Subject: RE: What is the differents between Switch and Router?

From: Vernon Schryver <[email-masked]>
Date: Sat, 17 Mar 2001 15:14:50 -0700 (MST)

> In telecommunications, a switch is a network device that selects
> a path or circuit for sending a unit of data to its next destination.
> A switch may also include the function of the router, a device or
> program that can determine the route and specifically what adjacent
> network point the data should be sent to. In general, a switch is
> a simpler and faster mechanism than a router, which requires
> knowledge about the network and how to determine the route.
>
>Ref : http://whatis.techtarget.com/WhatIs_Definition_Page/0,4152,213079,00.html

Somehow, I find definitions from marketing outfits like techtarget.com
other than authoritative.  However an outfit "squarely focused on the
information technology space" might not be a marketing outfit.  I'm unable
to translate http://techtarget.com/html/about-ttc.htm into English.

That definition of of "switch" is a classic example of why it is best
to ignore marketing and sales people when they talk technical.  Usable
bridges have always needed know something about network topology, which
is why such protocols as Spanning Tree exist.  Besides, doesn't Spanning
Tree tell more about network topology than IP distance vector protocols,
for nearby connections if not distant networks?  A box running at wire
speed cannot be any faster, so are all modern routers "switches?"  Whether
an IP router using RIP is more complicated than a bridge using Spanning
Tree is not clear to me.  A "switch" that does not "determine the
route and specifically what adjacent network point the data should be
sent to" doesn't sound very useful.

Or maybe that definition of "switch" is a proof that "switch" in packet
contexts is content-free marketing blather or worse, nonsense intended to
bamboozle the suckers.

] From: Harald Alvestrand <[email-masked]>

] ...
] That's my reason to use the TTL decrement; if someone shows me a device 
] where a packet comes in on one interface with a certain TTL, and it comes 
] out on another interface with a lower TTL but no other significant changes, 
] I call it a router.
]
] If it is incapable of not doing other significant damage, I call it an 
] application layer gateway, a NAT box, or something else, but not a router.

What about the changes to the IP header required to deal with an
IP record-route option?  There seem to be three types of routers:
 1. those that deal with LSRR and RR options correctly
 2. those that mess them up, including generating pure garbage IP packets
 3. those that ignore them.
As far as I can tell, Type 1 and Type 2 are each more common than Type 3.

Vernon Schryver    [email-masked]

---

From: Harald Alvestrand <[email-masked]>
Date: Sun, 18 Mar 2001 16:06:18 +0100

At 15:14 17/03/2001 -0700, Vernon Schryver wrote:
>IP record-route option?

if it's incapable of dealing with a datagram that does not have a 
record-route option, it is indeed a strange beast :-)

>   There seem to be three types of routers:

the field of Internet-connected boxes can be divided into 3 types of devices:

- Correctly functioning routers (conforming to full RFC 1812 and friends)
- Buggy routers (including your cat 2 and 3 above)
- Non-routers

Some people surmise that the first category is empty....

note: I have not yet found anything that allows me to tell the difference 
between a switch and something that is not a switch. That is one reason why 
I prefer to avoid the term.

metanote: the field of categorizations can be divided into two categories:
- those that are useful
- those that are useless
the last category is demonstrably larger than the first.....

metametanote: the categorization above may be a member of its second category.

--
Harald Tveit Alvestrand, [email-masked]
+47 41 44 29 94
Personal email: [email-masked]

---

From: Garrett Wollman <[email-masked]>
Date: Mon, 19 Mar 2001 10:05:34 -0500 (EST)

> I prefer to avoid the term.

Hmmm.  I always thought that the definition of ``switch'' in a packet
context was fairly well-understood:

	A switch is a device which accepts a packet, performs some
	computation on the contents of the packet combined with
	internal state, and uses the result of that computation to
	determine where (if anywhere) to forward the packet.

Thus, a router is a kind of switch (one which implements the
internetwork layer of the Internet reference model), but so is a
bridge and so is a NAT.

I'm not sure whence comes the marketing definition of ``switch'' as
anything that processes packets faster than a competitor's product,
but I've long since given up trying to understand the thought
processes of marketing people.

-GAWollman