VanLUG Email Archive
Alan Hodgson (ahodgson@simkin.com)
Fri, 13 Aug 1999 21:13:06 -0700
On Fri, Aug 13, 1999 at 07:21:55PM -0700, Simon A. de Weerdt wrote:
> I'm not sure where the dividing line between a switch and router is drawn
That's easy. A switch operates at the physical layer. It decides
whether to transfer packets to other ports based on the MAC address of
the destination (and whether or not it's ever seen traffic from the
MAC address of the destination so it knows what port that system
is connected to). A switch is really just a fast multi-port bridge.
As far as a switch is concerned, all devices are connected to the
same physical "network". A switch is not aware of the network
protocol of a particular packet, nor does it need to know.
(and yes, I know you can segment most switches, but in essence
that just makes it multiple separate switches).
A router operates at the Network layer. A router is aware of the
network protocols in use, and decides whether to move packets to
a different segment based on the network address of the destination.
Routers make use of routing protocols within the various protocol
families to make the decisions where to route various packets.
Further ... a switch doesn't usually modify a packet. It just re-
transmits it on the destination port, since the packet is already MAC-
addressed to the end-point destination. A router, on the other hand,
receives packets MAC-addressed to itself, but with a remote network/host
address at the network layer. The router decides on the next-hop,
whether that's another router or the end-point system, and creates
a new MAC-layer packet with the MAC address of the next-hop system
and then transmits that on the correct segment. Of course, these aren't
hard rules - switches that "switch" traffic from one topology to another
have to re-frame the packets, especially when the source packet is
too large for the destination topology.
And then there are so-called Layer 3 switches. I'm having some
trouble coming up with a good definition for them, other than to say
that they combine the functionality of switches with the functionality
of routers (for some protocols), with very fast (ie. wire-speed)
routing capabilities. They also seem to generally support the basic
routing protocols for IP and IPX, at least.
-- Linux: because a PC is a terrible thing to waste.
This archive was generated by hypermail 2.0b3 on Sat 14 Aug 1999 - 04:16:57