L2 vs L3 Etherchannel?

Discussion in 'Cisco' started by ciscortp@hotmail.com, Apr 14, 2006.

  1. Guest

    What is the real difference between L2 and L3 etherchannels? I can
    understand that with Layer 2 Etherchannels you can bundle two links
    into 1 big pipe, but why would I need to do this at Layer 3. Isn't it
    easier to just load share using routing over the multiple equal cost
    links?

    Also, if you are doing Layer 2 etherchannels, do you have to configure
    the Po1 interface at all? I have seen people placing commands this
    interface such as "trunk encap dot1q" without actually making it a
    trunk ie using the "switchport mode trunk | dynamic" command.

    And if you make it a layer 3 etherchannel, How is the IP addressing
    worked out. Do the switch interfaces have ip addresses in addition to
    the Po1 interface or is there only an IP address on the Po1 interface.
    If anyone has sample configurations used in a typical enterprise
    network, that would be greatly appreciated.

    Thanks in advance.
     
    , Apr 14, 2006
    #1
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  2. Tim F Guest

    I think that depends greatly on your configuration.

    Think of the port-channel mainly as a layer 1 type of configuration in which
    you logically bind together two physical interfaces. Whether you make it a
    switched or routed interface occurs at another layer.

    To me, port-channel seems more efficient than other means of load-balancing
    multiple interfaces. But I have encountered one gotcha. With the absence
    of keepalives between interfaces in port-channel, a medium in the middle,
    such as a Gig-E DWDM system, can maintain carrier but drop traffic resulting
    in a severely degraded network. At least with a routing protocol,
    keepalives will keep the session alive only if there is traffic flow. But
    if you are sure that carrier up means traffic flow, I would recommend
    port-channels hands down. Cisco may have implemented port-channel
    keepalives as I have not used these for over 2 years.

    I do not know the answer to your last question. My guess is that the IPs on
    the individual physical interfaces do not matter because once it becomes
    part of the port-group, the interface is no longer directly addressable.

    Best wishes
    Tim

    <> wrote in message
    news:...
    > What is the real difference between L2 and L3 etherchannels? I can
    > understand that with Layer 2 Etherchannels you can bundle two links
    > into 1 big pipe, but why would I need to do this at Layer 3. Isn't it
    > easier to just load share using routing over the multiple equal cost
    > links?
    >
    > Also, if you are doing Layer 2 etherchannels, do you have to configure
    > the Po1 interface at all? I have seen people placing commands this
    > interface such as "trunk encap dot1q" without actually making it a
    > trunk ie using the "switchport mode trunk | dynamic" command.
    >
    > And if you make it a layer 3 etherchannel, How is the IP addressing
    > worked out. Do the switch interfaces have ip addresses in addition to
    > the Po1 interface or is there only an IP address on the Po1 interface.
    > If anyone has sample configurations used in a typical enterprise
    > network, that would be greatly appreciated.
    >
    > Thanks in advance.
    >
     
    Tim F, Apr 14, 2006
    #2
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  3. stephen Guest

    "Tim F" <> wrote in message
    news:LUO%f.421$Jk2.8@fed1read03...
    > I think that depends greatly on your configuration.
    >
    > Think of the port-channel mainly as a layer 1 type of configuration in

    which
    > you logically bind together two physical interfaces. Whether you make it

    a
    > switched or routed interface occurs at another layer.
    >
    > To me, port-channel seems more efficient than other means of

    load-balancing
    > multiple interfaces. But I have encountered one gotcha. With the absence
    > of keepalives between interfaces in port-channel, a medium in the middle,
    > such as a Gig-E DWDM system, can maintain carrier but drop traffic

    resulting
    > in a severely degraded network. At least with a routing protocol,
    > keepalives will keep the session alive only if there is traffic flow. But
    > if you are sure that carrier up means traffic flow, I would recommend
    > port-channels hands down. Cisco may have implemented port-channel
    > keepalives as I have not used these for over 2 years.


    use 802.1ad instead - the control protocol (LACP) tests the underlying links
    using keepalives. whether it can detect a link that trashes just some
    traffic is a different Q...

