QTECH
Software Configuration Manual
8-114
8.2.7
Configure priority when selecting DR
The priority of router interface determines the competency in selecting“designated router”. The superior
priority is firstly considered in conflict. Designated router (DR) is not determined by human, but selected by all
routers in the network interface. The router in this network interface whose Priority > 0 can be the candidate. Choose
the one with the superior priority to be the so called DR. If the priority is the same, choose the one with larger router
ID. The vote is the Hello packet. Each router writes its own DR into Hello and sends it to each router in the network
interface. When two of them declairing that they are the DR, choose the one with superior priority. If they have the
same priority, choose the one with the larger router ID. The one with the priority being 0, he will not be selected to be
DR or BDR.
If DR is failure because of some fault, routers must select DR again at the same time. It costs a long time.
During this time, the calculation of router is not correct. In order to shorten it, BDR
（
Backup Designated Router
）
is
brought up. BDR is a abackup for DR. Select BDR at the same time as DR. It establishes neighborship and exchange
routing information with the routers in the network interface. After the failure of DR, BDR is about to be DR because
the neighborship has been established. There will be reselected a new BDR which will not be effected the calculation
of router though it needs a long time.
Caution :
DR is not always the router with the superlative priority and BDR is not always the one with the second
superlative priority. After selecting DR and BDR, a new router adds, no matter how superlative its priority is, it will
not be DR.
DR is the definition in a network interface which is for router interface. A router may be DR in an interface
and may be BDR or DRother in another interface.
Selecting DR in broadcast or NBMA interface, it is unnecessary to select DR in poit-to-poit or
poit-to-multipoit interface.
Configure it in interface configuration mode.
Configure the priority of interface to select “designated router”
ip ospf priority
value
Restore the default value
no ip ospf priority
By default, the priority of VLAN interface to select “designated router” is in the range of 0
～
255
Example :
!Configure priority of VLAN interface 3 to be 100
QTECH(config-if-vlanInterface-3)#ip ospf priority 100
8.2.8
Configure Hello time interval
Hello packet is a geneally used packet which is periodically sent to neighbor router to search and maintain
neighborship and select DR and BDR. User can configure time interval of sending Hello packet. The smaller the
hello-interval is, the faster the changes of network is found. The hello-interval of routers in the same network must be
the same.
After enabling a router, it sends Hello packet to the neighbor node whose priority is larger than 0 (the routers
can be selected as DR or BDR ). After selecting of DR and BDR, they will send Hello packet to all neighbors to set
up neighborship. If neighborship fails, router periodically send Hello packet according to the time interval of
poll-interval command until neighbor router can be used again. The value of poll-interval is three times of the value
of hello-interval. When the time interval of sending Hello packet is changed, configure the value of poll-interval.
Configure it in interface configuration mode :
Configure time interval of sending hello packet
ip ospf hello-interval
seconds
Restore the default time interval of sending hello packet
no ip ospf hello-interval
By default, the time interval of point-to-point, broadcast interface sending Hello packet to be 10 seconds
and point-to-nultipoint, nonbroadcast interface sending Hello packet to be 30 seconds.
Example :
!Configure time interval of VLAN interface 3 sending hello packet to be 15
QTECH(config-if-vlanInterface-3)#ip ospf hello-interval 15