Ryan
Actually in the example I sent the interfaces were already up.
Since there appears to be some interest in this possibility I have done
a more complete test and here are results which show that the
overlapping
addresses are accepted and provide connectivity.
I am not authoritative about why it is allowed but I think I remember
reading at some point in the docs that this was intended to support a
backup scenario.
I know that it does work but I do not know if there would be surprises
for someone who did this in a live environment. It is not an
implementation that I would recommend.
* * * *
configure R1 serials
R1#
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int s 1/2
R1(config-if)#ip address 14.0.0.2 255.255.255.0
R1(config-if)#int s 1/4
R1(config-if)#ip address 14.0.0.1 255.255.255.0
R1(config-if)#^Z
R1#
Apr 12 11:25:40: %SYS-5-CONFIG_I: Configured from console by console
* * * *
demonstrate that the configuration accepted and the interfaces are up
R1#sh ip int brie
Interface IP-Address OK? Method Status
Protocol
Serial1/0 12.0.0.1 YES manual up
up
Serial1/1 unassigned YES unset up
up
Serial1/2 14.0.0.2 YES manual up
up
Serial1/3 12.0.0.129 YES manual up
up
Serial1/4 14.0.0.1 YES manual up
up
Serial1/5 13.0.0.1 YES manual up
up
Serial1/6 unassigned YES unset administratively down
down
Serial1/7 unassigned YES unset administratively down
down
Loopback0 192.168.1.1 YES manual up
up
Loopback10 192.168.10.1 YES manual up
up
Loopback11 192.168.11.1 YES manual up
up
Loopback12 192.168.12.1 YES manual up
up
Loopback13 192.168.13.1 YES manual up
up
Loopback14 192.168.14.1 YES manual up
up
Loopback15 192.168.15.1 YES manual up
up
R1#
* * * *
now trace to the address of one of the physical interfaces of the remote
router. Responses are received from both interfaces.
R1#
R1#trace
Protocol [ip]:
Target IP address: 14.0.0.3
Source address:
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]: 24
Minimum Time to Live [1]:
Maximum Time to Live [30]:
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Type escape sequence to abort.
Tracing the route to 14.0.0.3
1 14.0.0.4 16 msec
14.0.0.3 16 msec * 16 msec
14.0.0.4 16 msec * 16 msec
14.0.0.3 16 msec * 16 msec
14.0.0.4 16 msec * 16 msec
14.0.0.3 16 msec * 16 msec
14.0.0.4 16 msec * 16 msec
14.0.0.3 16 msec * 16 msec
14.0.0.4 16 msec *
R1#
If you are interested in more detail of the test I can provide it
off-list.
Rick
Ryan O'Connell wrote:
>
> On 11-Apr-2002 Rick Burts wrote:
> > Actually what you say is true for LAN interfaces but not for serial
> > interfaces. The IOS will allow serial interfaces to be configured
> > within the same subnet as the following example shows:
> >
> > Router_D#conf t
> > Enter configuration commands, one per line. End with CNTL/Z.
> > R1(config)#int serial1/0
> > R1(config-if)#ip address 192.168.1.1 255.255.255.240
> > R1(config-if)#int serial1/1
> > R1(config-if)#ip address 192.168.1.2 255.255.255.240
> > R1(config-if)#
>
> That surprises me. Does it work if you "no shut" both interfaces? I suspect it
> won't. If it does, not sure if it's intentional or a bug, 'cos I can't see how
> the routing is supposed to work on that. Any idea why it's permitted?
>
> --
> Ryan O'Connell - CCIE #8147
> <ryan@complicity.co.uk>
> http://www.complicity.co.uk
-- Rick Burts CCIE 4615 CCSI Email: rburts@netcraftsmen.net Chesapeake NetCraftsmen 1290 Bay Dale Drive, #312 443.994.0675 Arnold, MD 21012-2325 WWW: http://www.netcraftsmen.netWith 7 CCIEs on staff Chesapeake NetCraftsmen offers services in network consulting and training. Our services include Network Design, Implementation, Troubleshooting as well as Network Management.
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