Verilink 9000 Series (34-00271) Product Manual - Manual (Page 310)

288 C HAPTER 18: IP R OUTER
Basically, RIP allows a router to exchange routing information with a neighboring
router. As a router becomes aware of any change in the internetwork layout, this
information is immediately broadcast to any neighboring routers. To assure that the
internetwork is always synchronized, RIP packets contain all routing information
known to the router.
RIP is ideal for small- to medium-sized networks.
OSPF Summary
Description Open Shortest Path First Routing (OSPF) is a shortest path first (SPF) or link-state
protocol. OSPF is an internal gateway protocol (IGP) that distributes routing
information between routers in a single autonomous system (AS). OSPF chooses
the least cost path as the best path. Suitable for complex networks with a large
number of routers, OSPF provides equal cost multipath 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 describing 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 external and type 2 external. Intra-area paths have
destinations within the same area, inter-area paths have destinations in other OSPF
areas and Autonomous System External (ASE) routes are routes to destinations
external to the AS. Routes imported into OSPF as Type 1 routes are supposed to
be from IGPs whose external metrics are directly comparable to OSPF metrics.
When a routing decision is being made, OSPF adds the internal cost to the AS
border router to the external metric. Type 2 ASEs are used for External Gateway
Protocols (EGPs) whose metrics are not comparable to OSPF metrics. In this case,
only the internal OSPF cost to the AS Border Router is used in the routing
decision.
From the topology database, each router constructs a tree of the shortest paths with
itself as the root. This shortest-path tree gives the route to each destination in the
AS. Externally derived routing information appears on the tree as leaves. The
link-state advertisement format distinguishes between information acquired from
external sources and information acquired from internal routers, so there is no
ambiguity about the source or reliability of routes. Externally derived routing
information (for example, routes learned from EGP or BGP) is passed
transparently through the autonomous system and is kept separate from OSPF's
internally derived data. Each external route can also be tagged by the advertising
router, enabling a passing of additional information between routers on the borders
of the autonomous system.