Performing IS-IS Routing Operations参考.ppt

The IS-IS Protocol Performing IS-IS Routing Operations OSI Addresses OSI network layer addressing is implemented with NSAP addresses. An NSAP address identifies a system in the OSI network; an address represents an entire node, not an interface. Various NSAP formats are used in various systems, because different protocols may use different representations of NSAP. NSAP addresses are a maximum of 20 bytes: Higher-order bits identify the interarea structure. Lower-order bits identify systems within area. Typical NSAP Address Structure The simplest NSAP format used by most companies running IS-IS as their IGP is as follows: Area address (must be at least 1 byte) AFI set to 49 Locally administered; thus, you can assign your own addresses. Area ID The octets of the area address after the AFI. System ID Cisco routers require a 6-byte system ID. NSEL Always set to 0 for a router. Identifying Systems in IS-IS: Area Address Identifying Systems in IS-IS: System ID System ID in the address used to identify the IS; it is not just an interface. Cisco supports only a 6-byte system ID. System ID is used in Level 1 routing and has to be unique within an area. System ID has to be unique within Level 2 routers that form the routing domain. General recommendation: use domain-wide unique system ID. This may be MAC (for example, 0000.0c12.3456) or IP address (for example, 1921.6800.0001) taken from an interface. OSI Addressing: NET Addresses NSAP address includes NSEL field (process or port number) NET: NSAP with a NSEL field of 0 Refers to the device itself (equivalent to the Layer 3 OSI address of the device) Used in routers because they implement the network layer only (base for SPF calculation) Subnetwork Point of Attachment (SNPA) and Circuit SNPA is equivalent to Layer 2 address; for example: Virtual circuit ID (DLCI on Frame Relay) MAC address on LAN interfaces Interfaces uniquely identified by circuit ID: On point-to-point interfaces, SNPA is used. On LANs, circuit ID c