RE: [nsp] two interfaces on the same LAN with same subnet

From: Scott.Keoseyan@BroadWing.com
Date: Thu Nov 15 2001 - 19:39:58 EST


Is this connecting from a switch to a router? If you just put two
interfaces in a bridge-group and attached them to the same VLAN I suppose
you'd have a spanning-tree loop and one of the ports would be blocked. If
you try to place an IP address from the same subnet on two different
interfaces, the router is going to complain that the subnet already exists
on the other interface and therefore not allow you to do it.

What you need to do, if you have a cisco router with fast-ethernet, is
configure fast-etherchannel to the switch. Place these ports on the switch
in the same vlan, and number the virtual interface you create for the router
(interface port-channel x) with a single IP. Take the physical
fast-ethernet ports on the router and place them into the channel-group you
want them to be in (this number matches the port-channel interface
number)... set the speed to 100mbps and the duplex to full on all 4
interfaces as well. This will more or less double your bandwidth into the
switch.

Unfortunately you can't do this with every model router, but most switches
do support it.

Scott

> -----Original Message-----
> From: Andy Harding [mailto:andyh@verio.net]
> Sent: Wednesday, November 14, 2001 12:40 PM
> To: cisco-nsp@puck.nether.net
> Cc: cisco-nsp@puck.nether.net
> Subject: RE: [nsp] two interfaces on the same LAN with same subnet
>
> hi Neil,
>
> if memory serves, you need a bridge-group - effectively a vlan in old
> nomenclature:
>
> bridge <bridge-group-#> protocol ieee # valid bridge-group-#s 1-63
> # protocol is stp - ieee|dec
> int f0/0
> bridge-group <bridge-group-#>
> ip address 10.0.0.1 255.255.255.0
>
> int f0/1
> bridge-group <bridge-group-#>
> ip address 10.0.0.2 255.255.255.0
>
> hth
>
> -andy
>
>
>
> Neil J. McRae wrote on 14 November 2001:
> >
> > I'm sure this has been discussed before but I can't find any info on.
> >
> > We need to have two interfaces on the same subnet into the same
> > switch but we can't use ISL/VLAN/.Q etc. So we effectively double
> > the bandwidth to that LAN.
> >
> > Anyone solved this problem?
> >
> > Neil.
> >
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