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secondary IP address on VLAN

hi,

i have allocated a secondary IP Address to the VLAN which is already having a IP .

what i need to know that could it create any problem..in any sense (broadcast, etc)

1 REPLY
Hall of Fame Super Silver

Re: secondary IP address on VLAN

Gaurav

I do not see how a secondary address in and of itself could cause any problem, broadcast or anything else. If you added the secondary address because the number of end stations in the VLAN exceeded the size of the existing subnet, then the increased traffic from the number of end stations may be something you need to consider.

End stations in one subnet that want to communicate with end stations in the other subnet may need to use the router (or layer 3 switch) to reach each other. So there may be an increase in the amount of traffic in and out of the interface. But I doubt that it would be enough to be a problem. I especially think it will not be a problem because many of the end stations will communicate directly without needing the router. Some end stations may ARP for the destination and since they are in the same broadcast domain (that is one definition of VLAN is it not) they will get an ARP response and communicate directly. Other stations will send the initial packet to their default gateway on the router and the router will generate an ICMP redirect and the stations may communicate directly after this.

There are a few things to remember about using secondary addresses. If there is more than one router with interfaces on this subnet/VLAN both routers should be careful to list the same subnet as the primary address. If the router runs a routing protocol the router will source its routing packets with the address of the primary address. If the routing protocol is EIGRP or OSPF they will form adjacencies on the primary address and will not form adjacencies on the secondary address.

The above comments are not problems with secondary addresses, they are just things that you need to be aware of to get the best performance from secondary addresses.

HTH

Rick

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