Multiaccess OSPF Networks
Summary
Configure the OSPF interface priority to influence the DR/BDR election in a multiaccess network. Start learning CCNA 200-301 for free right now!!
Note: Welcome: This topic is part of Module 2 of the Cisco CCNA 3 course, for a better follow up of the course you can go to the CCNA 3 section to guide you through an order.
Table of Contents
OSPF Network Types
Another type of network that uses OSPF is the multiaccess OSPF network. Multiaccess OSPF networks are unique in that one router controls the distribution of LSAs. The router that is elected for this role should be determined by the network administrator through proper configuration.
OSPF may include additional processes depending on the type of network. The previous topology used point-to-point links between the routers. However, routers can be connected to the same switch to form a multiaccess network, as shown in the figure. Ethernet LANs are the most common example of broadcast multiaccess networks. In broadcast networks, all devices on the network see all broadcast and multicast frames.
OSPF Designated Router
Recall that, in multiaccess networks, OSPF elects a DR and BDR as a solution to manage the number of adjacencies and the flooding of link-state advertisements (LSAs). The DR is responsible for collecting and distributing LSAs sent and received. The DR uses the multicast IPv4 address 224.0.0.5 which is meant for all OSPF routers.
A BDR is also elected in case the DR fails. The BDR listens passively and maintains a relationship with all the routers. If the DR stops producing Hello packets, the BDR promotes itself and assumes the role of DR.
All other routers become a DROTHER (a router that is neither the DR nor the BDR). DROTHERs use the multiaccess address 224.0.0.6 (all designated routers) to send OSPF packets to the DR and BDR. Only the DR and BDR listen for 224.0.0.6.
In the figure, R1, R5, and R4 are DROTHERs. Click play to see the animation of R2 acting as DR. Notice that only the DR and the BDR process the LSA sent by R1 using the multicast IPv4 address 224.0.0.6. The DR then sends out the LSA to all OSPF routers using the multicast IPv4 address 224.0.0.5.
Role of the DR
OSPF Multiaccess Reference Topology
In the multiaccess topology shown in the figure, there are three routers interconnected over a common Ethernet multiaccess network, 192.168.1.0/24. Each router is configured with the indicated IPv4 address on the Gigabit Ethernet 0/0/0 interface.
Because the routers are connected over a common multiaccess network, OSPF has automatically elected a DR and BDR. In this example, R3 has been elected as the DR because its router ID is 3.3.3.3, which is the highest in this network. R2 is the BDR because it has the second highest router ID in the network.
Verify OSPF Router Roles
To verify the roles of the OSPFv2 router, use the show ip ospf interface command.
Click each button see the output for the show ip ospf interface command on each router.
Verify DR/BDR Adjacencies
To verify the OSPFv2 adjacencies, use the show ip ospf neighbor command, as shown in the example for R1. The state of neighbors in multiaccess networks can be as follows:
- FULL/DROTHER – This is a DR or BDR router that is fully adjacent with a non-DR or BDR router. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.
- FULL/DR – The router is fully adjacent with the indicated DR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.
- FULL/BDR – The router is fully adjacent with the indicated BDR neighbor. These two neighbors can exchange Hello packets, updates, queries, replies, and acknowledgments.
- 2-WAY/DROTHER – The non-DR or BDR router has a neighbor relationship with another non-DR or BDR router. These two neighbors exchange Hello packets.
The normal state for an OSPF router is usually FULL. If a router is stuck in another state, it is an indication that there are problems in forming adjacencies. The only exception to this is the 2-WAY state, which is normal in a multiaccess broadcast network. For examples, DROTHERs will form a 2-WAY neighbor adjacency with any DROTHERs that join the network. When this happens, the neighbor state displays as 2-WAY/DROTHER.
Click each button see the output for the show ip ospf neighbor command on each router.
Default DR/BDR Election Process
How do the DR and BDR get elected? The OSPF DR and BDR election decision is based on the following criteria, in sequential order:
- The routers in the network elect the router with the highest interface priority as the DR. The router with the second highest interface priority is elected as the BDR. The priority can be configured to be any number between 0 – 255. If the interface priority value is set to 0, that interface cannot be elected as DR nor BDR. The default priority of multiaccess broadcast interfaces is 1. Therefore, unless otherwise configured, all routers have an equal priority value and must rely on another tie breaking method during the DR/BDR election.
- If the interface priorities are equal, then the router with the highest router ID is elected the DR. The router with the second highest router ID is the BDR.
Recall that the router ID is determined in one of the following three ways:
- The router ID can be manually configured.
- If no router IDs are configured, the router ID is determined by the highest loopback IPv4 address.
- If no loopback interfaces are configured, the router ID is determined by the highest active IPv4 address.
