The Link-State Database; The Shortest Path First Tree; Internal Versus External Routing - Lenovo Flex System Fabric CN4093 Application Manual

The Link-State Database

The Shortest Path First Tree

Internal Versus External Routing

© Copyright Lenovo 2015
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.
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
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 do 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 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.
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.
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.
425
Chapter 31: OSPF