1

During a networking-lecture I was told about the Link-State and Distance Vector protocol. Also we were told about Autonomous Systems and the BGP protocol.

Our lecturer told us that with the Link-State protocol, each router has a complete picture of the network, however, I do have a question about the "network" in this case. Now this is arising some confusion for me.

Does he mean that with the Link-State protocol, each router has a complete picture of the LAN or the entire Autonomous System or some other defined network? Any help would be much appreciated.

2

With OSPF (a common Enterprise link-state protocol), Each router has a complete view of the topology for the areas it is associated with. In smaller networks, all routers can belong to the one single area (commonly area 0), so all routers would have full topology information for the entire network/AS.

As the network grows, the network can be split up into separate areas to aid scaling. There are two levels of hierarchy, area 0 is the backbone area and all other areas must be connected to the backbone. A router can sit solely within a single area (all its interfaces are assigned to the area). In this case it will only have full topology information for that area. Areas are interconnected to the backbone by routers which have links in different areas (one of these must be the backbone area). These routers are called Area Border Routers and provide a service to the areas it is connected to of summarising the topology information from one area into another.

For example, if you have area 1, connected to area 0, connected to area 2.

  • All the routers only in area 1 have full topology information for area 1
  • All the routers only in area 2 have full topology information for area 2
  • All the routers only in area 0 have full topology information for area 0
  • ABRs that straddle areas 0 and 1 have full topology information for both of those areas
  • ABRs that straddle areas 0 and 2 have full topology information for both of those areas
  • An ABR connecting area 1 to area 0 will work out its best (lowest cost) path to each network within area 1 and create type 3 summary LSAs and advertise those into area 0.
  • The ABR connecting area 1 and area 0 will also summarise all the networks from area 0 (and the summary routes in area 0 summarised from area 2) into area 1
  • An ABR connecting area 2 to area 0 will work out its best (lowest cost) path to each network within area 2 and create type 3 summary LSAs and advertise those into area 0.
  • The ABR connecting area 2 and area 0 will also summarise all the networks from area 0 (and the summary routes in area 0 summarised from area 1) into area 2

So each area now has full topology information for all the routers and networks within the areas it is a member of, and it has summarised information for all the other networks within the AS, which are created by the area ABR. The summarised information only shows the ABRs best cost path to the networks in the other areas, it does not allow the router to build up a full picture of the entire network and work out its best path to the network number from the topology graph.

The example above is for standard areas, there is the concept of stub areas, totally stubby areas, not so stubby areas etc that all have different rules for how they summarise internal or external routes.

2
  • Thanks for your response. Your description really help me out in understanding the difference between the protocols! Just to be sure, in a area, multiple protocols can be used by different routers right? Or are all the routers in an area using the same protocol. – Toine-L Nov 4 '17 at 18:18
  • Hi, they all need to speak the same protocol to be part of the same OSPF domain, so all the routers in all the areas need to speak OSPF. You can have other protocols in your network, but they form their own domain. A router called an Autonomous System Border Router (OSPF term) can run two routing protocols, and redistribute the information from one protocol into the other protocol, to connect the two domains – Karl Billington Nov 4 '17 at 18:27
2

The answer is "it depends." The term "network" can have different meanings depending on the context.

Usually it is better to speak of routing domains. The routing domain is all the routers that participate in the particular routing protocol (in large networks you can have several routing protocols, and therefore several routing domains).

The two link state protocols in use today, OSPF and IS-IS, both have a hierarchical domain structure. That is, they can divide a large routing domain into smaller subdomains to improve efficiency and convergence time. In OSPF, the subdomains are called areas, and in IS-IS, they're called levels.

To use OSPF as an example, you can have a "network," consisting of a single area (domain). All routers in the area have a full topology map of the area. With multiple areas, OSPF routers have a topology map only of the routers in their area. They also learn a route to the backbone area (also called area 0), that links the multiple areas together.

1
  • Thanks for taking your time to respond to my question, I really appreciate it. Your answer put me on the right path to start reading more about this protocol and the structure. – Toine-L Nov 4 '17 at 18:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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