Lan routing - 2 subnets using single fast ethernet port

Discussion in 'Cisco' started by AlexC, Oct 16, 2003.

  1. AlexC

    AlexC Guest


    I thinking of doing simple lan routing for 2 subnets, I'm also
    of getting a cisco 1721 as the router, I plan to put the fast ethernet
    port on the subnet A, then create a sub-interface on subnet B.

    cisco 1721
    | |
    | 4 port switch |
    | |
    | |
    | |
    | |
    subnet A subnet B ( 20 Nodes)
    (about 30 Nodes)

    I tested this scenario with a Linux(with only 1 NIC). What i did was
    make eth0 IP sits on subnet A, and eth0:0 sits on subnet B, and let
    them be the default gateway for respecetive subnet nodes, it works

    Now my question is, will this work on the Cisco 1721? (i believe it
    be, just playing safe :) )

    Also, by using only 1 fast ethernet port, will this create bottleneck
    or performance issue?

    AlexC, Oct 16, 2003
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  2. AlexC

    Mike Guest

    I don't believe it will. I believe that you need at least a 1751. I
    don't think the 172x supports trunking and that's what you need for
    router on a stick. I think. It also depends on the switch, can you
    configure a port to trunk.

    When you bonded two addresses to your single physical NIC you still
    only had one collision domain.

    If all you want to do is replicate what the Linux box was doing then
    you can do it with a 1721. You would just apply a "secondary" ip
    address to the ethernet interface.

    I think
    Mike, Oct 16, 2003
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  3. AlexC

    AlexC Guest

    If all you want to do is replicate what the Linux box was doing then
    does that mean it can be done uing 1721? Would it create a bottleneck
    on network throuput?
    AlexC, Oct 16, 2003
  4. Configure "ip route-cache same-interface" and there shouldn't be any
    performance impact.
    Barry Margolin, Oct 16, 2003
  5. AlexC

    AlexC Guest

    Configure "ip route-cache same-interface" and there shouldn't be any
    that is something new to me, btw I also spotted another low end model
    Cisco 837, which has 4 ethernet ports. Anyone know if these 4 ports
    can be configure different IPs on each interface? like

    interface Ethernet0
    interface Ethernet1
    interface Ethernet2
    interface Ethernet3

    AlexC, Oct 17, 2003
  6. Of course they can, that's the normal way that routers are used.
    Barry Margolin, Oct 17, 2003
  7. On the 837, I believe the four Ethernet ports are four ports of a built-in
    10/100 Ethernet switch, therefore only a single L3 interface would be
    available in the router's configuration.


    Matthew Melbourne, Oct 17, 2003
  8. AlexC

    AlexC Guest

    I suddenly realize that the single port solution might not work if
    we need to use DHCP to assign IP address to these 2 different networks,
    we simple wont be able know which nodes "should" belong to which network
    as the DHCP request will all come in via the same internet to the router,
    anybody deal with this problem before?
    AlexC, Oct 20, 2003
  9. AlexC

    Ivan Ostres Guest

    Yes, that is a common problem. Most of the time, people don't really care
    which host gets address from which subnet. For hosts that need always the
    same address, it's a better solution to set static addresses anyway...

    Ivan Ostres, Oct 20, 2003
  10. AlexC

    Andre Beck Guest

    On a 836 testing 12.3(3)XM (or something like that), four new interfaces
    (FastEthernet1 - 4) appeared all of a sudden. The Ethernet0 remained
    as a routed interface. I didn't investigate further whether this was just
    done so you can modify individual switchport characteristics, or whether
    this would allow to turn them into routed ports. Enabling HSRP on this
    boxes e0 broke tunneling badly (Fa1 went down and up again when the HSRP
    group switched to this box, and at the same time, all tunnels with a
    source address borrowed from e0 just died), so I didn't really feel to
    push it any further.
    Andre Beck, Oct 23, 2003
  11. AlexC


    Feb 27, 2009
    Likes Received:

    I want to know how to connect 2 different subnet through LAN card only without cisco router.and i want to know configuration of this simple scenario.

    REena Ahuja
    reena_ahuja24, Feb 28, 2009
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