Do all routers really must know all routes to every router?

This is my complicated and long question. First let's talk about the context.

Network topology: (RT stands for router) enter image description here

PC A --- RT A --- RT C --- RT B --- PC B

(RT C has a WAN NIC connected to "the cloud")

With this situation :

PC A must send a packet to PC B
Default routes direct packets to the cloud
We haven't access to RT C's configuration
RT C only knows how to join network A, not network B
RT A knows about network B
RT B knows about network A

RT C's routing table:

Destination NIC     Gateway
0.0.0.0     WAN     Cloud
Network A   LAN A   RT A's WAN

RT A's routing table:

Destination NIC     Gateway
0.0.0.0     WAN     LAN A
Network B   WAN     LAN A

RT B's routing table:

Destination NIC     Gateway
0.0.0.0     WAN     LAN B
Network A   WAN     LAN B

I would like to permit PC A and PC B to communicate, but I don't have access to RT C. Networks B and BC are new.

Can PC A send a packet to RT B's WAN NIC (which is possible) and "ask RT B to direct the packet to PC B" ?

I believe replacing RT B with a VPN server should do the trick, but I would like to know if it is possible to make it without establishing a new connection.

Philipili

Posted 2012-10-12T17:51:02.300

Reputation: 80

You say it is possible for PC A to send a packet to RT B’s WAN NIC. How is this possible if RT A and RT B are not on the same network, and RT C doesn’t know that Network B exists? – Scott – 2012-10-12T22:01:34.860

Because of the default route of RT A if I remember well. – Philipili – 2012-10-13T09:23:32.947

I don’t understand that. You say RT A’s default route is out the WAN NIC to LAN A, which is one of RT C’s addresses. And RT C doesn’t know that Network B exists — or even Network BC. – Scott – 2012-10-15T16:13:53.197

Well, it knows network BC, because it is a network directly connected to it. – Philipili – 2012-10-16T06:25:27.493

Do all routers really must know all routes to every router?

Answers