Help me understand /30 Internet space

[mikey71497]

I am having a hard time grasping how a customer would set this up. I am a MetroE tech with a MSO company, and when a customer buys a internet circuit from us, we nail up a /30. The customer needs two interfaces on their router in order to get the internet to work. One ethernet interface will be the actual /30 P2P IP addy or the "WAN" and the other ethernet interface will face their local LAN. My question is, how do you get traffic from the local LAN to egress out the /30 interface which will eventually switch through our core and to the outside world. Does the customer need to setup a default route from one interface to the other? Can that even be done on the same device?

 

[HHunt]

I think this is the usual solution:
Give the LAN interface an RFC1918 IP, offer IPs in the same network over DHCP, with the router's LAN IP as default route. The router then does NAT and routing.

If I'm not mistaken, a /30 contains two useful addresses ... I think it's possible to arrange for one of the computers on the LAN to get the other IP, if it's really important. If you have to, try this: Set up an RFC1918 network like above. Set up the router and PC for PPPoE, and use that to assign the other free IP to the PPP interface on the PC.

 

[Vito_Corleone]

A /30 allows two devices on a link. There are two usable IP. The scenario you describe could be a number of things.

You could have SP---/30---CustomerR1----RFC1918-LAN. The LAN could be 1918 space, and the customer could be NATing everything to their IP on the /30.

You could have SP---/30---CustomerR1---Public(/26)-DMZ/Extranet/etc---CustomerR2---RFC1918-LAN. In this case, you have a /30 between the customer and the SP and some public space behind the customer's router. The SP would have a static route (or even dynamic routing) for the /26 to the customer side of the /30. This is where external services would sit.

Last scenario (that I'll post here, though there could be tons more). SP--/28--CustomerR1---RFC1918-LAN. In a case like this, the customer is given a block of 16 IPs (13 usable), which they're using to NAT the private space on the LAN.

Lots of ways to connect a customer to the internet. To answer the other part of your question, yes, a default route will point to the LAN/inside of the customer router. Assuming the customer is single homed to the internet, all traffic will flow to that router, then over the /30 to the SP.

 

[HHunt]

Oh yes, duh - the other IP in a /30 is on the ISP end. Nevermind me.x)