secondary ip address

[fujin.huang]

Hi,

Is there any document describe how to use secondary ip address? it seems can be used for discontiguous network, frame relay, etc. What is the guideline of using secondary ip address? is there any cisco tech notes on this topic?

Thanks for the help

Fujin

[jkemery]

Try this from a search on "Secondary IP address" on CCO:

"The optional keyword secondary allows you to specify an unlimited number of secondary addresses. Secondary addresses are treated like primary addresses, except the system never generates datagrams other than routing updates with secondary source addresses. IP broadcasts

and ARP requests are handled properly, as are interface routes in the IP routing table.

Secondary IP addresses can be used in a variety of situations. The following are the most common applications:

There might not be enough host addresses for a particular network segment. For example, your subnetting allows up to 254 hosts per logical subnet, but on one physical subnet you need to have 300 host addresses. Using secondary IP addresses on the switches allows you to have two logical subnets using one physical subnet.

Two subnets of a single network might otherwise be separated by another network. This situation is not permitted when subnets are in use. In these instances, the first network is extended, or layered on top of the second network by using secondary addresses.

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Note If any switch on a network segment uses a secondary address, all other switches on that same segment must also use a secondary address from the same network or subnet. Inconsistent use of secondary addresses on a network segment can very quickly cause routing loops.

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Example

In the following example, 131.108.1.27 is the primary address and 192.31.7.17 and 192.31.8.17 are secondary addresses for main Ethernet 0 interface.

Switch# configure terminal

Switch(config)# interface ethernet 0

Switch(config-if)# ip address 131.108.1.27 255.255.255.0

Switch(config-if)# ip address 192.31.7.17 255.255.255.0 secondary

Switch(config-if)# ip address 192.31.8.17 255.255.255.0 secondary

http://www.cisco.com/univercd/cc/td/doc/product/atm/c8540/12_0/4a_11a/command/i.htm#xtocid2996614

[sampathsr]

we got two sets of /24 subnets fro our ISP. One of the addresses is primary and the other is being used as the secondary address.

Gives us the flexibility to use all the addresses even though we have only one ethernet port on the router!

[jblancke]

I have done this on about 75 routers, We are slowly readdressing our network. The most success has come on the Ethernet and leased lines. We have had no luck on frame relay with point to point interfaces. I have also seen some troubles with EIGRP not finding all the routes (the secondaries). We have had very little troubles on the Ethernet, just a VPN that stoped working after the secondaries. The other area that I seen trouble is in the case with a switch between two routers. All primary addresses should be on the same subnet on a per link basis. All secondaries should be on the same subnet on a per link basis. Remember to add the network to your routing protocol.On leased line ( not frame relay) I have been able to readdress the link with secondaries with out the link going down. The users didn't even know it was readdressed, With a 1 minute outage they light my phone up like a christmas tree. I think that secondary addressing is a very usefull feature. If you go to www.cisco.com and do search on secondary addressing you will find most of what you need and will learn ways it has been used in the past.

Regards

James