Riverstone Networks WICT1-12 - Manual (Page 217)

Riverstone Networks RS Switch Router User Guide Release 8.0 14-1
14 OSPF CONFIGURATION GUIDE
Open Shortest Path First Routing (OSPF) is a shortest path first or link-state protocol. The RS supports OSPF
Version 2.0, as defined in RFC 2328. OSPF is an interior gateway protocol that distributes routing information
between routers in a single autonomous system. OSPF chooses the least-cost path as the best path. OSPF is
suitable for complex networks with a large number of routers because it provides equal-cost multi-path routing
where packets to a single destination can be sent via more than one interface simultaneously.
In a link-state protocol, each router maintains a database that describes the entire AS topology, which it builds out
of the collected link state advertisements of all routers. Each participating router distributes its local state (i.e., the
router's usable interfaces and reachable neighbors) throughout the AS by flooding. Each multi-access network that
has at least two attached routers has a designated router and a backup designated router. The designated router
floods a link state advertisement for the multi-access network and has other special responsibilities. The
designated router concept reduces the number of adjacencies required on a multi-access network.
OSPF allows networks to be grouped into areas. Routing information passed between areas is abstracted,
potentially allowing a significant reduction in routing traffic. OSPF uses four different types of routes, listed in
order of preference:
• Intra-area
• Inter-area
• Type 1 ASE
• Type 2 ASE
Intra-area paths have destinations within the same area. Inter-area paths have destinations in other OSPF areas.
Both types of Autonomous System External (ASE) routes are routes to destinations external to OSPF (and usually
external to the AS). Routes exported into OSPF ASE as type 1 ASE routes are supposed to be from interior
gateway protocols (e.g., RIP) whose external metrics are directly comparable to OSPF metrics. When a routing
decision is being made, OSPF will add the internal cost to the AS border router to the external metric. Type 2 ASEs
are used for exterior gateway protocols whose metrics are not comparable to OSPF metrics. In this case, the
external cost from the AS border router to the destination is used in the routing decision.
The RS supports the following OSPF functions:
• Definition of areas, including stub areas and NSSAs (RFC 1587).
• Opaque LSAs (RFC 2370)
• Authentication: Simple password and MD5 authentication methods are supported within an
area.
• Configuration of virtual links
• Configuration of parameters at the area, interface or global level. Parameters that can be
configured include retransmission interval, interface transmit delay, router priority, router dead
and hello intervals, and authentication key.