Secondary IP on interface

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Sep 2nd, 2009
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Hi,


In our network in intial deployment we had some requirement to use the secondary IP on the same interface to have seperate broadcast domain


but is there the right way of doing it on the interface


and also when we need to run HSRP on the interface we are using the standby IP for the same


can u suggest us is there will any disadvantage using the secondary IP

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ktwaddell Wed, 09/02/2009 - 01:28
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Hi


For us we normally use a secondary ip address when we are running out of ip addresses in the primary subnet, but what ever reason we can't re-address that LAN, so we bang a secondary ip address in.


As far as I know, there is only 1 way to configure it, so that is the right way.


and sorry I didn't understand what you are asking on the 3rd question.

Joseph W. Doherty Wed, 09/02/2009 - 03:50
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A secondary IP address resides in the same broadcast domain (except, perhaps, for a directed IP broadcast) as the primary IP address.


If you really want to isolate network segments, you'll want to use multiple VLANs on the interface (usually trunks for L2, VLAN subinterfaces for L3).

Wilson Samuel Wed, 09/02/2009 - 04:14
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Hi Vinoth,


Technically Broadcast Domain remains same unless there is a physical router involved (after all the packets are hitting the same Network Card).


As far as I understand, using Secondary IP Address has one primary disadvantage and that is one can't use any Routing Protocol for that Subnet/Network Add.


Also, this issue could (if this is an issue in the first place) by using Subinterfaces, that way one can still use the Routing Protocols.


However technically its still the same Broadcast Domain (because Broadcast Packets are still hitting the Interface at L-2)


Hope this helps.


Please rate all helpful posts.

Richard Burts Wed, 09/02/2009 - 07:44
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Samuel


Perhaps we can clarify what you mean when you say:"Secondary IP Address has one primary disadvantage and that is one can't use any Routing Protocol for that Subnet/Network Add."


When you say that you can not use a routing protocol are you saying that you can not advertise the subnet of the secondary using a routing protocol, or are you saying that you can not establish routing protocol neighbor relationship using the secondary address?


The first is not true: you can advertise the subnet of the secondary using a routing protocol. The second is true: the routing protocol will not use the secondary address to establish a neighbor relationship.


HTH


Rick

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