Subject: Re: switch vs router

From: "J. Noel Chiappa" <[email-masked]>
Date: Mon, 19 Mar 2001 20:04:45 -0500

> From: "Mike O'Dell" <[email-masked]>

    > the distinction is often based on the namespace used for making the
    > forwarding decision
    > the term "router" is *usually* applied to a device which is examining
    > an L3 token ... a switch is usually examining a token other than an IP
    > destination address - often an L2 address token

This is sort of the definition I prefer - although I tend to put it as "which
header are they looking at - the internetwork header, or the header used by
the access network".

(And, as Mo notes elsewhere, at one point there was a theory that boxes that
did "routing" were slower - a statement not entirely without reason, as a
router generally has to discard one local header, examine the internetwork
header, and create a new local header - certainly more work than just
examining a local header, and going. Unless you're allowed to throw some
modest number of mega-gates at the problem, yes, "routing" does tend to be
slower.)

Note that it may be hard to tell, from external examination, exactly which of
these two definitions (routing and switching) a given box is doing. However,
definitions which involve looking for mods to the packet may fail; I've known
"routers" which didn't decrement the TTL. And which TTL does one mean - MPLS
has a TTL, too, but an MPLS-only box isn't a router...

	Noel

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From: "Claudio Isaias Bernal Montalvo" <[email-masked]>
Date: Tue, 20 Mar 2001 11:58:15 -0600

I hope the following lines can help to clarify the point:

Layer-2 switching is hardware based,wich means it uses the MAC address from the
host´s NIC cards to filter the network. Switches use Application-Specific
intergrated Circuits( ASIC´s) to build and maintain filter tables instead of
that routers uses ip adresses  wich is layer 3 information to buid and maintain
routing tables

Layer-2 switches are fast because they don´t look at the network layer header
information( like routers do), looking instead at the frame´s harware address
before deciding to either forward the frame or drop it.

What makes layer-2 switching so efficient is that there is no modification to
the data packet, only to the frame encapsulating the packet.since no
modification of the data packet is performed( routers make decitions after check
ip network address layer-3 information), the switching process is faster and
less-error prone than routing.

Bridged networks break up collision domains, but the network is still one large
broadcast domain. Layer-2 switches (bridges) cannot break up broadcast domains,
wich can cause less performance and limit the size of the network. Broadcast and
multicasts, along with the slow convergence of spanning tree, can cause major
problems as the network grows. Because of these problems, layer-2 switches
cannot completely replace routers( layer-3 devices) in the internetwork.

Atte: Claudio Bernal
IP Conectivity Specyalist
Alestra - ATT
Monterrey,Nuevo Leon, Mexico.