Routing Protocol Overview
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
24.3
During boot-up, an OSPF router initializes its routing-protocol-specific data structures and
tables. When the lower-layer protocols with which it interfaces are functional, it sends the
OSPF Hello protocol packets to find neighboring routers. A router sends Hello packets as
keep-alive packets, informing other routers about its continuing functionality. Two routers are
adjacent when their link state databases are synchronized.
Multi-access networks have more than two routers. On multi-access networks, the hello
protocol chooses a designated router and a designated backup-router. The designated router
generates LSAs for the entire multi-access network, and reduces network traffic and the size of
the topological database. The designated router also determines the adjacency of routers and
the synchronization of their topological databases. The data on a router's adjacencies or state
changes are provided by periodic transmission of an LSA. Failed routers are detected, and
topology is changed quickly by comparison of adjacencies to link states. Each router calculates a
shortest path tree, with itself as a root, from the topological database generated from these
LSAs. This shortest path tree creates a routing table.
PIM-SM
The AlliedWare Plus
TM
Protocol Independent Multicast–Sparse Mode (PIM-SM) module is a
multicast routing protocol module that uses the underlying unicast Routing Information Base
(RIB) to determine the best next-hop neighbor to reach the root of the multicast data
distribution tree, the Rendezvous Point (RP), or the source. It builds unidirectional-shared trees
per group, and optionally creates shortest-path trees per source.
VRRP
Mission-critical applications running on fault-tolerant networking equipment, such as routers
and switches, require redundancy and high availability. This section provides an architectural
overview of Virtual Router Redundancy Protocol (VRRP) implementation in the AlliedWare
Plus
TM
OS.
Typically, end hosts are connected to the enterprise network through a single router (first-hop
router) that is in the same Local Area Network (LAN) segment. The most popular method of
configuration is for the end hosts to statically configure this router as their default gateway. This
minimizes configuration and processing overhead. The main problem with this configuration
method is that it produces a single point of failure if the enterprise network’s first-hop router
fails.
VRRP attempts to solve this problem by introducing the concept of a virtual router, composed
of two or more VRRP routers on the same subnet. The concept of a virtual IP address is also
introduced, which is the address that end hosts configure as their default gateway. Only one of
the routers (called the Master) forwards packets on behalf of this IP address. In the event that
the Master fails, one of the other routers (Backups) assumes forwarding responsibility for it.