Secondary IP on subinterface

[pbxnkey]

I have a router that has 2 IP's on the fastethernet with different subnets. I need to change the interface to a subinterface . I don't see an option to create a secondary IP on the subinterface. Is there a way around this other than creating a subinterface for each subnet?

 

[VinceWhirlwind]

I was having a chat about secondary IP addresses with a colleague just this morning: they used to be quite common in the past, people would have multiple subnets on the same broadcast segment. It was a carry over from times when network infrastructure didn't support VLANs. Now, you have 1 subnet = 1 VLAN, so no secondary IP addresses. On routers, each VLAN/subnet is therefore hanging off its own subinterface.
In your situation, you essentially need to redesign your network in terms of where that second subnet is deployed.

 

[iggsterman]

Secondary IP address capability is quite alive and used when needed. You can put it on a physical interface or sub-interface, does not matter. Below is the example of 2 routers with IOS 15.1 and IOS 12.4 and how to make sub-ifs with 2 addresses each. I also have had it working on IOS 12.1, 12.2 and 12.3. Bear in mind that this is typically done for transition purposes. A healthier design would be VLAN carrying a single subnet. However, techically, NOTHING prevents one from sticking >1 subnet into a VLAN, except for design considerations, or course.
Let me know if this is not clear and I hope this helps.

ROUTER 1 (7200)
----
cre-gw1-in#sh ver | i IOS
Cisco IOS Software, 7200 Software (C7200-ADVENTERPRISEK9-M), Version 15.1(4)M3a, RELEASE SOFTWARE (fc1)
...........

cre-gw1-in#sh run | sec interface
interface FastEthernet0/0
no ip address
duplex full
interface FastEthernet0/0.1
encapsulation dot1Q 1 native <-----------------------FOR THE TRUNK TO COME UP
interface FastEthernet0/0.10
encapsulation dot1Q 10
ip address 10.10.2.1 255.255.255.0 secondary
ip address 10.10.1.1 255.255.255.0
interface FastEthernet0/0.20
encapsulation dot1Q 20
ip address 10.20.2.1 255.255.255.0 secondary
ip address 10.20.1.1 255.255.255.0
...........
The below will only show ONE address per if/subif:

cre-gw1-in#sh ip int b
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
FastEthernet0/0.1 unassigned YES unset up up
FastEthernet0/0.10 10.10.1.1 YES manual up up
FastEthernet0/0.20 10.20.1.1 YES manual up up
====================

ROUTER 2 (3660)
-----
cre-gw5-in#sh ver | i IOS
Cisco IOS Software, 3600 Software (C3660-IS-M), Version 12.4(16), RELEASE SOFTWARE (fc1)
...........

cre-gw5-in#sh run | sec interface
interface FastEthernet0/0
no ip address
duplex auto
speed auto
interface FastEthernet0/0.1
encapsulation dot1Q 1 native
interface FastEthernet0/0.10
encapsulation dot1Q 10
ip address 10.10.2.2 255.255.255.0 secondary
ip address 10.10.1.2 255.255.255.0
interface FastEthernet0/0.20
encapsulation dot1Q 20
ip address 10.20.2.2 255.255.255.0 secondary
ip address 10.20.1.2 255.255.255.0
...........

cre-gw5-in#sh ip int b
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset up up
FastEthernet0/0.1 unassigned YES manual up up
FastEthernet0/0.10 10.10.1.2 YES manual up up
FastEthernet0/0.20 10.20.1.2 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
...........
From the 1st router:

cre-gw1-in#ping 10.10.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/29/32 ms
cre-gw1-in#ping 10.10.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/28/36 ms
cre-gw1-in#ping 10.20.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.20.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/32/40 ms
cre-gw1-in#ping 10.20.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.20.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/27/28 ms

 

[pbxnkey]

Actually it does help. I did not have a spare router to test so I was using Packet Tracer and it did not allow me to add a secondary address on a subinterface. I will have to see if I can dig up a spare router.

 

[iggsterman]

Have you tried GNS3? It is very helpful.

 

[pbxnkey]

My laptop is too slow and it slows even more when I use gns3