Configuring OSPF loopback address and router priority

Configuring OSPF loopback address and router priority
2.3.2

This page will explain the purpose of an OSPF loopback interface. Students will also learn how to assign an IP address to a loopback interface.

When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID. If there is no active interface, the OSPF process will not start. If the active interface goes down, the OSPF process has no router ID and therefore ceases to function until the interface comes up again.

To ensure OSPF stability there should be an active interface for the OSPF process at all times. A loopback interface, which is a logical interface, can be configured for this purpose. When a loopback interface is configured, OSPF uses this address as the router ID, regardless of the value. On a router that has more than one loopback interface, OSPF takes the highest loopback IP address as its router ID.

To create and assign an IP address to a loopback interface use the following commands:

Router(config)#interface loopback number

Router(config-if)#ip address ip-address subnet-mask

It is considered good practice to use loopback interfaces for all routers running OSPF. This loopback interface should be configured with an address using a 32-bit subnet mask of 255.255.255.255. A 32-bit subnet mask is called a host mask because the subnet mask specifies a network of one host. When OSPF is requested to advertise a loopback network, OSPF always advertises the loopback as a host route with a 32-bit mask.

In broadcast multi-access networks there may be more than two routers. OSPF elects a designated router (DR) to be the focal point of all link-state updates and link-state advertisements. Because the DR role is critical, a backup designated router (BDR) is elected to take over if the DR fails.

If the network type of an interface is broadcast, the default OSPF priority is 1. When OSPF priorities are the same, the OSPF election for DR is decided on the router ID. The highest router ID is selected.

The election result can be determined by ensuring that the ballots, the hello packets, contain a priority for that router interface. The interface reporting the highest priority for a router will ensure that it becomes the DR.

The priorities can be set to any value from 0 to 255. A value of 0 prevents that router from being elected. A router with the highest OSPF priority will be selected as the DR. A router with the second highest priority will be the BDR. After the election process, the DR and BDR retain their roles even if routers are added to the network with higher OSPF priority values.

Modify the OSPF priority by entering global interface configuration ip ospf priority command on an interface that is participating in OSPF. The command show ip ospf interface will display the interface priority value as well as other key information.

Router(config-if)#ip ospf prioritynumber

Router#show ip ospf interfacetype number

The Lab Activity will teach students to configure loopback interfaces for OSPF as well as observe the election process for DR and BDR.

The next page will discuss the OSPF cost metric.

Steps in the operation of OSPF / Configuring OSPF routing process

Steps in the operation of OSPF
2.3.1

This page will explain how routers communicate in an OSPF network. When a router starts an OSPF routing process on an interface, it sends a Hello packet and continues to send Hellos at regular intervals. The set of rules that govern the exchange of OSPF Hello packets is called the Hello protocol. On multi-access networks, the Hello protocol elects a designated router (DR) and a backup designated router (BDR). The Hello carries information about which all neighbors must agree to form an adjacency and exchange link-state information. On multi-access networks the DR and BDR maintain adjacencies with all other OSPF routers on the network.  Adjacent routers go through a sequence of states. Adjacent routers must be in the full state before routing tables are created and traffic routed. Each router sends link-state advertisements (LSA) in link-state update (LSU) packets. These LSAs describe all of the routers links. Each router that receives an LSA from its neighbor records the LSA in the link-state database. This process is repeated for all routers in the OSPF network. When the databases are complete, each router uses the SPF algorithm to calculate a loop free logical topology to every known network. The shortest path with the lowest cost is used in building this topology, therefore the best route is selected. Routing information is now maintained. When there is a change in a link-state, routers use a flooding process to notify other routers on the network about the change. The Hello protocol dead interval provides a simple mechanism for determining that an adjacent neighbor is down. - This page concludes this lesson. The next lesson will explain more about OSPF. The first page will discuss the configuration of OSPF. Configuring OSPF routing process 2.2.8 This page will teach students how to configure OSPF. OSPF routing uses the concept of areas. Each router contains a complete database of link-states in a specific area. An area in the OSPF network may be assigned any number from 0 to 65,535. However a single area is assigned the number 0 and is known as area 0. In multi-area OSPF networks, all areas are required to connect to area 0. Area 0 is also called the backbone area. OSPF configuration requires that the OSPF routing process be enabled on the router with network addresses and area information specified. Network addresses are configured with a wildcard mask and not a subnet mask. The wildcard mask represents the links or host addresses that can be present in this segment. Area IDs can be written as a whole number or dotted decimal notation. To enable OSPF routing, use the global configuration command syntax: Router(config)#router ospfprocess-id The process ID is a number that is used to identify an OSPF routing process on the router. Multiple OSPF processes can be started on the same router. The number can be any value between 1 and 65,535. Most network administrators keep the same process ID throughout an autonomous system, but this is not a requirement. It is rarely necessary to run more than one OSPF process on a router. IP networks are advertised as follows in OSPF: Router(config-router)#network address wildcard-mask area area-id Each network must be identified with the area to which it belongs. The network address can be a whole network, a subnet, or the address of the interface. The wildcard mask represents the set of host addresses that the segment supports. This is different than a subnet mask, which is used when configuring IP addresses on interfaces. The Lab Activity will help students configure and verify OSPF routing. This next page will teach students how to configure an OSPF loopback interface.

