Designated and Backup Designated Routers

As stated in the Day 12 module, OSPF elects a Designated Router (DR) and/or a Backup Designated Router (BDR) on Broadcast and Non-Broadcast network types. It is important to understand that the BDR is not a mandatory component on these network types. In fact, OSPF will work just as well when only a DR is elected and there is no BDR; however, there will be no redundancy if the DR fails and the OSPF routers need to go through the election process again to elect a new DR.

On the segment (on Broadcast and Non-Broadcast network types), each individual non-DR/BDR router establishes an adjacency with the DR and, if one has also been elected, the BDR, but not with any other non-DR/BDR routers on the segment. The DR and BDR routers are fully adjacent with each other and all other routers on the segment. The non-DR/BDR routers send messages and updates to the AllDRRouters Multicast group address 224.0.0.6. Only the DR/BDR routers listen to Multicast messages sent to this group address. The DR then advertises messages to the AllSPFRouters Multicast group address 224.0.0.5. This allows all other OSPF routers on the segment to receive the updates.

It is important to understand the sequence of message exchanges when a DR and/or a BDR router have been elected. As an example, imagine a Broadcast network with four routers, which are R1, R2, R3, and R4. Assume that R4 has been elected DR, and R3 has been elected BDR. R2 and R1 are neither DR nor BDR and are therefore referred to as DROther routers in Cisco OSPF terminology. A configuration change is made on R1, and R1 then sends an update to the AllDRRouters Multicast group address 224.0.0.6. R4, the DR, receives this update and sends an acknowledgement back to the AllSPFRouters Multicast group address 224.0.0.5. R4 then sends this update to all other non-DR/BDR routers using the AllSPFRouters Multicast group address. This update is received by the other DROther router, R2, and R2 sends an acknowledgement to the AllDRRouters Multicast group 224.0.0.6. This is illustrated in Figure 39.1 below:

Figure 39.1 – OSPF DR and BDR Advertisements

NOTE: The BDR simply listens to the packets sent to both Multicast groups.

In order for a router to be the DR or the BDR for the segment, the router must be elected. This election is based on the following:

• The highest router priority value
• The highest router ID

By default, all routers have a default priority value of 1. This value can be adjusted using the ip ospf priority <0-255> interface configuration command. The higher the priority, the greater the likelihood the router will be elected DR for the segment. The router with the second-highest priority will then be elected BDR. If a priority value of 0 is configured, the router will not participate in the DR/BDR election process. The highest router priority and router ID are important only if OSPF processes loads at the same time on all routers participating in the DR/BDR election process. Otherwise the router that finishes loading the OSPF process first will become the DR on the segment.

When determining the OSPF router ID, Cisco IOS selects the highest IP address of configured Loopback interfaces. If no Loopback interfaces are configured, the software uses the highest IP address of all configured physical interfaces as the OSPF router ID. Cisco IOS software also allows administrators to specify the router ID manually using the router-id [address] router configuration command.

It is important to remember that with OSPF, once the DR and the BDR have been elected, they will remain as DR/BDR routers until a new election is held. For example, if a DR and a BDR exist on a Multi-Access network and a router with a higher priority or IP address is added to the same segment, the existing DR and BDR routers will not change. If the DR fails, the BDR will assume the role of the DR, not the new router with the higher priority or IP address. Instead, a new election will be held and that router will most likely be elected BDR. In order for that router to become the DR, the BDR must be removed or the OSPF process must be reset using the clear ip ospf command, forcing a new DR/BDR election. Once elected, OSPF uses the DR and the BDR routers as follows:

• To reduce the number of adjacencies required on the segment
• To advertise the routers on the Multi-Access segment
• To ensure that updates are sent to all routers on the segment

To better understand these fundamental concepts, reference the basic OSPF network topology illustrated in Figure 39.2 below:

Figure 39.2 – OSPF DR and BDR Fundamentals

Referencing Figure 39.2, each router on the segment establishes an adjacency with the DR and the BDR but not with each other. In other words, non-DR/BDR routers do not establish an adjacency with each other. This prevents the routers on the segment from forming N(N-1) adjacencies with each other, which reduces excessive OSPF packet flooding on the segment.

