So I've got this weird assignment, where each branch office has to be able to access the main office (and Internet) through a serial connection.

Since switches don't have a serial connections socket, I am using routers. I also use them for subnetting.

The issue is, since PC A <-> Router A and Router A <-> Router HQ are on different subnets, I cannot access (ping) the Main router from PC A.

I've never configured default gateway on a router, but I think in this instance it's the only solution.

Based on this article/tutorial, it should be possible, but it didn't work for me, and to be honest I didn't really understand it.


The question is, what is the best solution in this scenario and why? The objective is to be able to successfully ping from any subnet to any subnet ie. PC A <-> PC HQ


So each branch is connected to the main via a "serial link". I suppose what you mean is that at each branch there is a router with 10/100/1G/10G Base-T on one side, and a serial connection on the other, with the serial connection being to some kind of DSL or or other modem or MUX. Then over this serial connection, the router uses PPP to create an IP connection to the main. And at the main you have one or more of these routers sitting and their 10/100/1G/10G Base-T connections are all connected to one switch. (This detail is important I think.)

In this scenario, each Branch must have its own subnet. You can do this easily enough with the 10.x.x.x address space. Alternatively, if the company has, say, a single class C range, you can subdivide a portion of that (the downside is that all subnet masks on all PCs throughout the company will have to change) so that each branch has its own subnet with a CIDR of, say, /26 (giving you 30 addresses at each Branch).

Router A will have two Static routes, one of which is the Default route and points to the IP address of Main's router. The other points to the subnet for the specific branch. Now, in actuality, the PPP protocol establishment on the router should take care of the default route for you -- it should dynamically create that route every time the link is created. You might need to configure that somewhere on the router. But it will create the routing rule when needed. The other route (for the local branch) should also be handled automatically by its being on the same LAN as the branch -- simply configure the router have its internal IP address in the Branch's LAN/subnet.

The PCs in the Branch will each have a default route this same IP Address of Router A. If you've configured the Branch to have the subnet of, the Router might have the address and that becomes the gateway address for all the PCs. Simple.

The complex part comes with the switch / router configuration at Main. This maybe where you got lost. Its default route will be to the company's internet gateway (if it's a separate router -- if not, then the one the ISP gives them). Also, for each Branch, it must have a route to that branch's subnet. So the configuration on the switch / router at Main will look like:

SUBNET         GATEWAY             COMMENT
default           # Company external Internet firewall  ?                   # to branch A  ?                   # to branch B  ?                   # to branch C

What are the values where the question marks are? That depends on the kind of switch / router... I can't answer that without knowing more specifics. But in general, they will be associated with the link to the respective Branch's router. When the Branch's router connects via PPP to the Main's router, each side will have an IP address. Usually the proper gateway is to the Main's dynamically-assigned IP address. But sometimes it's to the Branch's. Sometimes it's simply the port designation ("Serial-Port-A").

  • thanks a lot for the explanation.. I have the network subnetted already. The issue I am having is actually configuring the branch routers to route traffic beyond themselves - to the main router. As you mentioned, static route is probably what I need to look at. The serial link connections between the main router and all branch routers have a /30 subnet. All branches use VLSM, so it varies depending on the # of clients. But the base network block is (but this is not relevant to the question). And yes, you are absolutely correct with the NIC setup. – Mike94 Apr 15 '15 at 18:36
  • What model routers are these? I can peek into the manuals and maybe give you more specifics. Also, please be sure to 'accept' my answer if it was helpful -- i need the rep. – Otheus Apr 16 '15 at 10:37
  • Here's the whole pka if you'd want to have a look.. that will be easier I guess. Thank you. mediafire.com/download/hxm70kpkgl83dkt/2015_03_25_Diagram.pkt – Mike94 Apr 16 '15 at 12:10
  • Uh, no thanks. I'm not going through a packet tracer. The question is, what kind of routers are these? – Otheus Apr 16 '15 at 16:03
  • It's 2621 XM... – Mike94 Apr 16 '15 at 16:38

If I were you, I would use routing protocols like eigrp or ospf. These protocols are flexible. or instead, you can use default route.

  • I've looked at eigrp but it seems quite complicated.. is default route the same as static route, mentioned by Otheus ? – Mike94 Apr 17 '15 at 13:49

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