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FTOS Configuration Guide for the C-Series - Force10 Networks

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An OSPF backbone is responsible <strong>for</strong> distributing routing in<strong>for</strong>mation between areas. It consists of all<br />

Area Border Routers, networks not wholly contained in any area, and <strong>the</strong>ir attached routers.<br />

The Backbone is <strong>the</strong> only area with an default area number. All o<strong>the</strong>r areas can have <strong>the</strong>ir Area ID assigned<br />

in <strong>the</strong> configuration.<br />

Figure 375 shows Routers A, B, C, G, H, and I are <strong>the</strong> Backbone.<br />

A Stub Area (SA) does not receive external route in<strong>for</strong>mation, except <strong>for</strong> <strong>the</strong> default route. These areas do<br />

receive in<strong>for</strong>mation from inter-area (IA) routes. Note that all routers within an assigned Stub area must be<br />

configured as stubby, and no generate LSAs that do not apply. For example, a Type 5 LSA is intended <strong>for</strong><br />

external areas and <strong>the</strong> Stubby area routers may not generate external LSAs. Stubby areas cannot be<br />

traversed by a virtual link.<br />

A Not-So-Stubby Area (NSSA) can import AS external route in<strong>for</strong>mation and send it to <strong>the</strong> Backbone. It<br />

cannot received external AS in<strong>for</strong>mation from <strong>the</strong> Backbone or o<strong>the</strong>r areas. It can be traversed by a virtual<br />

link.<br />

Totally Stubby Areas are referred to as No Summary areas in <strong>FTOS</strong>.<br />

<strong>Networks</strong> and Neighbors<br />

As a link-state protocol, OSPF sends routing in<strong>for</strong>mation to o<strong>the</strong>r OSPF routers concerning <strong>the</strong> state of <strong>the</strong><br />

links between <strong>the</strong>m. The state (up or down) of those links is important.<br />

Routers that share a link become neighbors on that segment. OSPF uses <strong>the</strong> hello protocol as a neighbor<br />

discovery and keep alive mechanism. After two routers are neighbors, <strong>the</strong>y may proceed to exchange and<br />

synchronize <strong>the</strong>ir databases, which creates an adjacency.<br />

Router Types<br />

Router types are attributes of <strong>the</strong> OSPF process. A given physical router may be a part of one or more<br />

OSPF processes. For example, a router connected to more than one area, receiving routing from a BGP<br />

process connected to ano<strong>the</strong>r AS acts as both an Area Border Router and an Autonomous System Router.<br />

Each router has a unique ID, written in decimal <strong>for</strong>mat (A.B.C.D). The router ID does not have to be<br />

associated with a valid IP address. However, Force 10 recommends that <strong>the</strong> router ID and <strong>the</strong> router’s IP<br />

address reflect each o<strong>the</strong>r, to make troubleshooting easier.<br />

<strong>FTOS</strong> <strong>Configuration</strong> <strong>Guide</strong>, version 7.7.1.0 531

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