function of Secondary

[manojkumarnb]

Dear All,

I have got a querry,

To give an IP address to an interface of a router we use the command as

ip address ip-address mask.

This is clear. But I found in a book just as follows also,

ip address ip-address mask secondary

So my querry is what is the function of secondary in this command? For what we are using secondary word in this command? Do it has got any particular function? Please help me.

[Danilo Dy]

Cisco IOS software supports multiple IP addresses per interface. You can specify an unlimited number of secondary addresses. 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, suppose your subnetting allows up to 254 hosts per logical subnet, but on one physical subnet you must have 300 host addresses. Using secondary IP addresses on the routers or access servers allows you to have two logical subnets using one physical subnet.

* Many older networks were built using Level 2 bridges, and were not subnetted. The judicious use of secondary addresses can aid in the transition to a subnetted, router-based network. Routers on an older, bridged segment can easily be made aware that many subnets are on that segment.

* Two subnets of a single network might otherwise be separated by another network. You can create a single network from subnets that are physically separated by another network by using a secondary address. In these instances, the first network is extended, or layered on top of the second network. Note that a subnet cannot appear on more than one active interface of the router at a time.

Note: If any router on a network segment uses a secondary address, all other routers on that same segment must also use a secondary address from the same network or subnet.

[devang_etcom]

problems, if any, with secondary addresses occur mainly in conjunction with routing protocols, such as OSPF, which cannot establish adjacencies on secondary addresses. Other than that, you can add as many as the software allows you to. I guess the reason why multiple secondary addresses are being used, rather than multiple logical interfaces, is that you do not have to bother with trunking, inter-VLAN routing etc. when using secondary addressing. It is just a simple way of being able to add more hosts. I would be careful, though, to not add to many hosts, since that, and especially with regard to VoIP, can cause problems with delay and dropped packets, which VoIP usually doesn't handle very well.

secondary addressing dependency in routing protocols I would add another aspect to be careful about when configuring secondary addressing: be careful that all routers on the segment agree which subnet is the primary address.

Also be aware that any packet sourced by the router from that interface will use the primary address as the source address and will not ever source anything from the secondary address.

regards

Devang