why we need 'bridge'

Discussion in 'Cisco' started by Mark, Sep 9, 2011.

  1. Mark

    Mark Guest

    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
     
    Mark, Sep 9, 2011
    #1
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  2. "Mark" <> writes:
    >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:

    ...
    >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.


    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..
     
    Doug McIntyre, Sep 9, 2011
    #2
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  3. Mark

    Mark Guest

    "Doug McIntyre" <> wrote in message
    news:4e6a61b7$0$74948$...
    > "Mark" <> writes:
    >>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:

    > ..
    >>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.

    >
    > 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
     
    Mark, Sep 9, 2011
    #3
  4. "Mark" <> writes:
    >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.


    Indeed, WAN bridges was the primary use for bridge groups, not
    necessarily LAN to LAN bridging..
     
    Doug McIntyre, Sep 11, 2011
    #4
  5. Mark

    Stephen Guest

    On Fri, 9 Sep 2011 15:34:37 -0400, "Mark"
    <> wrote:

    >"Doug McIntyre" <> wrote in message
    >news:4e6a61b7$0$74948$...
    >> "Mark" <> writes:
    >>>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:

    >> ..
    >>>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.

    >>
    >> 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.


    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
    >
    >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
    >

    --
    Regards

    - replace xyz with ntl
     
    Stephen, Sep 14, 2011
    #5
  6. Mark

    Stephen Guest

    On Mon, 12 Sep 2011 15:37:04 -0700, Aaron Leonard <>
    wrote:

    >>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

    >
    >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.


    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......
    >
    >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.


    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.
    >
    >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.
    >
    >Aaron

    --
    Regards

    - replace xyz with ntl
     
    Stephen, Sep 14, 2011
    #6
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