1720 - Two IP's on same FastEthernet interface

[lhuegele]

Is it possible to have two IP addresses assigned to the one FastEthernet interface? Our router is a Cisco 1720 running 12.1 IOS. I want to use this router as the gateway for two internal subnet's. Also, what is the integrated AUX port used for?

Thanks for any help!
Larry

 

[svermill]

Since I don't think the 1720 with 12.1 code does trunking, you're stuck with a secondary address (not recommended in most situations, but it'll do in a pinch).

'ip address 10.1.1.2 secondary'

The aux port is often used as a dialin or dialout port. It can be dialed into as a console for remote management or it can be configured to actually pass "user" data.

 

[lhuegele]

Thanks for the info svermill, but it won't take the secondary address for some reason. It doesn't recognize the "secondary" part of the command.

 

[svermill]

I should have said:

'ip route 10.1.1.2 255.255.255.0 secondary'

or perhaps more generically:

'ip route x.x.x.x y.y.y.y secondary'

Sorry.

 

[Crabzilla]

On our 2600 and 3600 routers running IOS 12 the statement is

ip address 192.168.212.52 255.255.255.0 secondary

The IP ROUTE statement is for adding static routes not secondary addresses.

 

[svermill]

Man I just couldn't get it right yesterday. Thanks Crabzilla!

 

[mstreule]

Hi,

The command is:

interface fas0/0
ip address <address> <mask> secondary

Be sure also to add
&quot;ip route-cache same-interface&quot;
If you don't do so, the packets between this subnets will be process switched (using CPU power) and this will slow down everything.

If you have trouble with the syntax of IOS, you can use the &quot;?&quot; at every place. (Much easier then learn all the syntax stuff - you need your brain for more interesting stuff).

The AUX port can be used to connect a modem if you want &quot;out-of-band&quot; management of this router (connect to a console even if your WAN is broken).

Good luck,
Martin

 

[lhuegele]

Thanks to everyone for helping on this, especially svermill! Even though you said route, I knew you meant address and it worked great! I haven't added Martin's ip route-cache line yet, but if I have any speed problems, I'll be sure to add it!

Happy New Year to everyone!
Larry

 

[Reisinger]

I would suggest creating a sub interface and not a secondary to accomplish this. A subinterface will allow you more flexibility down the road to add acces-lists route-maps or even ISL trunking on your single FA interface. Just a suggestion but I think you will see the benefits later on.

 

[l3stuff]

Please add the &quot;ip route-cache same-interface&quot; command....

 

[lhuegele]

I added the &quot;ip route-cache same-interface&quot; command but haven't noticed any difference as of yet...everything has been working great! Thanks to everyone.
Laryr

 

[svermill]

It would only apply to traffic between the two attached subnets. So unless you have a server in one that is being accessed from the other (or something along those lines), I doubt you would see much difference. But it's good policy anyway.

 

[lhuegele]

Actually, all of our servers are on the first subnet. The original designers of our network never anticipated we'd need more than 254 IP addresses. I really appreciate the help of this forum and others on Tek-Tips! Cheers!

 

[svermill]

That being the case, you might not have noticed a performance difference per se, but you likely offloaded a lot of preasure on the CPU. If you were to 'sh proc' with and without the route-cacheing, you likely would see some change (assuming you did this while there was some measurable amount of traffic crossing the subnets). Without the command, every packet that crosses the subnet boundary gets attention from the processor. Not good on a low-end router.

 

[lhuegele]

Thank you for the knowledge and info. Last thing I want is a sluggish or crashed router. <smile>

Cheers!