|
Previous
|
Content
|
Next
|
|
| 2.1. OSPF
functional summary |
|
 |
|
| A separate copy of OSPF's routing
algorithm runs in each area. Routers having interfaces to multiple areas run
multiple copies of the algorithm. Routing algorithm is as follows: |
| Intra-area routing |
| When a router starts, it first initializes the
routing protocol data structure. Then, it waits for indication from
the lower-level protocols that its interfaces are functional. |
| Being the interfaces functional, the router
sends Hello packets to its neighbors, and in turn receives Hello
packets. On broadcast and P2P networks, the router sends its
Hello packets to the multicast address AllSPFRouters. On
non-broadcast networks, some configuration may be necessary in order to
flood Hello packets. |
| The router will attempt to form adjacencies
with some of its newly adquired neighbors. LS-databases are
synchronized between pairs of adjacent routers. On broadcast and
NBMA networks, the Designated Router (DR) determines which
routers should become adjacents. Adjacencies control the distribution of
routing information; routing updates are sent and received only on
adjacencies. |
|
|
|
| A router periodically advertises its
link-state. Link-state is also advertised when a router's state
changes. Router's adjacencies are reflected in its Link-State
Advertisements (LSAs). The relationship between adjacencies
and link-state allows the protocol to detect dead routers in a timely
fashion. |
| LSAs are flooded throughout the area.
The flooding algorithm ensures that all routers in an area have the
same LS-database. The database consists of the collection of LSAs
originated by routers belonging to the area. From this database, each router
calculates a shortest-path tree with itself as root. From this
tree a routing table is built for the protocol. |
| Inter-area routing |
| In order to be able to route to destinations
outside of the area, the area border routers (ABRs) inject
additional routing information into the area. This information is a
distillation of the rest of the AS's topology. |
| Each ABR is connected to the backbone;
each of them summarizes the topology of its attached non-backbone areas
for transmission on the backbone and hence to all other ABRs.
Each ABR then has a complete topological information concerning the
backbone and the area summaries from each of the other ABRs.
From this information the router calculates paths to all inter-area
destinations. The router then advertises these paths into its attached
areas. This enables the internal routers to pick the best exit route when
forwarding to inter-area destinations. |
| AS external routes |
| Routers that have information regarding other
ASs can flood this information throughout the AS. This
information is distributed verbatim to every router, except for those
belonging to stub areas. |
|
|
|
|
|
Previous
|
Content
|
Next
|