For example, without the concept of a DR/BDR on the segment, each individual router would need to establish an adjacency with every other router on the segment. This would result in 4(4-1) or 12 adjacencies on the segment. However, with the DR/BDR, each individual router needs to establish an adjacency with only these two routers and no other non-DR and BDR routers. The DR and the BDR also establish an adjacency between themselves. This reduces the number of adjacencies required on the segment and on each individual OSPF router, which in turn reduces resources consumption (e.g., memory and processor utilisation) on the routers.

Regarding the second point, OSPF views a link as a connection between two routers or nodes. In Multi-Access networks, such as Ethernet, multiple routers can reside on the same segment, as illustrated in Figure 39.2. On such networks, OSPF uses the Network Link State Advertisement (Type 2 LSA) to advertise the routers on the Multi-Access segment. This LSA is generated by the DR and is flooded only within the area. Because the other non-DR/BDR routers do not establish adjacencies with each other, this LSA allows those routers to know about the other routers on the Multi-Access segment.

To further clarify this point, referencing Figure 39.2, assuming that all routers on the segment have the default OSPF priority value of 1 (and load the OSPF process at the same time), R4 is elected as the DR for the segment because it has the highest router ID. R3 is elected as the BDR for the segment because it has the second-highest router ID. Because R2 and R1 are neither the DR nor the BDR, they are referred to as DROther routers in Cisco terminology. This can be validated using the show ip ospf neighbor command on all routers, as follows:

NOTE: The DROther routers remain in the 2WAY/DROTHER state because they exchange their databases only with the DR and BDR routers. Therefore, because there is no full database exchange between the DROther routers, they will never reach the OSPF Full adjacency state.

Because R4 has been elected DR, it generates the Network LSA, which advertises the other routers on the Multi-Access segment. This can be verified using the show ip ospf database network [link state ID] command on any router on the segment or the show ip ospf database network self-originate command on the DR only. The following illustrates the output of the show ip ospf database network self-originate command on the DR (R4):

Referencing the output above, the DR (R4) originates the Type 2 (Network) LSA representing the 192.168.1.0/24 subnet. Because multiple routers exist on this subnet, this 192.168.1.0/24 subnet is referred to as a transit link in OSPF terminology. The Advertising Router field shows the router that originated this LSA. The Network Mask field shows the subnet mask of the transit network, which is 24-bit or 255.255.255.0.

The Attached Router field lists the router IDs of all routers that are on the network segment. This allows all of the routers on the segment to know what other routers also reside on the segment. The output of the show ip ospf database network [link state ID] command on R1, R2, and R3 reflects the same information, as illustrated in the following outputs:

The functionality of the Network LSA and how it is correlated to another LSA, specifically the Router LSA (Type 1), will be described in detail later in this module. For this section, primary emphasis should be placed on understanding that the DR generates and advertises the Network LSA on the Multi-Access segment to advertise other routers that reside on the same segment. This is because routers on the segment establish an adjacency only with the DR and BDR routers and not with each other. Without an adjacency with each other, the routers will never know about other non-DR/BDR routers on the Multi-Access segment.

Finally, regarding the third point made on DR/BDR routers, the DR/BDR routers ensure that all routers on the segment have complete databases. Non-DR/BDR routers send updates to the Multicast group address 224.0.0.6 (AllDRRouters). The DR then advertises these updates to other non-DR/BDR routers by sending the update to the Multicast group address 224.0.0.5 (AllSPFRouters). Figure 39.3 below illustrates an update from R1 (a DROther) to the DR group address referencing the routers illustrated in Figure 39.2:

Figure 39.3 – DROther Update to DR/BDR Group Address

R4 (DR) receives this update and in turn sends the same to Multicast group address 224.0.0.5. This group address is used by all OSPF routers, ensuring that all other routers on the segment receive this update. This update from the DR (R4) is illustrated in Figure 39.4 below:

Figure 39.4 – DR Update to OSPF Group Address

NOTE: You can see that this is the Update from R1 because the Advertising Router field in both Figures 39.3 and 39.4 contains the router ID (RID) of R1, which is 1.1.1.1.

NOTE: The other LSAs used by OSPF will be described in detail later in this module.