OSPFv2 Multiaccess Reference Topology
In the figure, all Ethernet router interfaces have a default priority of 1. As a result, based on the selection criteria listed above, the OSPF router ID is used to elect the DR and BDR. R3 with the highest router ID becomes the DR; and R2, with the second highest router ID, becomes the BDR.
The DR and BDR election process takes place as soon as the first router with an OSPF-enabled interface is active on the multiaccess network. This can happen when the preconfigured OSPF routers are powered on, or when OSPF is activated on the interface. The election process only takes a few seconds. If all of the routers on the multiaccess network have not finished booting, it is possible that a router with a lower router ID becomes the DR.
OSPF DR and BDR elections are not pre-emptive. If a new router with a higher priority or higher router ID is added to the network after the DR and BDR election, the newly added router does not take over the DR or the BDR role. This is because those roles have already been assigned. The addition of a new router does not initiate a new election process.
DR Failure and Recovery
After the DR is elected, it remains the DR until one of the following events occurs:
- The DR fails.
- The OSPF process on the DR fails or is stopped.
- The multiaccess interface on the DR fails or is shutdown.
If the DR fails, the BDR is automatically promoted to DR. This is the case even if another DROTHER with a higher priority or router ID is added to the network after the initial DR/BDR election. However, after a BDR is promoted to DR, a new BDR election occurs and the DROTHER with the highest priority or router ID is elected as the new BDR.
Click each button for an illustration of various scenarios relating to the DR and BDR election process.
In this scenario, the current DR (R3) fails. Therefore, the pre-elected BDR (R2) assumes the role of DR. Subsequently, an election is held to choose a new BDR. Because R1 is the only DROTHER, it is elected as the BDR.
In this scenario, R3 has re-joined the network after several minutes of being unavailable. Because the DR and BDR already exist, R3 does not take over either role. Instead, it becomes a DROTHER.
In this scenario, a new router (R4) with a higher router ID is added to the network. DR (R2) and BDR (R1) retain the DR and BDR roles. R4 automatically becomes a DROTHER.
In this scenario, R2 has failed. The BDR (R1) automatically becomes the DR and an election process selects R4 as the BDR because it has the higher router ID.
The ip ospf priority Command
If the interface priorities are equal on all routers, the router with the highest router ID is elected the DR. It is possible to configure the router ID to manipulate the DR/BDR election. However, this process only works if there is a stringent plan for setting the router ID on all routers. Configuring the router ID can help control this. However, in large networks this can be cumbersome.
Instead of relying on the router ID, it is better to control the election by setting interface priorities. This also allows a router to be the DR in one network and a DROTHER in another. To set the priority of an interface, use the command ip ospf priority value, where value is 0 to 255. A value of 0 does not become a DR or a BDR. A value of 1 to 255 on the interface makes it more likely that the router becomes the DR or the BDR.
Configure OSPF Priority
In the topology, the ip ospf priority command will be used to change the DR and BDR as follows:
- R1 should be the DR and will be configured with a priority of 255.
- R2 should be the BDR and will be left with the default priority of 1.
- R3 should never be a DR or BDR and will be configured with a priority of 0.
Change the R1 G0/0/0 interface priority from 1 to 255.
R1(config)# interface GigabitEthernet 0/0/0 R1(config-if)# ip ospf priority 255 R1(config-if)# end R1#
Change the R3 G0/0/0 interface priority from 1 to 0.
R3(config)# interface GigabitEthernet 0/0/0 R3(config-if)# ip ospf priority 0 R3(config-if)# end R3#
The following example, shows how to clear the OSPF process on R1. The clear ip ospf process command also must be entered on R2 and R3 (not shown). Notice the OSPF state information that is generated.
R1# clear ip ospf process Reset ALL OSPF processes? [no]: y R1# *Jun 5 03:47:41.563: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached *Jun 5 03:47:41.563: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached *Jun 5 03:47:41.569: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done *Jun 5 03:47:41.569: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done
The output from the show in ospf interface g0/0/0 command on R1 confirms that R1 is now the DR with a priority of 255 and identifies the new neighbor adjacencies of R1.
R1# show ip ospf interface GigabitEthernet 0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
Internet Address 192.168.1.1/24, Area 0, Attached via Interface Enable
Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DR, Priority 255
Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
Backup Designated router (ID) 2.2.2.2, Interface address 192.168.1.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:00
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 2
Last flood scan time is 0 msec, maximum is 1 msec
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 2.2.2.2 (Backup Designated Router)
Adjacent with neighbor 3.3.3.3
Suppress hello for 0 neighbor(s)
R1#
Syntax Checker – Configure OSPF Priority
Use the Syntax Checker to configure a different OSPF priority scenario for R1, R2, and R3.
Use the ip ospf priority command to change the DR and BDR as follows:
- R1 should be the BDR and will be configured with a priority of 10.
