why we need 'bridge'

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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.


Re: why we need 'bridge'
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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..

Re: why we need 'bridge'
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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


Re: why we need 'bridge'
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Indeed, WAN bridges was the primary use for bridge groups, not
necessarily LAN to LAN bridging..

Re: why we need 'bridge'
On Fri, 9 Sep 2011 15:34:37 -0400, "Mark"

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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)

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stephen_hope@xyzworld.com - replace xyz with ntl

Re: why we need 'bridge'
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We have the concept of L2 ports (aka "switchports") and L3 ports ("routed
interfaces".)  The former ports naturally bridge (forward at layer 2) amongst
themselves if they are configured to be in the same VLAN.  The latter port
flavor naturally route (forward at layer 3), but can be made to bridge if put
into the same bridge-group as shown above.

In olden times, back when a variety of L3 protocols was prevalent, a Cisco
router interface could be configured to route some subset of protocols (by
configuring it with an L3 address in the given protocol), and then bridge
other protocols (by putting the interface into a bridge-group.)  Nowadays,
with everyone running IPv4 everywhere, you use bridge-group to extend an
IPv4 broadcast domain ("subnet"), and you use routed interfaces to delimit
the broadcast domain.

The prevalence of switchports is the distinguishing mark of a device
that we call a "switch"; the prevalence of routed interfaces is what
causes us to call a device a "router".  Although nowadays a device like
the 881 has 4 switchports (Fa0..3) but only 1 routed port (Fa4), but is
nonethleless called a "router" rather than a "switch" - go figure.

I seem to recall that _Interconnections_ by Radia Perlman had a useful
discussion of this sort of stuff.


Re: why we need 'bridge'

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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......
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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
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stephen_hope@xyzworld.com - replace xyz with ntl

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