It's to tuff to describe the actual of backbone router with in a single word. Also it's depending on network infrastructure. Basically when a router is carrying so many same links and / or multiple links (LL, MPLS, FR etc.) to other end and / or so many Branch Office, as well as Regional Offices are connected that router is known as backbone router.
If you go for OSPF term then it will be such routers whiches are the part of the OSPF backbone. By definition, this includes all area border routers, since those routers pass routing information between areas. However, a backbone router may also be a router that connects only to other backbone (or area border) routers, and is therefore not part of any area (other than Area 0).
To summarize: an area border router is always a backbone router, but a backbone router is not necessarily an area border router.
It might be obvious by now that the term "backbone" router is a relative one. It depends on the kind of network. Examples of customer networks have already been mentioned. The customer router that connects a customer to the ISP would never be called a "backbone" router from the point of view of the provider. In an OSPF ISP network the customer routers would not normally be part of area 0 (backbone area) of the provider. In most cases the ISP would not even run an IGP with the customer (BGP instead). Even major ISP routers that are connected to many customer CPEs could be considered as non-backbone (and be outside area 0).
The customer network that is overlayed above an ISP Frame Relay cloud or any other L2 or L3 type connectivity would have its own IGP and from its point of view some customer routers could be considered as backbone routers.
In conclusion, this term is a relative one. It depends on the importance of the routers for each administrative domain considered. Usually the most important routers are the most powerful ones, because they interconnect many pieces of the network together and their non-stop operation is vital. Normally, if such a "backbone" router fails, a "significant" part of the network is isolated. The size of the "significant" part is relative to business size.
In general, a backbone router is a router that connects multiple networks in a single administration domain - a gateway to all the networks in the same domain. It can also connects to other backbone routers in different administration domain to form an internetwork.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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