Secondary IP Addresses

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
EDGE(config-if)#ip add secondary

Very simple commands. Let’s verify:

interface FastEthernet0/0
 ip address secondary
 ip address

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.

Spanning VLANs Across the WAN

A thread on Networking Forum led to some interesting conversation and a little mini-lab on my part. If you’re interesting in seeing a use for AToM in the real world, check out the thread.

This is the topology I ended up with:

To summarize, basically we ran a GRE tunnel across the WAN, then LDP and an xconnect across that. It was pretty fun to play with. So if you’re interested, check out the thread.


Colby Glass has been in IT since 2002. He is currently a Systems Engineer (presales) with a Cisco Gold partner and holds the CCNP R/S, CCNP DC, CCDP, CCIP, JNCIA-ER.

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