ProLiant BL p-Class GbE2 Interconnect Switch Application Guide

OSPF 113
Neighbors and adjacencies
In areas with two or more routing devices, neighbors and adjacencies are formed.
Neighbors are routing devices that maintain information about each others’ health. To establish neighbor
relationships, routing devices periodically send hello packets on each of their interfaces. All routing devices that
share a common network segment, appear in the same area, and have the same health parameters (hello and
dead intervals) and authentication parameters respond to each other’s hello packets and become neighbors.
Neighbors continue to send periodic hello packets to advertise their health to neighbors. In turn, they listen to
hello packets to determine the health of their neighbors and to establish contact with new neighbors.
The hello process is used for electing one of the neighbors as the area’s Designated Router (DR) and one as the
area’s Backup Designated Router (BDR). The DR is adjacent to all other neighbors and acts as the central contact
for database exchanges. Each neighbor sends its database information to the DR, which relays the information to
the other neighbors.
The BDR is adjacent to all other neighbors (including the DR). Each neighbor sends its database information to
the BDR just as with the DR, but the BDR merely stores this data and does not distribute it. If the DR fails, the BDR
will take over the task of distributing database information to the other neighbors.
Link-State Database
OSPF is a link-state routing protocol. A link represents an interface (or routable path) from the routing device. By
establishing an adjacency with the DR, each routing device in an OSPF area maintains an identical Link-State
Database (LSDB) describing the network topology for its area.
Each routing device transmits a Link-State Advertisement (LSA) on each of its interfaces. LSAs are entered into the
LSDB of each routing device. OSPF uses flooding to distribute LSAs between routing devices.
When LSAs result in changes to the routing device’s LSDB, the routing device forwards the changes to the
adjacent neighbors (the DR and BDR) for distribution to the other neighbors.
OSPF routing updates occur only when changes occur, instead of periodically. For each new route, if an
adjacency is interested in that route (for example, if configured to receive static routes and the new route is
indeed static), an update message containing the new route is sent to the adjacency. For each route removed
from the route table, if the route has already been sent to an adjacency, an update message containing the route
to withdraw is sent.
Shortest Path First Tree
The routing devices use a link-state algorithm (Dijkstra’s algorithm) to calculate the shortest path to all known
destinations, based on the cumulative cost required to reach the destination.
The cost of an individual interface in OSPF is an indication of the overhead required to send packets across it.
The cost is inversely proportional to the bandwidth of the interface. A lower cost indicates a higher bandwidth.
Internal versus external routing
To ensure effective processing of network traffic, every routing device on your network needs to know how to
send a packet (directly or indirectly) to any other location/destination in your network. This is referred to as
internal routing and can be done with static routes or using active internal routing protocols, such as OSPF, RIP,
or RIPv2.
It is also useful to tell routers outside your network (upstream providers or peers) about the routes you have access
to in your network. Sharing of routing information between autonomous systems is known as external routing.
Typically, an AS will have one or more border routers (peer routers that exchange routes with other OSPF
networks) as well as an internal routing system enabling every router in that AS to reach every other router and
destination within that AS.
When a routing device advertises routes to boundary routers on other autonomous systems, it is effectively
committing to carry data to the IP space represented in the route being advertised. For example, if the routing
device advertises 192.204.4.0/24, it is declaring that if another router sends data destined for any address in
the 192.204.4.0/24 range, it will carry that data to its destination.