Line card or another fabric channel even on same line card.

Ingress Port ASIC 1/2 and Egress Port ASIC 3/4 on same line card, would see packet going over Fabric.

Ingress Port ASIC 1/3 and Egress Port ASIC 2/4 on same line card, would see no packet leaving outside at all. Packet is thus locally switched.

Locally switched is relative term, depends on how you use and interpret it. However, in real sense locally switched makes more sense when we have DFC on the line card.

Fabric ASIC for 67xx Line card also does packet replication along with providing access to the crossbar fabric. Packet replication is required for Span, multicast and some other features as well.

HTH.

Regards,

Deepak

[szhongling]

Deepak, thanks again for your generous help.