Routing between subinterfaces

bjitima22 · ‎08-12-2005

I am working with a company that has run out of host addresses on their class C network. They have a static address on every computer, and do not wish to go through and change each computer to a different subnet allowing for more hosts. They asked that we add more class C networks to their router so that they could continue adding hosts on a new network and have the router send traffic between the networks.

They currently have 1.1.0.1 and 10.1.1.1 (secondary) assigned to the ethernet interface. The hosts reside on the 1.1.0.x network and three switch clusters reside on the 10.1.1.x network. The switches are 10.1.1.2, .3, and .4. The switches have a very basic configuration with an ACL that is set to permit ip any any.

I tried adding two subinterfaces to the router give them additional spacing with the following:

interface FastEthernet0/0.101

encapsulation dot1Q 101

ip address 1.1.1.1 255.255.255.0

and

interface FastEthernet0/0.1

encapsulation dot1Q 1

ip address 1.1.2.1 255.255.255.0

This did not work. We also tried editing the switches so that one port on each switch was configured as a trunk port.

interface FastEthernet0/1

switchport mode trunk

This was a suggestion from Cisco TAC. It did not work either.

Routing is configured as EIGRP with

network 1.0.0.0

network 10.0.0.0

configured.

I have attached configs for the router and the switch. If anyone has any ideas, please let me know.

Thanks

Ben Jitima

jitima2@marshall.edu

amit-singh · ‎08-12-2005

Hi,

The only this missing is the Native keyword on the sub interface fa 0/0.1. Try adding that on the interface and let us know.

nterface FastEthernet0/0.1

encapsulation dot1Q 1 native

ip address 1.1.2.1 255.255.255.0

regards,

-amit singh

noonanac · ‎08-12-2005

Hi,

I have a couple of responses which may help:

1) The quick and not so elegant way is I have noticed you do have a second class C subnet already there on 10.1.1.X range which you could use for clients.

And you should remove your sub-interfaces from your configuration.

2) You also have a spare Ethernet Interface you can use for another subnet.

**** The big issue with the above methods is that you pass all the traffic from one subnet to another over the router Ethernet interface(s), which will potentially swamp your router interfaces and delay your outgoing traffic. ********

3) A more elegant way is to use sub-interfaces entirely and you created all the subnets you wish to use ( sub-if 0.1 1.1.1.0/C, sub-if 0.2 10.1.1.0/C, sub-if 0.3 1.1.2.0/C......). On your switch connected to the router's ethernet port create a trunk, and create VLAN's for each of the IP subnets. Ensure your VLAN's are propagated between all your switches and that they are interconnected with trunk ports.

Check the document on VLAN's

http://www.cisco.com/univercd/cc/td/doc/product/l3sw/4908g_l3/ios_12/18w522a/config/vlancnfg.htm

4) Also you could make it very very simple and change your subnet from a class C to multiple of class C or even a class B, but you will have to change your client PC's subnet masks, too.

Andrew.

cool.cisco · ‎08-12-2005

I think you need to remove the ip address defined on fastethernet0/0 on the router and check it should work.

Another thing which Layer2 switch you are using. If its 3500xl series try using isl trunking on router.

Regards,