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SonicOS 5.8.1 Administrator Guide
LSA’s are then exchanged within LSU’s across these adjacencies rather than between
each possible pairing combination of routers on the segment. Link state updates are sent
by non-DR routers to the multicast address 225.0.0.6, the RFC1583 assigned ‘OSPFIGP
Designated Routers’ address. They are also flooded by DR routers to the multicast address
225.0.0.5 ‘OSPFIGP All Routers’ for all routers to receives the LSA’s.
• OSPF Packet types – The five types of OSPF packets are:
–
Hello (OSPF type 1) – Sent at a certain interval to establish and maintain relationships
with neighboring OSPF routers, and elect Designated Routers. (Sent during the
initialization and the 2-WAY phases on LSDB synchronization).
–
Database Description (OSPF type 2) – Sent between OSPF routers during the creation
of an adjacency. During the Exstart phase of LSDB synchronization, DD packets
establish an ISN (initial sequence number) used to track LSA’s, and they establish a
master/slave relationship between neighboring OSPF routers. In the Exchange phase
of LSDB synchronization, they contain short versions of Link State Advertisements.
Because DD exchanges can span multiple packets, they are exchanged in a poll
(master) and response (slave) fashion to ensure completeness.
–
Link State Request (OSPF type 3) – During the Loading phase of LSDB
synchronization, LSR packets are sent to request database updates from a neighbor.
This is the final step in the establishment of an adjacency.
–
Link State Update (OSPF type 4) – Sent in response to Link State Requests, LSU
packets flood adjacencies with Link State Advertisements to achieve LSDB
synchronization.
–
Link State Acknowledgement (OSPF type 5) – To ensure reliability of LSA flooding, all
updates are acknowledged.
• Link State Advertisements (LSA) – There are 7 types of LSA’s:
–
Type 1 (Router Link Advertisements) - Sent by an OSPF router to describe the links to
each area to which it belongs. Type 1 LSA’s are only flooded into a router’s area.
–
Type 2 (Network Links Advertisements) – Sent by the DR for an area describing the set
of routers within the network. Type 2 LSA’s are only flooded into a router’s area.
–
Type 3 (Summary Link Advertisements) – Sent across areas by ABR’s (Area Border
Routers) to describe the networks within an area. Type 3 LSA’s are also used for route
aggregation purposes, and are not sent to Totally Stubby Areas.
–
Type 4 (AS Summary Link Advertisements) – Sent across areas by ABR’s to describe
networks within a different AS. Type 4 LSA’s are not sent to Stub Areas.