I am a first year Netacad student and I am having an issue with a word problem.

The requirements for my network are as follows:

I will be using packet tracer to design and simulate the network.

  1. 3 offices in 3 cities.
  2. 5 Vlans per city. Each Allowing for 250 users.
  3. DHCP for end point management.
  4. Each user should be able to reach other users.
  5. Only one IPv4 address per city. Using PAT will be necessary.
  6. Use layer 2 security on switches for end user connections.
  7. OSPF for internal routing.
  8. All users need unrestricted access to internet.
  9. All devices in company should be managed by one device using SSH.
  10. Include 3 web servers (websites).

I am having trouble imagining how the topology should look. I am very much able to do the configurations but I would greatly help if someone could help me imagine how the topology might look. Each company has to connect to their own ISP through serial, and each ISP connected by serial as well.

  • Are you sure it's 5 VLANs per city? Shouldn't it be 5 VLANs per office? Because this way it looks like you are managing both office network and ISP network.
    – phandox
    Commented Nov 17, 2016 at 9:30
  • Yes, exactly. I was thinking I should have worded that differently. One office per city. 5 vlans in the office.
    – LBigbie
    Commented Nov 17, 2016 at 9:31
  • I am quite new to networking. I'm having a hard time how it would look from each offices edge router to the WAN connecting to each ISP, and then to the the other offices ISP and so on.
    – LBigbie
    Commented Nov 17, 2016 at 9:33
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 15, 2017 at 5:16

1 Answer 1


Ok, I think there are two variants to look into this, the problematic part where it divides is when you mentioned OSPF should be used for internal routing. If you want to reach other destination through ISP and use OSPF between sites the ISP routers needs to be part of your OSPF topology, so they would propagate all the networks with OSPF through the whole topology. This picture shows it. All routers are part of OSPF topology

Notice the orange region is also on links between offices, so they are part of OSPF.

The problem here is, that this wouldn't be really possible in real life. You can't manage ISP network, so the solution here is to create GRE Tunnels between every office. That way, the OSPF would use Tunnel interfaces, and from logical part of network it will look like the routers Office A, Office B, Office C are directly connected and can establish neighbours in OSPF through these Tunnel interfaces.

For PAT, I guess you also ment one IPv4 address per OFFICE, not per city. It makes much more sense, that means you will make PAT on every Office router and ISP network uses public addresses to connect between cities. You should also use some dynamic routing / static routing in ISP network, to make sure every ISP router can reach public address of office router ( if you go with the way of GRE tunnels. Having ISP routers part of OSPF topology loose the purpose of PAT, because they can directly see on the inside networks).

Within one office, users in different VLANs will be reachable by inter-VLAN routing (either on L3 switch or router-on-stick solution with subinterfaces). If they want to reach other site directly and you are using PAT, there needs to be GRE tunnel between sites allowing direct L3 connection. I guess you don't want to create rules in PAT for every user.

So to sum it up, I think the best solution here is to do GRE tunnels between sites and do OSPF on these interfaces to manage internal network. For external network (ISP) use again OSPF or EIGRP, whatever works for you.

  • Thank you Lukas! Although, I will not be using GRE or EIGRP as those are subjects that I actually have not really learned about. The routing will need to be done with all OSPF for my internal routing with some Static default routes etc. I also think the webservers will need to be independent of each office and will be reached through the ISP. So no GRE tunnels unfortunately. With this configuration, would the packets maintain the OSPF characteristics when sent over the WAN link? I'll need to clarify but I might only need to set up OSPF routing for each individual office.
    – LBigbie
    Commented Nov 18, 2016 at 9:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.