Secondary addressing and subinterfacing serve different purposes. In
my less than humble opinion, the major applications for secondary
addresses are workarounds for problems caused by the dinosaur of
classful addressing. These are discussed in detail in Chapter 5 of my
_Designing Addressing Architectures for Routing and Switching_, but
some applications include:
-- making more hosts available than are available with a fixed
subnet mask
-- fixing discontiguous networks
-- dealing with especially stupid host stacks that assume default masks
The underlying concepts of secondaries and subinterfaces are
different. Secondaries (called multinets in Bay-speak) map multiple
subnets/networks to a single broadcast domain, NBMA medium, or
point-to-point link. Subinterfaces separate the broadcast domains on
a medium with layer 2 multiplexing (e.g., DLCI, VLAN ID, ATM VC,
etc.).
There are some complex VLAN configurations where it can even be
useful to have secondaries on subinterfaces. Personally, I shudder at
the thought.
"What Problem are you trying to solve?"
***send Cisco questions to the list, so all can benefit -- not
directly to me***
Howard C. Berkowitz [EMAIL PROTECTED]
Technical Director, CertificationZone.com
Senior Product Manager, Carrier Packet Solutions, NortelNetworks (for ID only)
but Cisco stockholder!
"retired" Certified Cisco Systems Instructor (CID) #93005
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