At 5:33 PM +0000 6/26/03, Priscilla Oppenheimer wrote: >It's not a dumb question, as far as I can tell, but it's awfully hard to >parse due to your stream of consciousness style and lack of paragarpahs. >White space is a good thing. :-) > >There's nothing wrong with IP secondary addresses as far as I know. They can >come in handy. And yes you can have multiple ones. At some point you should >design your network more hierarchically and get rid of them perhaps, but >that's for another thread.
I think people tend to forget that secondaries and subinterfaces are not freely interchangeable, although they do have many common characteristics. Indeed, there are times when using secondaries on subinterfaces makes perfectly good sense. There are times to use secondaries alone (e.g., healing discontiguous networks on point-to-point lines). There are times to use VLANs alone (e.g., separating broadcast domains; connecting multiple physical LAN segments). The key difference is the L3:L2 relationship. Let me restrict this to broadcast media for simplicity. A subinterface assumes one subnet maps to one broadcast domain. A secondary address maps multiple subnets to the same broadcast domain. Mapping multiple subnets to a common broadcast domain can be bad for performance, especially if there are ill-formed multicast or broadcast implementations around. But if the multicasters are well disciplined, the combination of secondaries and subinterfaces, for example, can be very useful in supporting multiple DHCP servers on several subnets. A digression, I suppose, but one thing that I particularly hate is the ability to put a port into several VLANs. Cisco did this as a competitive response to a 3Com feature, and, as far as I'm concerned, it's to be condemned in IP networking. The feature made some sense in NetBIOS over data link, where you had to broadcast to resolve addresses, and there was no network layer. But now, if you are in an IP world, and someone says to use this technique to share a printer across several VLANs, you now put the constraint on the _host_ to be able to support secondaries. > >The one caveat is that packets in and out the same interface, for the few >cases that you mentioned that they do communicate (through this interface?), >you need to enable fast switching. It's not on by default. And.... hmmmm... >I can't remember the command. Maybe somebody else will remember it. ip | ipx route-cache same-interface Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=71472&t=71472 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