- R2 should never be a DR or BDR and will be configured with a priority of 0.
- R3 should be the DR and will be left with the default priority of 100.
On all routers, use g0/0/0 for the interface name.
You are logged into R1 in global configuration mode. Configure R1 with a priority of 10.
R1(config)#interface g0/0/0
R1(config-if)#ip ospf priority 10
R1(config-if)#
You are now logged into R2 in global configuration mode. Configure R2 with a priority of 0.
R2(config)#interface g0/0/
R2(config-if)#ip ospf priority 0
R2(config-if)#
You are now logged into R3 in global configuration mode. Configure R3 with a priority of 100.
R3(config)#interface g0/0/0
(config-if)#ip ospf priority 100
R3(config-if)#
You are still logged into R3 in interface configuration mode. Return to privileged EXEC mode. Because R3 is to be the DR, restart the OSPF process on it first.
R3(config-if)#end
R3#clear ip ospf process
Reset ALL OSPF processes? \[no\]:y
\*Jun 5 05:29:35.231: %OSPF-5-ADJCHG: Process 10, Nbr 1.1.1.1 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached \*Jun 5 05:29:35.231: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached \*Jun 5 05:29:35.235: %OSPF-5-ADJCHG: Process 10, Nbr 1.1.1.1 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done \*Jun 5 05:29:44.563: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done R3#
You are now logged into R1 in privileged EXEC mode. Because R1 is the be the DR, restart the OSPF process on it next.
R1#clear ip ospf process
Reset ALL OSPF processes? \[no\]:y
\*Jun 5 05:27:20.691: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached \*Jun 5 05:27:20.691: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached \*Jun 5 05:27:21.695: %OSPF-5-ADJCHG: Process 10, Nbr 2.2.2.2 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done \*Jun 5 05:27:20.951: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done R1#
You are now logged into R2 in privileged EXEC mode. R2 is to be DROTHER. Restart the OSPF process.
R2#clear ip ospf processReset ALL OSPF processes? \[no\]:y
\*Jun 5 15:37:08.978: %OSPF-5-ADJCHG: Process 10, Nbr 1.1.1.1 on GigabitEthernet0/0/0 from 2WAY to DOWN, Neighbor Down: Interface down or detached \*Jun 5 15:37:08.978: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached \*Jun 5 15:37:08.983: %OSPF-5-ADJCHG: Process 10, Nbr 1.1.1.1 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done \*Jun 5 15:37:19.477: %OSPF-5-ADJCHG: Process 10, Nbr 3.3.3.3 on GigabitEthernet0/0/0 from LOADING to FULL, Loading Done R2#
You are now logged into R1. Use the show ip ospf interface g0/0/0 command to verify the R1 is the BDR.
R1#show ip ospf interface g0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
Internet Address 192.168.1.1/24, Area 0, Attached via Interface Enable
Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State BDR, Priority 10
Designated Router (ID) 3.3.3.3, Interface address 192.168.1.3
Backup Designated router (ID) 1.1.1.1, Interface address 192.168.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:04
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 2
Last flood scan time is 0 msec, maximum is 1 msec
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 2.2.2.2
Adjacent with neighbor 3.3.3.3 (Designated Router)
Suppress hello for 0 neighbor(s)
R1#
You are now logged into R2. Use the show ip ospf interface g0/0/0 command to verify the R2 is a DROTHER.
R2#show ip ospf interface g0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
Internet Address 192.168.1.2/24, Area 0, Attached via Interface Enable
Process ID 10, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 3.3.3.3, Interface address 192.168.1.3
Backup Designated router (ID) 1.1.1.1, Interface address 192.168.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 2
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 1.1.1.1 (Backup Designated Router)
Adjacent with neighbor 3.3.3.3 (Designated Router)
Suppress hello for 0 neighbor(s)
R2#
You are now logged into R3. Use the show ip ospf interface g0/0/0 command to verify the R3 is the DR.
R3#show ip ospf interface g0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
Internet Address 192.168.1.3/24, Area 0, Attached via Interface Enable
Process ID 10, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DR, Priority 100
Designated Router (ID) 3.3.3.3, Interface address 192.168.1.3
Backup Designated router (ID) 1.1.1.1, Interface address 192.168.1.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:00
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1/1, flood queue length 0
Next 0x0(0)/0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 3
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 1.1.1.1 (Backup Designated Router)
Adjacent with neighbor 2.2.2.2
Suppress hello for 0 neighbor(s)
R3#
You successfully changed the OSPF priority.
Packet Tracer – Determine the DR and BDR
In this activity, you will complete the following:
- Examine DR and BDR roles and watch the roles change when there is a change in the network.
- Modify the priority to control the roles and force a new election.
- Verify routers are filling the desired roles.
Glossary: If you have doubts about any special term, you can consult this computer network dictionary.
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