Single-Area OSPF (Open Shortest Path First Protocol) 1st semester 1439-2017

Outline Link-State Routing Protocols Link-State Routing Process OSPF Configuration Loopback interface OSPF Configuration Mode OSPF router ID Enabling OSPF on Interfaces Wildcard Mask OSPF cost Reference BW Interface BW Verify OSPF

Types of Routing Protocols 7.1.4.1 3 Types of Routing Protocols

Link-State Routing Protocols In contrast to distance vector routing protocol operation, a router configured with a link-state routing protocol can create a complete view or topology of the network by gathering information from all of the other routers. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology A link-state routing protocol is like having a complete map of the network topology. The sign posts along the way from source to destination are not necessary, because all link-state routers are using an identical map of the network. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology.

Link-State Routing Process 7.4.2.1 5 Link-State Routing Process

7.4.2.2 6 Link and Link-State The first step in the link-state routing process is that each router learns about its own links, its own directly connected networks. 

7.4.2.3 7 Say Hello The second step in the link-state routing process is that each router is responsible for meeting its neighbors on directly connected networks.

7.4.2.4 8 Link State Updates The third step in the link-state routing process is that each router builds a link-state packet (LSP) containing the state of each directly connected link.

Flooding the LSP and Building the Link-State Database 7.4.2.5 9 Flooding the LSP and Building the Link-State Database The fourth step in the link-state routing process is that each router floods the LSP to all neighbors, who then store all LSPs received in a database.

Computing the Best Path 7.4.2.6 10 Computing the Best Path The final step in the link-state routing process is that each router uses the database to construct a complete map of the topology and computes the best path to each destination network.

Adding Routes to the Routing Table 7.4.2.7 11 Adding Routes to the Routing Table The best paths are inserted into the routing table

OSPF Administrative Distance

OSPF Configuration

Loopback Interface 15 4.1.3 Basic Settings on a Router 4.1.3.4 Configure a Loopback Interface 4.1.3.5 Packet Tracer – Configuring IPv4 and IPv6 Interfaces 15 Loopback Interface Loopback interface is a logical interface internal to the router. It is not assigned to a physical port (not connected to any other device), it is considered a software interface that is automatically in an UP state. Loopback interfaces are very useful because they will never go down, unless the entire router goes down. This helps in managing routers because there will always be at least one active interface on the routers Also it is important in the OSPF routing process.

Configure a Loopback Interface 4.1.3 Basic Settings on a Router 4.1.3.4 Configure a Loopback Interface 4.1.3.5 Packet Tracer – Configuring IPv4 and IPv6 Interfaces 16 Configure a Loopback Interface Any number between 0 and 2,147,483,647

Configuring Single-Area OSPFv2 OSPFv2 is enabled using the global configuration mode command router ospf process-id. The process-id value is locally significant, which means that it does not have to be the same value on the other OSPF routers to establish adjacencies with those neighbors any number between 1 and 65,535 The process-id value represents a number between 1 and 65,535 and is selected by the network administrator

Configuring Single-Area OSPFv2 Note: The list of commands has been altered to display only the commands that are used in this chapter. 18 Configuring Single-Area OSPFv2

Router IDs

Router IDs Every router requires a router ID to participate in an OSPF domain. The router ID can be defined by an administrator or automatically assigned by the router. The router ID is used by the OSPF-enabled router to: Uniquely identify the router Participate in the election of the DR Uniquely identify the router - The router ID is used by other routers to uniquely identify each router within the OSPF domain and all packets that originate from them. 2. Participate in the election of the DR – - In a multiaccess LAN environment, the election of the DR occurs during initial establishment of the OSPF network. - When OSPF links become active, the routing device configured with the highest priority is elected the DR. Assuming there is no priority configured, or there is a tie, then the router with the highest router ID is elected the DR. The routing device with the second highest router ID is elected the BDR.

Router IDs But how does the router determine the router ID?

Router IDs The router ID is explicitly configured using the OSPF router configuration mode command router-id rid. The rid value is any 32-bit value expressed as an IPv4 address. This is the recommended method to assign a router ID.

Router IDs If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any of configured loopback interfaces. This is the next best alternative to assigning a router ID. The IPv4 address of the loopback interface should be configured using a 32-bit subnet mask (255.255.255.255).

Router IDs If no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces. This is the least recommended method because it makes it more difficult for administrators to distinguish between specific routers. This means that the interface address does not need to be included in one of the OSPF network commands for the router to use that IP address as the router ID. If the router uses the highest IPv4 address for the router ID, the interface does not need to be OSPF-enabled. This means that the interface address does not need to be included in one of the OSPF network commands for the router to use that IP address as the router ID. The only requirement is that the interface is active and in the up state.

Router IDs Note: The router ID looks like an IP address, but it is not routable and, therefore, is not included in the routing table, unless the OSPF routing process chooses an interface (physical or loopback) that is appropriately defined by a network command.

Configuring an OSPF Router ID An OSPF router identifies itself to other routers using this router ID.

If the router ID is the same on two neighboring routers, the router displays an error message similar to the one below:

Modifying a Router ID 28 8.2.1.3/4/5/6 Sometimes a router ID needs to be changed. However, after a router selects a router ID, an active OSPF router does not allow the router ID to be changed until the router is reloaded or the OSPF process cleared. The reason is because the router already has adjacencies with other neighbors using the router ID. Those adjacencies must be renegotiated using the new router.

Clearing the OSPF process is the preferred method to reset the router ID. This forces OSPF on R1 to transition to the Down and Init states.

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