[nsp] Best Practice for Secondary IP addresses on interfaces

[Jon Allen Boone]

On Sunday, Jan 19, 2003, at 01:49 US/Eastern, Brian R. Watters wrote:

> We have a few routers (7206's) which interface via ethernet 100 and
> CAT2924 and Extreme 24 port switches .. What we would like is to get
> some real world (From an ISP's) perspective on just what the best
> practice is for routing block's of IP space out to CAT switches ..
> Secondary IP addresses on eth interfaces or VLAN's? Good or bad for 
> both
> .. In some cases we have 5 to 6 class C IP blocks being routed out over
> eth interfaces .. Any insight or direction would be great!
>

VLANs are a good idea, especially if you want to use OSPF/IS-IS routing 
protocols.  For example, you can determine which OSPF area an interface 
is to be put in based on it's primary address, but *not* based on it's 
secondary addresse(s).

Another consideration is that choosing to not use VLANs means that all 
devices on that Ethernet will be in the same broadcast domain - which 
could be a potential problem if you have to support legacy protocols 
that are notoriously "chatty" [i.e. AppleTalk or Novel IPX].

I tend to prefer separate VLANs to the use of secondaries where it's 
practical to do so.

--jon
CCIE #8338