— Exhibit —
[edit]
user@R1# show routing-options router-id
router-id 1.1.1.1;
[edit]
user@R1# show protocols ospf
area 0.0.0.0 {
interface ge-0/0/7.0;
}
[edit]
user@R2# show routing-options router-id
router-id 2.2.2.2;
[edit]
user@R2# show protocols ospf
area 0.0.0.0 {
interface ge-0/0/8.0 {
priority 200;
}
}
[edit]
user@R3# show routing-options router-id
router-id 222.255.255.255;
[edit]
user@R3# show protocols ospf
area 0.0.0.0 {
interface ge-0/0/8.0;
}
[edit]
user@R4# show routing-options router-id
router-id 239.255.255.255;
[edit]
user@R4# show protocols ospf
area 0.0.0.0 {
interface ge-0/0/6.0 {
priority 0;
}
}
— Exhibit —
Click the Exhibit button.
All four routers in the exhibit are in the same broadcast domain. The routers were powered on at
the same time.
Based on the configurations, which devices are the DR and the BDR
A.
R4 is the DR and R2 is the BDR.
B.
R2 is the DR and R3 is the BDR.
C.
R2 is the DR and R1 is the BDR.
D.
R3 is the DR and R2 is the BDR.
Correct:
R2 had a priority value assigned.
R3 had a higher router-id than R1
http://www.juniper.net/techpubs/en_US/junos12.1/topics/topic-map/ospf-router-designated-router.html
OSPF Designated Router Overview
Large LANs that have many routing devices and therefore many OSPF adjacencies can produce heavy control-packet traffic as link-state advertisements (LSAs) are flooded across the network. To alleviate the potential traffic problem, OSPF uses designated routers on all multiaccess networks (broadcast and nonbroadcast multiaccess [NBMA] networks types). Rather than broadcasting LSAs to all their OSPF neighbors, the routing devices send their LSAs to the designated router. Each multiaccess network has a designated router, which performs two main functions:
Originate network link advertisements on behalf of the network.
Establish adjacencies with all routing devices on the network, thus participating in the synchronizing of the link-state databases.
In LANs, the election of the designated router takes place when the OSPF network is initially established. When the first OSPF links are active, the routing device with the highest router identifier (defined by the router-id configuration value, which is typically the IP address of the routing device, or the loopback address) is elected the designated router. The routing device with the second highest router identifier is elected the backup designated router. If the designated router fails or loses connectivity, the backup designated router assumes its role and a new backup designated router election takes place between all the routers in the OSPF network.
OSPF uses the router identifier for two main purposes: to elect a designated router, unless you manually specify a priority value, and to identify the routing device from which a packet is originated. At designated router election, the router priorities are evaluated first, and the routing device with the highest priority is elected designated router. If router priorities tie, the routing device with the highest router identifier, which is typically the routing device’s IP address, is chosen as the designated router. If you do not configure a router identifier, the IP address of the first interface to come online is used. This is usually the loopback interface. Otherwise, the first hardware interface with an IP address is used.
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