Quick question about secondary addresses

Sat Jul 31 17:38:40 UTC 2004

Jon Lewis wrote:

> On Sat, 31 Jul 2004, Jesper Skriver wrote:
> 
>>On Fri, Jul 30, 2004 at 10:21:06AM -0700, Dan Lockwood wrote:
>>
>>>I'm in a debate with a guy over the use of 'ip address x.x.x.x s.s.s.s
>>>secondary' on Cisco gear.  I seem to remember reading that the use of
>>>secondary addresses is a bad idea, but I can't recall the details of
>>>why.  Process switched?
>>
>>No, traffic to hosts within a subnet configured as secondaries
>>will be CEF switched.
>>
>>The only "bad" thing I can think of with secondaries, is that it's often
>>not what you want, why not split it on layer 2 as well, and get the
>>benefit of a smaller broadcast domain ?
> 
> A few other possible issues:
> 
> 1) routing protocols (i.e. ospf) will not form adjacencies with devices in
> the secondary address subnets...so if you're doing this to get more
> address space on a particular ethernet without renumbering, if you need
> OSPF on the ethernet, all the OSPF speakers have to be in the primary
> subnet.
> 
> 2) If you're doing this to separate customers, it doesn't really.  They're
> all free to steal each others IPs.  Better solutions would be VLAN
> trunking back to the router with a subint for each subnet or a L3 switch
> effectively doing that all in one box.

I meant to add (but apparently didn't sent the reply where I thought I
did):

Depending on traffic flows, the "one-armed" routing (bouncing the
traffic from one IP net to the other off the router) can be a
significant issue for the router.

> 3) Human error.  More than once I've seen someone change an interface's
> primary IP by "adding a secondary" and hitting return before typing
> "secondary".  Maybe it would have been better/safer if the command were
> "secondary ip addr ..." :)

That is an especial treat when you do it the interface you are talking
to the router on.

I always set a secondary on the most-likely-to-be-the-managment
interface and left it there and used it for managment sessions.

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