Introduction; Rip; Ospf - Allied Telesis SwitchBlade x908 Series Software Reference Manual

Routing Protocol Overview

Introduction

This chapter introduces the basic routing protocols supported within the AlliedWare Plus
Operating System.

RIP

A distance-vector protocol, Routing Information Protocol (RIP) is an Interior Gateway Protocol
(IGP) that uses hop counts as its metrics. The AlliedWare Plus
1058 and 1723; the RIPv2 module supports more fields in the RIP packets, and supports
security authentication features.
At regular intervals of the routing update timer (a default value of 30 seconds), and at the time
of change in the topology, the RIP router sends update messages to other routers. The listening
routers update their route table with the new route, and increase the metric value of the path
by one (referred to as a hop count). The router recognizes the IP address advertising router as
the next hop, then sends the routing updates to other routers. A maximum allowable hop
count is 15. If a router reaches a metric value of 16 or more (referred to as infinity), the
destination is identified as unreachable. This avoids the indefinite routing loops. The split-
horizon and hold-down features are used to avoid propagation incorrect routing information.
The route becomes not valid when the route time-out timer expires; it remains in the table
until the route-flush timer expires.

OSPF

A link-state routing protocol, Open Shortest Path First (OSPF) is an interior gateway protocol
(IGP) that uses the Shortest Path First (SPF) Dijsktra algorithm.
OSPF sends link-state advertisements (LSAs) to all other routers within the same hierarchical
area. Data on attached interfaces, metrics used, and other variables, are included in OSPF LSAs.
As OSPF routers accumulate link-state data, they use the SPF algorithm to calculate the
shortest path to each node.
An Autonomous System (AS) or Domain is defined as a group of networks with common
routing infrastructure. OSPF can work in one AS; or receive or send routes from or to different
AS systems. Autonomous systems consist of areas. An area is a group of neighboring networks
or attached hosts. A router attached to multiple areas with its interfaces is called an Area
Border Router (ABR). It creates a distinct topological database: a group of LSAs received from
all routers in the same area, for each area. All the routers in the same area have an identical
topological database. OSPF routing traffic is restricted in the area because areas are unknown
to each other. The routing information is distributed between areas, area border routers,
networks, and connected routers by the OSPF backbone.
All backbone OSPF area routers use the same procedures and algorithms to maintain routing
information within the backbone that any area router would. The backbone topology is invisible
to all routers within an area. The individual area topologies are invisible to the backbone.
Sometimes the backbone is not a contiguous area. Virtual links function as if they were direct
links, and are configured between backbone routers that share a link to a non-backbone area.
28.2
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
AlliedWare Plus Operating System - Software Version 5.3.1
TM
RIP module supports RFCs
C613-50007-01 REV B
TM