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sebastiaan_ij
sebastiaan_ij
New Member
February 18, 2013
Question

Reasons to add a secondary IP address on a interface

  • February 18, 2013
  • 3 replies
  • 6172 views
Hello everybody, I have a question about the use of a second IP address on a interface. It basically is the why question, why would you do such a thing?? In this particularly case the main interface is part of a trunk, so it has a vlan ID configured. In my opinion if you add secondary IP addresses you also introduce different subnets (and different broadcast domains) in the same vlan, which is hardly a best practice. It will work I quess but it' s asking for trouble I think, the problem is I can' t think of a proper argument against it other than a gut feeling I have. What do you guys think? Am I overreacting or is there a real reason not to do it?

    3 replies

    emnoc
    emnoc
    New Member
    February 19, 2013
    Your gut feeling is correct, one benefit of secondary tho.... is when you migrating a network from old to new ip space and you want the migration transparent. Outside of that, I prefer to go what you described earlier, separate collisions and broadcast domains and just craft a 802.1q sub-intf
    ddskier
    ddskier
    New Member
    February 19, 2013
    It is also useful when you want the port to have a rout-able IP and also the announced IP when using BGP.
    Rick_H
    Rick_H
    New Member
    February 19, 2013
    Staggered network address migrations are the primary reason I' ve had to use the feature so far. This goes for both Fortinet and other vendor gear. Being able to re-address devices on a network in stages is a real boon with even medium-sized networks. I' ve also had to use secondary addresses in a scenario relative to VPN: I wanted to use an IP address other than the interface address as the VPN terminator. In this case, though, it was a secondary IP address in the same subnet rather than a wholly new one.
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