Hi, today I am going to talk about OSPF routers and the role of each router. Since OSPF is a vast protocol with a lot going on, different routers carry different duties to make sure that the OSPF AS functions the way that it is supposed to. Without these routers and their role OSPF would not be as efficient and scalable as it currently is.
Internal Router- The first router I am going to talk about is the Internal Router. The name pretty much speaks for itself, but it’s a router where all of its interfaces reside within one OSPF area. These routers are mainly responsible for only one LSDB, and for any intra-area routing.
Backbone Router- The next router I am going to talk about is the backbone router. The backbone router is any router that has at least one interface in the backbone area or area 0. These routers are responsible for forwarding inter-area routes to the correct destination ABR. Every ABR router is a backbone router as well, because by design they are required to have at least one interface in the backbone area. Do not be confused though, not every backbone router is an ABR because a router with all of its interface is area 0 is a backbone router but not an ABR.
ABR- An ABR (Area Border Router) is the router that allows the OSPF topology to have multiple existing OSPF areas not be completely isolated. If it weren’t for ABRs, internal routers would have no way to talk to internal routers in a completely different area. An ABR is a router that has at least one interface in the backbone-area and at least one more interface in a non-backbone area. The ABR is responsible for turning intra-area routes into Type 3 Summary LSAs and floods them through the area 0 interface. Other ABRs in the backbone will receive those Type 3 LSAs, install them into the LSDBs for their areas and flood them again in the backbone area. This ensures that every ABR router receives these routes and every area will have a path to another area. ABR is also responsible for carrying multiple LSDBs for every area it’s connected to, even including area 0. These LSDBs are completely separate from each other. ABR allows for the hierarchical structure of the OSPF to remain as it is by allowing the backbone area to be the primary transit path for any inter-area routing. ABR also runs SPF for every area that it is connected to as well, having every OSPF area basically having their own operation.
ASBR- The last but not least router is the ASBR (Autonomous System Border Router). This router is responsible for redistributing external routes into the OSPF AS. As you know OSPF is an IGP, so routing is only done within an AS. In order for this to be sustainable and to be able to reach networks that are not a part of the AS, ASBR essentially acts as the “default router” for the OSPF domain (Only if it is configured with default-information originate). Any routing that needs to go to a different network (AS) or the internet will be routed to the ASBR. The ASBR will essentially act as the gateway of last resort, and the exit door to networks outside of the OSPF domain. Any route can be redistributed into the OSPF domain such as BGP, EIGRP, or even ISIS. These routes will be seen on the routing table as E1/E2, and LSA packets that are distributed into the OSPF domain from another protocol are called Type 5 External LSAs. Another caveat is that the ASBR can distribute a default route to the whole OSPF domain with one command which is “default-information originate.” Once this command is configured, every router in the topology will adopt that router’s IP address as the gateway of last resort.
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