Re: [nsp] Fabric Line Cards

From: Timothy Brown (
Date: Fri May 31 2002 - 10:50:41 EDT

On Fri, May 31, 2002 at 01:04:47PM +0200, wrote:
> This paper on the 6500 Architecture is good:
> except it completely ignores exactly this scenario.
> Presumably the header compresssion used to send packet headers over the
> switching bus
> when switching packets between fabric cards is disabled when mixing with
> non-fabric cards?
> This would pull performance down to something like Supervisor1/PFC
> performance of 15Mpps.
> If you use Distributed forwarding on the fabric cards you may of course
> achieve higher switching speeds,
> when switching between fabric cards.

This information may already be understood; sorry if you already are aware:

According to Cisco's website (7.2 CatOS docs)

When you install a Switch Fabric Module in a Catalyst 6500 series switch,
the traffic is forwarded to and from modules in one of the following modes:

Flow-through mode. In this mode, data passes between the local bus and
the supervisor engine bus. This mode is used for traffic to or from
nonfabric-enabled modules.

Truncated mode. In this mode, only the truncated data (the first 64 bytes
of the frame) is sent over the switch fabric channel if both the destination
and the source modules are fabric-enabled modules. If either the source or
destination is not a fabric-enabled module, the data goes through the switch
fabric channel and the data bus. The Switch Fabric Module does not get
involved when traffic is forwarded between nonfabric-enabled modules.

Compact mode. In this mode, a compact version of the DBus header is
forwarded over the switch fabric channel, delivering the best possible
switching rate. Nonfabric-enabled modules do not support the compact
mode and will generate CRC errors if they receive frames in compact mode.
This mode is only used if nonfabric-enabled modules are not installed in
the chassis.

If you enter the truncated keyword and your system does not contain
nonfabric-enabled modules, the system is placed in compact mode.

If two or more fabric-enabled modules are installed in your system,
forwarding between these modules occurs in truncated mode.


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