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OSPF and IS-IS: Choosing an IGP for Large-Scale Networks by Jeff Doyle

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4.1. Router and Area IDs

An essential requirement for the proper functioning of a link state protocol, as you saw in Chapter 2, is that each router must be able to uniquely identify itself within the routing domain. This is the purpose of the OSPF router ID (RID) and the IS-IS system ID (SysID). In addition, the router must be able to identify its general position within the routing domain. This is the purpose of the area ID (AID).

4.1.1. OSPF Router IDs

OSPF uses a 32-bit number for its RID, represented in the same dotted-decimal format as an IP address. A router can find its RID in one of two ways: The RID can be administratively specified in the OSPF configuration, or an IP address configured on one of the router’s interfaces can be used ...

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