/30 vs /24

[dacruzer1]

Hello All!

A network with a hub and spoke WAN.  The remote routers are connected via a Metro E. and then connects to the hub router.

My question is:  Is there an advantage of using a /30 for each connection or just use a /24 besides saving IP addresses ?

I hope this make sense.

 Thanks in advanced !

-  Dacruzer

[Joseph W. Doherty]

Answer also depends on the topology support and routing protocol used.

When you use a /24, by default, all the connected hosts will assume they have direct access to all the other hosts on that network.  I.e. all will assume they are in the same L2 broadcast domain.

As to "saving" IP addresses, maybe, maybe not.  With /30s you may actually leave more address space free as you only allocate for the needed p2p links.  With a /24, even for just one pair of hosts, you've allocated the whole /24.  Also, some newer routing protocols support /31s for p2p too.  With those, you don't "waste" an IP for the network and broadcast addresses.  If /31s are supported, from an IP usage viewpoint, likely they're the most "efficient" way to allocate IPs.

[Richard Burts]

Dacruzer

You tell us that the remote routers are connected by Metro E. It would help if we knew more about that Metro E. Does it use QinQ where each remote site has its own vlan? Or is it an Ethernet single broadcast domain. As Joseph points out, if it is a single broadcast domain then you need to use /24 and not /30. 

Another way of looking at this question is whether the remote connections will come into a single logical L3 interface (single IP address on the hub) or will come into individual subinterfaces of the physical interface?