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Let router A connects two LANs L1 and L2 into an internetwork, and router B connects two LANs L3 and L4 into an internetwork.

Can the two internetworks be connected into an internetwork?

Is it done by connecting router A and B?

If yes, is the connection between A and B treated the same as an connection between A and LAN L1?

Let me clarify my last question. I assume routers A and B are just gateways between networks for internetworking, without having non-gateway features that are commonly added to personal routers. I am not familiar with gateway only routers, except reading a little on Tanenbaum's Computer Network book. But my last question comes from a comparison to personal routers. In my apartment, my personal router has a WAN port and several LAN ports. Back to the case here, do routers A and B (as gateways only) have different ports similarly? What ports of routers A and B are used to connect between A (or B) and the LANs and to connect between A and B?

Thanks.

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    This may be off-topic as it's essentially question about basic home equipment. Part of what makes them home equipment is that they make certain shortcuts and assumptions, which is why they have those specific port designations. Commercial routers can have a varying number of ethernet ports, each configured to multiple purposes.
    – Radhil
    Oct 6, 2015 at 2:08
  • I am not asking about home case. I just draw comparison to home case.
    – Tim
    Oct 6, 2015 at 2:26

2 Answers 2

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Routers switch layer-3 packets. It doesn't matter what interface. When a packet comes into an interface, if the router has a route for the packet, it switches out the interface toward the destination (assuming something like an ACL isn't blocking it). This could be toward an LAN, another router, etc.

There is a problem with comparing to consumer-grade equipment since that type of router typically has only a single physical router interface. Interfaces labeled LAN are usually switch interfaces that are connected to a single logical router interface rather than a physical router interface.

Logical interfaces count as router interfaces to/from which routers switch layer-3 packets. The problems with switch interfaces is that they only switch layer-2 frames, not layer-3 packets.

Routers A and B would need a link between them, but they would gladly exchange layer-3 packets. Somehow, Router A would need to know to switch packets to router B, either through a statically defined route, or through exchanging routes with Router B, and vice versa.

A home router has a built-in default route toward the WAN port, whereas a business-grade router needs to be explicitly told how to route anything not directly connected, even if it is a configured default route.

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i guess you speak about topology like this enter image description here

regarding to your first question , answer is sure
regarding to your second question L1,L2 can connect L3,L4 by routing configuration on both routers not just connect Router A to router B
regarding to your third question it is not clear enough but router A will use its own port P3 to reach the networks behind Router B and so router B will do it will use P4 to know about networks behind router A

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  • Thanks. for your reply to my third question, are the ports P3 and P1 on router A different?
    – Tim
    Oct 6, 2015 at 2:29
  • different in the sense similar to WAN port vs LAN port? or can P3 and P1 be treated the same, and thus exchanged?
    – Tim
    Oct 6, 2015 at 2:38
  • @Tim, as I explained, on your home router, the LAN port is not really a router port, but a switch port. Business grade routers can have many types of physical interfaces, ethernet, token ring, serial , etc. The ports on a router could all be the same or different. You can't compare the LAN side of a home router with a business-grade router.
    – Ron Maupin ♦
    Oct 6, 2015 at 2:45
  • @Ron: Thanks. ok, I see no LAN port aka switch port. Are the connections to P3 and P1 in the drawing exchangeable then?
    – Tim
    Oct 6, 2015 at 2:52
  • Maybe, maybe not. P1 could be an ethernet interface while P3 is a serial interface, or they could both be either one or another interface type altogether.
    – Ron Maupin ♦
    Oct 6, 2015 at 2:53

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