Link-State PDU (LSP)

Link-State PDU (LSP)
The link-state PDU (LSP) is used in much the same way that an OSPF router uses its LSA
packets. The LSP is used to advertise routing information. There are two possible types of
LSPs available:
Level 1 LSP The Level 1 LSP is used to advertise Level 1 link-state routing information
between Level 1 routers. It contains data about the routing information that the advertising
Level 1 router knows. Level 1 LSPs are used to form the Level 1 link-state database.
Level 2 LSP Level 2 LSPs are used to advertise the link-state routing information a Level 2
router knows about. This information is used to help form the Level 2 link-state database.
It is very possible that a router will utilize both types of LSPs. If a router is an L1/L2 router,
it will use the Level 1 LSP to help it form its Level 1 link-state database, and it will use the
Level 2 LSP to help it form its Level 2 link-state database. After a router receives all of the LSPs,
it utilizes the SPF algorithm to select the routes to populate its routing table.
In a broadcast multi-access network, routers will multicast LSPs. Level 1 LSPs are multicast
to the MAC address 0180.C200.0014. This MAC address is known as AllL1ISs. MAC address
0180.C200.0015, known as AllL2ISs, is where routers will multicast all Level 2 LSPs on a
broadcast multi-access network. Routers use unicast instead of multicast on point-to-point networks,
because only one unicast will ever be necessary for each LSP.