[네트워크] OSPF Router LSA Packet Format

• Link State ID for Router LSAs is the originating router's Router ID.

• V, or Virtual Link Endpoint bit, is set to one when the originating router is an endpoint of one or more fully adjacent virtual links having the described area as the transit area.

• E, or External bit, is set to one when the originating router is an ASBR.

• B, or Border bit, is set to one when the originating router is an ABR.

• Number of Links specifies the number of router links the LSA describes. The Router LSA must describe all of the originating router's links, or interfaces, to the area in which the LSA is flooded.

  Subsequent fields in the Router LSA describe each link and appear one or more times, corresponding to the number in the Number of Links field. This discussion covers the Link Type field first, although that field does not appear until after the Link Data field. Understanding link type first is important because the descriptions of the Link ID and Link Data fields vary according to the value of the Link Type field.

• Link Type describes the general type of connection the link provides. Table 8-9 lists the possible values of the field and the associated connection types.

  Table 8-9. Link type values.

  Link Type	Connection
  1	Point-to-point connection to another router
  2	Connection to a transit network
  3	Connection to a stub network
  4	Virtual link

  
  

• Link ID identifies the object to which the link connects. This is dependent on the link type, as shown in Table 8-10. Note that when the connected object is another router, the Link ID is the same as the Link State ID in the header of the neighboring router's LSA. During the routing table calculation, this value is used to find the neighbor's LSA in the link-state database.

  Table 8-10. Link ID values.

  Link Type	Value of Link ID Field
  1	Neighboring router's Router ID
  2	IP address of the DR's interface
  3	IP network or subnet address
  4	Neighboring router's Router ID

  
  

• Link Data also depends on the value of the Link Type field, as shown in Table 8-11.

  Table 8-11. Link data values.

  Link Type	Value of Link Data Field
  1	IP address of the originating router's interface to the network[*]
  2	IP address of the originating router's interface to the network
  3	Network's IP address or subnet mask
  4	The MIB-II ifIndex value for the originating router's interface

  
  

  ^[*] If the point-to-point link is unnumbered, this field will instead carry the MIB-II ifIndex value of the interface.

• Number of TOS specifies the number of Type of Service metrics listed for this link. Although TOS is no longer supported in RFC 2328, the TOS fields are still included for backward compatibility with earlier OSPF implementations. If no TOS metrics are associated with a link, this field is set to 0x00.

• Metric is the cost of the link (interface).

  The next two fields are associated with a link corresponding to the number (#) of TOS field. For example, if # of TOS = 3, there will be three 32-bit words containing three instances of these fields. If # of TOS = 0, there will be no instances of these fields.

  Note that Cisco supports only TOS = 0.

• TOS specifies the Type of Service to which the following metric refers.^[24] Table 8-12 lists the TOS values (as specified in RFC 1349), the bit values of the corresponding TOS field in the IP header, and the corresponding value used in the OSPF TOS field.

  ^[24] Philip Almquist, "Type of Service in the Internet Protocol Suite," RFC 1349, July 1992.

  Table 8-12. OSPF TOS values.

  RFC TOS Value	IP Header TOS Field	OSPF TOS Encoding
  Normal Service	0000	0
  Minimize Monetary Cost	0001	2
  Maximize Reliability	0010	4
  Maximize Throughput	0100	8
  Minimize Delay	1000	16

  
  

• TOS Metric is the metric associated with the specified TOS value.