Choice OSPF DR & DBR

As a CCNA/CCNP candidate, you are expected to understand the purpose and function of the OSPF protocol. The CCNA/CCNP exam will ask you to determine the purpose of the DR and BDR along with the election procedure.

OSPF Designated Router and Backup Designated Router Elections

The OSPF routing algorithm consumes a lot of CPU. Each and every time the OSPF process executes the first shortest path algorithm, it will have an impact on the router. In a point-to-point network, OSPF events are limited to the periodic Hello packet (default 10 seconds on a PPP, HDLC, 30 seconds Frame Relay NBMA), 30 minutes for each route entry in the link state database, and triggered events.

In a multi-access broadcast network where routers are connected to a common broadcast segment, i.e. many routers in a vlan, all routers should create an adjacency with all other routers in the same broadcast domain. Also, the event that causes an LSA to be generated would require the advertising router to create an LSA to be multicast to all neighbors. The more routers there are in the broadcast multicast network, the more adjacencies are required. Two routers require the following number of adjacencies: 2(2-1) / 2 = 1

Four routers require the following number of adjacencies: 4(4-1)/2=6

Ten routers require the following number of adjacencies: 10(10-1)/2=45

Note: DR and BDR are not chosen on point-to-point links because there can only be one adjacency and the point of choosing DR and BDR is for efficiency. The more routers you have on a segment, the more adjacencies you have, the more bandwidth you have, and the more processing power you have on the routers. We need to reduce the amount of work each router has to do on the next by choosing one router as the Designated Router with which all other routers form adjacencies. When you use a Designated Router in your multi-access broadcast network, the number of adjacencies is reduced along with the reduction in processing and bandwidth. All routers will form full adjacencies with the designated router, when you run the command on a non-DR cisco router: R1#show ip ospf neighbor

Neighbor ID Pri Status Dead Time Address Interface

192.168.1.3 1 FULL/DR 00:00:39 192.168.1.3 Ethernet0/0

You will see that your router has formed a “FULL” neighbor adjacency with the DR and the output also defines which neighbor is the DR. The router ID of the neighbor in the output above is 192.168.1.3 and that neighbor is the DR

Any event that occurs is announced to the DR using the multicast address of 224.0.0.6

When the DR announces any event to non-DR devices, it is done using the address 224.0.0.5

Choice of Designated and Designated Backup Router:

The DR is elected based on the router with the highest router priority. The default router priority is 1, the range is 0 to 255. Setting the router priority to 0 means that the interface cannot participate in any DR or BDR elections. If you set the router priority of a current DR or a current BDR to 0, it will relinquish the role immediately. Routers that are not elected as DR or BDR still form neighbor adjacencies, although they will not exchange any routing information with each other, only hello’s are exchanged and these are sent to the multicast address of 224.0.0.5

Despite the name “router priority”, it is in fact a priority that is set at an interface level as follows:

R1(conf)#interface fastethernet 0/0 R1(conf-if)#ip ospf priority 1

In the event that two or more interfaces sharing a common broadcast domain have the same router priority, the OSPF router ID is used as a tiebreaker. The election process is as follows:

1. To participate in the election, any router with its OSPF priority set between 1 and 255 can attempt to become a DR by entering its own router ID in the DR field of its sent hello. You can see the election process by running the “debug ip ospf adj” command, the elections occur during the bidirectional process. 2. Each and every router in the common broadcast domain examines the received Hellos and observes the priority settings of other routers. The candidate must be the DR by putting the DR’s router ID in the DR field of their Hello packets. Therefore, the first criteria to upgrade is a router with the highest priority 4. If the Priorities of the router are identical, the router with the highest RID is chosen as DR 5. The one with the next highest priority or the next Highest Router ID in case there is a tie in router priority value becomes the BDR, (Backup Designated Router). 6. If a new router enters the network after the election of DR and/or BDR, or if some existing router improves its priority, it will not be able to get ahead of the existing DR or even BDR, remember that DR and BDR roles do not they are preventative. 7. If the DR fails, the existing BDR will take over as DR and another election will be held for the BDR. 8. If the previous DR comes back online, it will not be able to resume its previous role.

When a router that is neither the DR nor the BDR needs to announce an event to its neighbors on the common broadcast network, it will send the LSA (Type 1) to the DR using the address 224.0.0.6 (the BDR passively listens to this exchange and keeps a relationship with all routers), the DR will then advertise the event to all adjacent routers in the common LSA broadcast network (Type 2) All routers will then acknowledge with an ACK packet ensures that each has a synchronized copy of the LSDB.

Data on the RD and the BDR

1. These roles are not preventive, once a device is DR or BDR, the only events that can cause it to relinquish the roles are the following:

on. Power off the interface b. Set Priority to 0 c. Reboot the router d. Restart the OSPF router process (clear ip ospf process) e. Disconnect the interface cable.

2. The router that becomes the DR is the first to boot, start its OSPF processes, and beat the 40-second timeout, before seeing another router sending its own hello packet claiming the DR role. The wait timer can be seen in the output below, eighth line down.

R1_x#show OSPF IP interface

Fastethernet0/0 is up, line protocol is up Internet Address 192.168.x.1/24, Area 0 Process ID 1, Router ID 192.168.3.1, Network Type BROADCAST, Cost: 10 Transmission Delay is 1 second, DR State, Priority 1 Designated Router (ID) 192.168.3.1, Interface Address 192.168.3.1 Backup Designated Router (ID) 192.168.3.2, Interface Address 192.168.3.2 Configured Timer Intervals, Hello 10, Dead 40, Wait 40, Retransmit 5

3. A router should only be a DR for a LAN segment. 4. Set the DR by setting its router priority to 255. 5. Set the BDR by setting its router priority to 250 6. Set the router priority of all other routers to 1, never set them to 0 because in case lose both DR and BDR, routers with Router Priority of 0 will never become fully adjacent neighbors, and in turn will never exchange routing information.