|
Because adjacency is required for OSPF routers to
share routing information, a router will try to become adjacent to at least one
other router on each IP network to which it is connected. Some routers may try
to become adjacent to all their neighbor routers, and others may try with only
one or two. OSPF routers determine which routers to become adjacent to based on
what type of network connects them.
OSPF interfaces automatically recognize three
types of networks: broadcast multiaccess, nonbroadcast multiaccess (NBMA), and
point-to-point networks .
An administrator can configure a fourth network type, a point-to-multipoint
network. The four network types are
listed in Figure .
The type of network dictates how OSPF routers
should relate to each other. An administrator may have to override the detected
network type in order for OSPF to operate properly.
Some networks are defined as multiaccess because
you can not predict just how many routers are connected to them - it may be one,
two, or more. A campus that uses a switched Ethernet core may have half a dozen
routers connected to the same backbone network. A school district might have 10,
12, or 25 remote-site routers connected via Frame Relay PVCs to the same IP
subnet.
Because a significant number of routers can exist
on a multiaccess network, OSPF's designers developed a system to avoid the
overhead that would be created if every router established full adjacency with
every other router. This system restricts who can become adjacent to whom by
employing the services of one of the following:
- Designated router (DR) - For every
multiaccess IP network, one router will be elected the DR. This DR has two
main functions: first, to become adjacent to all other routers on the
network, and second, to act as a spokesperson for the network. As
spokesperson, the DR will send network LSAs for all other IP networks to
every other router. Because the DR becomes adjacent to all other routers on
the IP network, it is the focal point for collecting routing information (LSAs).
- Backup designated router (BDR) - The DR
could represent a single point of failure, so a second router is elected as
the BDR to provide fault tolerance. Thus, the BDR must also become adjacent
to all routers on the network and must serve as a second focal point for
LSAs, as shown in Figure
.
However, unlike the DR, the BDR is not responsible for updating the other routers
or sending network LSAs. Instead, the BDR keeps a timer on the DR's update
activity to ensure that it is operational. If the BDR does not detect
activity from the DR before the timer expires, the BDR takes over the role
of DR and a new BDR is elected.
On point-to-point networks, only two nodes exist.
Therefore, a focal point for routing information is not needed. No DR or BDR is
elected. Both routers become fully adjacent to one and other.
|