[c-nsp] Routing-induced latency

[Rodney Dunn]

On Thu, Feb 02, 2006 at 12:31:55PM +0000, christian.macnevin at uk.bnpparibas.com wrote:
> Hi all,
> 
> In a campus environment I'm working on, I'm consistently being asked to 
> 'reduce latency' between devices. So here's my question.
> In (usually Catalyst) hardware, should you expect to see any latency 
> induced by routing? By which I mean, if all routes are present
> in the table, and you're running MLS CEF, I would expect that the 
> switching is being done in hardware, and we should be introducing
> zero latency. The older kit in here is not running CEF, so will be fast 
> switching, I presume, which may sitll incur processor overhead.

The forwarding latency by hardware with no congestion will be faster than
if it's switched in hardware.

> 
> Is that assumption correct? That in the absence of a converging network, 
> there should in theory be no latency induced by switching
> between two VLANs on the same switch as opposed to switching between two 
> hosts on the same VLAN?

There is always a forwarding latency through a box. Extremely small but
it's there. You can never eliminate that just like you can't eliminate
propagation delay on the wire (light travels only so fast until we figure
out a way to speed it up). 

The main thing most people focus on is how fast the network can reconverge
when there is a failure. It sounds like someone needs to give you
some requirements that are more clear other than "make it go faster".

> 
> Cheers
> Christian
> 
> 
> This message and any attachments (the "message") is 
> intended solely for the addressees and is confidential. 
> If you receive this message in error, please delete it and
> immediately notify the sender. Any use not in accord with
> its purpose, any dissemination or disclosure, either whole
> or partial, is prohibited except formal approval. The internet
> can not guarantee the integrity of this message.
> BNP PARIBAS (and its subsidiaries) shall (will) not
> therefore be liable for the message if modified. 
> 
> **********************************************************************************************
> 
> BNP Paribas Private Bank London Branch is authorised
> by CECEI & AMF and is regulated by the Financial Services
> Authority for the conduct of its investment business in
> the United Kingdom.
> 
> BNP Paribas Securities Services London Branch is authorised 
> by CECEI & AMF and is regulated by the Financial Services 
> Authority for the conduct of its investment business in 
> the United Kingdom.
>   
> BNP Paribas Fund Services UK Limited is authorised and 
> regulated by the Financial Services Authority
> 
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/