Cisco Catalyst - Switching with Supervisor and Line Cards

Question

Regarding the Cisco Catalyst 4500, or 6500, does the frames switching and the packets switching (with CEF) are performed by the Supervisor Card or by the Line Card ? (Also, does a Line Card has CAM and TCAM ?)

Also, if a frame or packet from the Line Card 1, has to travel to the Line Card 2 to the right outbound interface, do the frames and packets could travel directly to the Line Card 2 without passing thought the Supervisor Card ?

Answer 1

It depends on the specific line card. Some line cards have more intelligence and features than other line cards.

With the line cards that support CEF on the card, you can enable distributed CEF.

IP Switching Cisco Express Forwarding Configuration Guide

When to Enable or Disable Central CEF on a Router

Enable central Cisco Express Forwarding operation when line cards are not available for Cisco Express Forwarding switching or when you need to use features not compatible with distributed Cisco Express Forwarding switching. When central Cisco Express Forwarding operation is enabled, the Cisco Express Forwarding Forwarding Information Base (FIB) and adjacency tables reside on the RP, and the RP performs express forwarding.

Disable central Cisco Express Forwarding on a router when you want to turn off central Cisco Express Forwarding on the router and on all interfaces on the router. You might want to do this if your router and router interfaces are configured with a feature that central Cisco Express Forwarding or distributed Cisco Express Forwarding does not support.

To disable central Cisco Express Forwarding on a router and on all interfaces on the router, use the no ip cef command.

When to Enable dCEF on a Line Card

Enable distributed Cisco Express Forwarding on a line card when you want the line card to perform express forwarding so that the RP can handle routing protocols or switch packets from legacy interface processors. When distributed Cisco Express Forwarding is enabled, line cards maintain an identical copy of the FIB and adjacency tables. The line cards perform express forwarding between port adapters, thus relieving the RP of involvement in the switching operation. distributed Cisco Express Forwarding uses an interprocess communication (IPC) mechanism to ensure synchronization of FIB tables and adjacency tables on the RP and line cards.

The Cisco ASR 1000 Series Routers operate only in distributed Cisco Express Forwarding mode. In other routers you can mix various types of line cards in the same router, and all of the line cards you are using need not support Cisco Express Forwarding. When a line card that does not support Cisco Express Forwarding receives a packet, the line card forwards the packet to the next higher switching layer (the RP) or forwards the packet to the next hop for processing. This structure allows legacy interface processors to exist in the router with newer interface processors.

Note: When you enable distributed Cisco Express Forwarding globally, all interfaces that support distributed Cisco Express Forwarding are enabled by default.

When to Enable or Disable CEF on an Interface

You need to decide whether or not you want Cisco Express Forwarding operation on an interface. In some instances, you might want to disable Cisco Express Forwarding or distributed Cisco Express Forwarding on a particular interface because that interface is configured with a feature that Cisco Express Forwarding or distributed Cisco Express Forwarding does not support. Because all interfaces that support Cisco Express Forwarding or distributed Cisco Express Forwarding are enabled by default when you enable Cisco Express Forwarding operation globally, you must use the no form of the ip route-cache cefcommand to turn off Cisco Express Forwarding operation on a particular interface. To reenable Cisco Express Forwarding, use the ip route-cache cef command. To reenable distributed Cisco Express Forwarding, use the ip route-cache distributed command.

Disabling Cisco Express Forwarding or distributed Cisco Express Forwarding on an interface disables Cisco Express Forwarding switching for packets forwarded to the interface, but has no effect on packets forwarded out of the interface.

When you disable Cisco Express Forwarding or distributed Cisco Express Forwarding, Cisco IOS XE software switches packets received on the interface using the next fastest switching path. For Cisco Express Forwarding, the next fastest switching path is switching on the RP. For distributed Cisco Express Forwarding, the next fastest switching path is Cisco Express Forwarding on the RP.

The input interface determines the Cisco IOS XE switching path that a packet takes. Consider the following when enabling or disabling switching methods on a particular interface:

• You need Cisco Express Forwarding to be enabled on the incoming interface for packets to be Cisco Express Forwarding switched.
• Because Cisco Express Forwarding makes the forwarding decision on input, you need to use the no ip route-cache cefcommand on the
  ingress interface if you want to disable Cisco Express Forwarding.

Answer 2

The 4500 does not support switching on the line card at all. All packets are received on the line card, travel over the backplane, reach the supervisor where the ASIC is located where they get switched, then travel over the backplane again to the linecard with the egress interface on the way out.

The 6500 supports switching on the line card (aka distributed switching), but you have to check specific combinations of linecards/supervisors/IOS versions.

Yes, linecards that are capable of distributed switching do contain on-board ASICs to do the job.