[nsp] Best Practice for Secondary IP addresses on interfaces

Brian Wallingford brian at meganet.net
Sun Jan 19 19:27:59 EST 2003

On Sun, 19 Jan 2003, Jon Allen Boone wrote:

: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).

I wholeheartedly agree.  Use of VLANs also provides more flexibility wrt
ACLs and IGPs (i.e., you can do a passive-interface fX/Y.NNN, while
allowing others to announce routing info).


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