This is going to be a short post. I feel guilty for neglecting my blog (stupid ITIL), so I’m throwing this one together real quick.

There was a thread on Tech Exams recently, the poster was trying to figure out how to connect two subnets to a single Ethernet interface on a router. This was due to changing the address scheme within his company. Here’s what it looked like:

Very simple topology. The PC is part of the new address scheme and the printer is using a static IP from the old scheme. One of the easiest solutions here is to use a secondary IP on the Fa0/0 interface of R1. Here’s how it works:

EDGE(config)#int fa0/0
EDGE(config-if)#ip add 192.168.10.1 255.255.255.0
EDGE(config-if)#ip add 10.1.1.1 255.255.255.0 secondary

Very simple commands. Let’s verify:

interface FastEthernet0/0
 ip address 10.1.1.1 255.255.255.0 secondary
 ip address 192.168.10.1 255.255.255.0

It worked, the interface now has two IPs. Now we would just configure the IPs on the hosts (or DHCP server) using the proper subnet and gateway. Once the hosts are configured, everything will work as desired.

One thing to keep in mind, secondary IPs do not appear in the output of “sh ip int b”, but the secondary network will be shown with “sh ip route”:

EDGE#sh ip int b fa0/0
Interface                  IP-Address      OK? Method  Status                Protocol
FastEthernet0/0         192.168.10.1  YES manual     up                      up
 
EDGE#sh ip route 10.1.1.0
Routing entry for 10.1.1.0/24
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via FastEthernet0/0
      Route metric is 0, traffic share count is 1

Remembering that secondary IPs do not appear in “sh ip int b” can be important for troubleshooting.

Last we’ll verify that the gateways are pingable:

EDGE#ping 192.168.10.1
Sending 5, 100-byte ICMP Echos to 192.168.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
 
EDGE#ping 10.1.1.1
Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

That’s it for this one. Secondary IPs are a simple concept, but many people are not exposed to them. We use them quite a bit in my company at a lot of remote sites. It’s kind of a hack job, but they can be pretty useful.

Colby

Colby Glass has been in IT since 2002. He is currently a network engineer with a Cisco Gold partner and holds the CCNP R/S, CCNP DC, CCDP, CCIP, JNCIA-ER and ITILv3: Foundations certifications.

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