Specifications

Chapter 2. IBM System Networking Switch 10Gb Ethernet switch features 65
Each routing device transmits a Link-State Advertisement (LSA) on each of its
active
interfaces. LSAs are entered into the LSDB of each routing device. OSPF uses flooding to
distribute LSAs between routing devices. Interfaces may also be
passive. Passive interfaces
send LSAs to active interfaces, but do not receive LSAs, hello packets, or any other OSPF
protocol information from active interfaces. Passive interfaces behave as stub networks,
allowing OSPF routing devices to be aware of devices that otherwise participate in OSPF
(either because they do not support it, or because the administrator chooses to restrict OSPF
traffic exchange or transit).
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 adjacent neighbor is interested in that route, an update message that contains the
new route is sent to the neighbor. For each route removed from the route table, if the route
has already been sent to an adjacent neighbor, an update message that contains the route to
withdraw is sent.
Shortest Path First
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 processing 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 action is referred to as
internal routing and can be done with static routes or
using active internal routing protocols, such as OSPF or RIP.
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 has one or more border routers (peer routers that exchange routes with
other OSPF networks), and an internal routing system that enables 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 committing to carry data to the IP space represented in the route that is 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 carries that data to
its destination.