OSPF Hello protocol

OSPF Hello protocol
2.2.6 This page will introduce hello packets and the Hello protocol.
When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals. The rules that govern the exchange of OSPF hello packets are called the Hello protocol.
At Layer 3 of the OSI model, the hello packets are addressed to the multicast address 224.0.0.5. This address is “all OSPF routers”. OSPF routers use hello packets to initiate new adjacencies and to ensure that neighbor routers are still functioning. Hellos are sent every 10 seconds by default on broadcast multi-access and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, the default time is 30 seconds.
On multi-access networks the Hello protocol elects a designated router (DR) and a backup designated router (BDR).
Although the hello packet is small, it consists of the OSPF packet header. For the hello packet the type field is set to 1.
The hello packet carries information that all neighbors must agree upon before an adjacency is formed, and link-state information is exchanged.
The Interactive Media Activity will help students identify the fields in an OSPF packet header.
The next page will describe the OSPF routing process.

OSPF network types

OSPF network types
2.2.5 This page will introduce the three types of OSPF networks.
A neighbor relationship is required for OSPF routers to share routing information. A router will try to become adjacent, or neighbor, to at least one other router on each IP network to which it is connected. OSPF routers determine which routers to become adjacent to based on the type of network they are connected to. Some routers may try to become adjacent to all neighbor routers. Other routers may try to become adjacent to only one or two neighbor routers. Once an adjacency is formed between neighbors, link-state information is exchanged.
OSPF interfaces automatically recognize three types of networks:

• Broadcast multi-access, such as Ethernet
• Point-to-point networks
• Nonbroadcast multi-access (NBMA), such as Frame Relay

A fourth type, point-to-multipoint, can be manually configured on an interface by an administrator. 
In a multi-access network, it is not known in advance how many routers will be connected. In point-to-point networks, only two routers can be connected.
In a broadcast multi-access network segment, many routers may be connected. If every router had to establish full adjacency with every other router and exchange link-state information with every neighbor, there would be too much overhead. If there are 5 routers, 10 adjacency relationships would be needed and 10 link-states sent. If there are 10 routers then 45 adjacencies would be needed. In general, for n routers, n*(n-1)/2 adjacencies would need to be formed.
The solution to this overhead is to hold an election for a designated router (DR). This router becomes adjacent to all other routers in the broadcast segment. All other routers on the segment send their link-state information to the DR. The DR in turn acts as the spokesperson for the segment. The DR sends link-state information to all other routers on the segment using the multicast address of 224.0.0.5 for all OSPF routers.
Despite the gain in efficiency that electing a DR provides, there is a disadvantage. The DR represents a single point of failure. A second router is elected as a backup designated router (BDR) to take over the duties of the DR if it should fail. To ensure that both the DR and the BDR see the link-states all routers send on the segment, the multicast address for all designated routers, 224.0.0.6, is used.
On point-to-point networks only two nodes exist and no DR or BDR is elected. Both routers become fully adjacent with each other.
The Interactive Media Activity will help students recognize the three types of OSPF networks.
The next page will describe the OSPF Hello protocol.