Subnet size between Core and WAN Layer

Mikey John · ‎08-17-2016

Hi,

For a medium to large sized networks, what is the general subnet size we should "ideally" go for between the Core layer and WAN layer devices? There would be OSPF running between my Core and WAN layer, and I generally use /30s (including cross links) between these two layers. I have attached a sample diagram for your reference.

Does it make sense to use a /29 instead? Like for example, Wan1, Core1 and Wan2 on a /29 network and Wan1, Core2 and Wan2 on another /29 network?

Even if we look at expanding a network (and include Multi VRF), we would still have 2 x Core switches and 2 x WAN routers, and using /30s between these networks would suffice. We would be wasting IPs if we use /29s.

Appreciate if someone could help figure out the Pros and Cons of /30s vs /29s.

Thanks

Mikey

Richard Burts · ‎08-17-2016

Mikey

Your drawing is quite clear how you have set up the network using /30. It is not clear how you would set it up for /29. The logical thing would be that core 1 and core 2 would each have a separate vlan with two interfaces connecting to the two WAN routers. Or is there some other setup that you have in mind? I believe that it is more simple and better to use /30 subnets.

A /30 is a block of 4 IP addresses. You would need 4 of these subnets so you would use 16 IP addresses. A /29 is a block of 8 IP addresses. You would need 2 of these subnets so you would use 16 IP addresses. There is not really any efficiency or any waste comparing the use of IP addresses in either approach.

HTH

Rick

HTH

Rick

Mikey John · ‎08-17-2016

Thanks Rick. Yes, my intent is the same with /29s. Create two VLANs on the core switches for the two uplink interfaces.

I would agree with you that it is simpler with /30s. I can't find any reason not to use them :)

Thanks

Mikey

Joseph W. Doherty · ‎08-17-2016

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You could consider going the other way, i.e. use /31s for your P2Ps.

As to using P2P vs. /29s, Rick described that from a IP space usage, it's a wash between /29s and /30s, but using P2P vs. /29 has a few other advantages and a disadvantage. With P2P OSPF adjacency outage made be tied to the physical link where as with VLANs you usually rely on dead timers. If you do multicast, multicast may flood to all multicast routers on transit networks larger than /30. (Cisco does have RGMP to counter this issue.) A disadvantage of using P2P, more networks to pass around your topology.