    Cisco have implemented BFD (bidirectional forwarding detection) which is
    intended as a fast reacting connectivity check
    >
    > I do not know the answer to your last question. My guess is that the IPs

    on
    > the individual physical interfaces do not matter because once it becomes
    > part of the port-group, the interface is no longer directly addressable.
    >
    > Best wishes
    > Tim
    >
    > <> wrote in message
    > news:...
    > > What is the real difference between L2 and L3 etherchannels? I can
    > > understand that with Layer 2 Etherchannels you can bundle two links
    > > into 1 big pipe, but why would I need to do this at Layer 3. Isn't it
    > > easier to just load share using routing over the multiple equal cost
    > > links?


    l3 vs l2 is about the logical interface to the device, and also affects how
    the balancing algorithm decides which packets go on which link.

    you can use L3 ECMP in combination with Etherchannel (or 802.1ad) - had to
    do this once to handle a 16 * GigE link.....

    Dont forget you cannot do ECMP unless there is a L3 box at each end of the
    pipe (and that you can run a routing protocol that supports multi path, and
    that both boxes can support ECMP).

    There are also some nice resilience protocols that allow a pair of boxes to
    "pretend" to support 2 links within a single load balanced 802.1ad layer 2
    pipe - Nortel Passport 8600s have 1 version. The elegent bit is that
    anything that supports 802.1ad can sit on the end, but benefit from from the
    resilience of 2 links to separate central switches, without needing a
    routing protocol or any other complication.
    > >
    > > Also, if you are doing Layer 2 etherchannels, do you have to configure
    > > the Po1 interface at all? I have seen people placing commands this
    > > interface such as "trunk encap dot1q" without actually making it a
    > > trunk ie using the "switchport mode trunk | dynamic" command.


    cisco switches by default mainly support dynamic trunking protocol, which
    can sort out which ports go to the same destination switch and build the
    port group on the fly.

    Frankly i think that anyone who depends on this kind of "feature" in a live
    network is just not paranoid enough....

    DTP isnt going to work with other devices, such as other vendor switches, or
    even a conventional Cisco router
    > >
    > > And if you make it a layer 3 etherchannel, How is the IP addressing
    > > worked out. Do the switch interfaces have ip addresses in addition to
    > > the Po1 interface or is there only an IP address on the Po1 interface.
    > > If anyone has sample configurations used in a typical enterprise
    > > network, that would be greatly appreciated.
    > >


    the IP address is bound to the set of links, not the individual interfaces.

    > > Thanks in advance.

    --
    Regards

    - replace xyz with ntl
     
    stephen, Apr 14, 2006
    #3
  4. Guest

    wrote:
    > What is the real difference between L2 and L3 etherchannels? I can
    > understand that with Layer 2 Etherchannels you can bundle two links
    > into 1 big pipe, but why would I need to do this at Layer 3.


    Think of the situation where the component links of an etherchannel on
    a switch are connected to a interfaces on a router. The router will
    have to treat those interfaces as a bundle since that is how the switch
    is treating them. Hence the router too will have to support
    etherchannel and have a corresponding interface that represents the
    bundle of component links. Since this is a router, that etherchannel
    interface will have an IP address and operate at layer3. And that is
    exactly what a layer3 etherchannel is.

    > Isn't it easier to just load share using routing over the multiple equal cost
    > links?


    There are some advantages to having a L3 etherchannel rather than
    having equal cost paths seen by L3 as two separate links. For
    instance, imagine that one of the links starts flapping. With an
    etherchannel, the routing protocol will not see that at all since the
    port-channel will always stay up. With independent equal-cost links
    rather than an etherchannel, the flapping links will cause routing
    updates and the attendant cpu activity trying to converge etc. In such
    a case, the etherchannel would give to load sharing and link redundancy
    with less overhead than if the links were treated as independent links.

    > Also, if you are doing Layer 2 etherchannels, do you have to configure
    > the Po1 interface at all? I have seen people placing commands this
    > interface such as "trunk encap dot1q" without actually making it a
    > trunk ie using the "switchport mode trunk | dynamic" command.
    >
    > And if you make it a layer 3 etherchannel, How is the IP addressing
    > worked out. Do the switch interfaces have ip addresses in addition to
    > the Po1 interface or is there only an IP address on the Po1 interface.
    > If anyone has sample configurations used in a typical enterprise
    > network, that would be greatly appreciated.


    No, the IP address will only be associated with the port-channel
    interface that represents the etherchannel and not on the component
    links. From the p.o.v of L3, the component links don't exist and only
    the port-channel interface is seen.

    Cisco da Gama
    http://ciscostudy.blogspot.com
     
    , Apr 14, 2006
    #4
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