Router Priority And Dr Election; Ospf Area - Zte ZXR10 5900E Series User Manual

ZXR10 5900E Series User Manual (IPv4 Volume)
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Neighbors do not necessarily have the adjacency relationship, on
broadcast and NBMA networks. If all routers (the number is n)
on a network have set up the adjacency relationship, each router
has (n-1) adjacency relationships and there are n (n-1)/2 adja-
cency relationships on the network. Tracking so many adjacency
relationships on a large multi-access network will impose a heavy
burden on each router. Routing information between each pair of
neighbor routers will waste a great deal of network bandwidth.
OSPF defines a Designated Router (DR) and a Backup Designated
Router (BDR). Designated Router (DR) has following duties:
� To represent a multi-access network and it's attached routers
to the rest of the internetwork.
� To manage the flooding process on the multi-access network.
DR and BDR must establish an adjacency relationship with each
OSPF router over the network. Each OSPF router only establishes
adjacency relationships with DR and BDR. If the DR stops working
then BDR take its place and becomes DR.

Router Priority and DR Election

Each multi-access interface of each router has a Router Priority,
which is an 8-bit unsigned integer ranging from 0 to 255. Default
priority is 1.
During the DR election, the router with the highest priority be-
comes the DR. If all routers have the same priority, the one with
the highest IP address will be elected as the DR. Routers with a
priority of 0 are ineligible to become the DR or BDR.

OSPF Area

A network is divided into several smaller OSPF areas to reduce the
information that each router stores and maintains. Each router
must have the complete information of its area. Areas can share
their information. Routing information can be filtered out on the
area edge to reduce the routing information stored in routers.
Each area is identified by a 32-bit unsigned number. Area 0 is used
to identify the backbone area. All the other areas must directly
connect to Area 0. An OSPF network must have one backbone
area. Based on its tasks in the area, a router can be of one or
multiple of the following roles, as shown in Figure 5 .