Ospf Packets; Intra-Area Link-State Advertisements - Avaya 8800 Configuration Manual

OSPF and RIP fundamentals

OSPF packets

All OSPF packets start with a 24-octet header that contains information about the OSPF
version, the packet type and length, the ID of the router that transmits the packet, and the ID
of the OSPF area from which the packet is sent. An OSPF packet is one of the following types:
• Hello packets are transmitted between neighbors and are never forwarded. The Hello
Protocol requires routers to send Hello packets to neighbors at predefined Hello intervals.
A neighbor router does not receive a Hello packet declares the other router dead.
• Database description (DD) packets are exchanged when a link is established between
neighboring routers which synchronize their link-state databases.
• Link-state request packets describe one or more link-state advertisements that a router
requests from its neighbor. Routers send link-state requests if the information received in
DD packets from a neighbor is not consistent with its own link-state database.
• Link-state update packets contain one or more link-state advertisements and are sent
following a change in network conditions.
• Link-state acknowledgement packets are sent to acknowledge receipt of link-state
updates and contain the headers of the received link-state advertisements.

Intra-area link-state advertisements

OSPF does not require each router to send its entire routing table to its neighbors. Instead,
each OSPF router floods only link-state change information in the form of LSAs throughout the
area or AS. LSAs in OSPF are one of the following five types:
• A router links advertisement is flooded only within the area and contains information about
neighbor routers and the LANs to which the router is attached. A backbone router can
flood router link advertisements within the backbone area.
• A network links advertisement is generated by a DR on a LAN, and lists all routers on that
LAN and flooding only within the area. A backbone DR can flood network links
advertisements within the backbone area.
• A network summary link advertisement is flooded into an area by an ABR that describes
networks that are reachable outside the area. An ABR attached to two areas generates
a different network summary link advertisement for each area. ABRs also generate area
summary link advertisements that contain information about destinations within an area
that are flooded to the backbone area.
48 Configuration — OSPF and RIP June 2011