why we need 'bridge'

Hi,

many layer2 ethernet equipment vendor, Cisco as example, have a concept (I
don't really know if it is correct to call it concept?) of a 'bridge', i.e.
on cisco CLI one needs to explicitly create bridge:

#conf t
#(config) bridge 1 protocol ieee
#interface fe0
#bridge-group 1
#interface fe1
#bridge-group 1
....

Why is it necessary ? As long as it is a switch, all the ports by default
are in switchable, or this allows to have a number of independent bridges
within one single physical device? Thanks in advance.

Mark

 

This sounds suspiciously like homework already, so you only get a hint.

Its unlikely that somebody setting up a cisco would bridge two ethers
together, but they support other kinds..

 

Thanks for reply. It's not a homework, although I'm student My guess
after some research was that cisco originally supported multiple
technologies (for ex. token ring, ethernet, fddi etc.), and it was naturally
to allow a customer to bridge these ports in a single bridging entity.

But I wasn't sure if my thoughts were right or wrong. Certainly I searched
cisco.com, but they don't give much theeoretical background in this item
though.


Mark

 

Indeed, WAN bridges was the primary use for bridge groups, not
necessarily LAN to LAN bridging..

 

there are even now some networks that need bridging and a router would
not have all interfaces on the same switch module for hardware L2
switching support - protocols like LAT have not died completely and
bridging OSI CLNS was common at 1 point in carrier networks for
managing SDH and similar kit

This is usually on a router between Ethernet LANs and WAN interfaces.

More recently Ethernet is taking over the high bandwidth WAN role, but
even there routers get used on the LAN to WAN border to allow QoS +
rate limiting etc in ways switches struggle with.

A network i used to work on used bridging between the loopback and ATM
PVCs for management traffic (i think - this is a while back)

finally

 

Put multiple switch modules such as the HWIC-4ESWs into a Cisco router
and the ports within the module switch L2 locally, but the modules are
not connected at layer 2......

Still got lots of OSI CLNS running for SDH and other management = note
carrier networks will be happily running kit well past the sell by
dates.