Like OSPFv2, the process-idР’В value is a number between 1 and 65,535 and is chosen by the network administrator. The networkР’В IPv4 router configuration mode command does not exist in IPv6. IPv6 routing protocols are enabled on an interface, and not from router configuration mode, like their IPv4 counterparts. Use the IPv6 router confirmation mode to configure global OSPFv3 parameters, such as a assigning a 32-bit OSPF router ID and reference bandwidth. The IPv6 router configuration mode prompt is different than the IPv4 router configuration mode prompt. Use the ipv6 router ospf process-idР’В global configuration mode command to enter router configuration mode. 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 networkР’В command. The only requirement is that the interface is active and in the up state. This means that the interface address does not need to be included in one of the OSPF networkР’В 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 is the least recommended method because it makes it more difficult for administrators to distinguish between specific routers. If no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces.This is the next best alternative to assigning a router ID. If the router ID is not explicitly configured, the router chooses the highest IPv4 address of any of configured loopback interfaces.This is the recommended method to assign a router ID. The ridР’В value is any 32-bit value expressed as an IPv4 address. The router ID is explicitly configured using the OSPF router-id ridР’В router configuration mode command.The routing device with the second highest router ID is elected the BDR.Р”В«ut how does the router determine the router ID? As illustrated in the figure, Cisco routers derive the router ID based on one of three criteria, in the following preferential order: Assuming there is no priority configured, or there is a tie, then the router with the highest router ID is elected the DR. When OSPF links become active, the routing device configured with the highest priority is elected the DR. Participate in the election of the DRР’В - In a multiaccess LAN environment, the election of the DR occurs during initial establishment of the OSPF network.Uniquely identify the routerР’В - The router ID is used by other routers to uniquely identify each router within the OSPF domain and all packets that originate from them.

The router ID is used by the OSPF-enabled router to:

The router ID can be defined by an administrator or automatically assigned by the router. In the picture above you can see that the routerРІР‚в„ўs physical interface Fa0/0 was divided into three subinterfaces that were then configured as trunk interfaces and given the IP addresses.Every router requires a router ID to participate in an OSPF domain. Notice how weРІР‚в„ўve configured the Fa0/1 port on a switch (the port connected to the routerРІР‚в„ўs Fa0/0 interface) as a trunk port. But first, here is the switch configuration: Each host is in a different VLAN, so we need to divide the physical routerРІР‚в„ўs interface Fa0/0 into logical interfaces, one for each VLAN. We have a simple network of three hosts, a switch and a router.

(config-subif)# ip address IP_ADDRESS SUBNET_MASK РІР‚вЂњ sets the IP address for the subinterface.

(config-subif)# encapsulation dot1q VLAN_ID РІР‚вЂњ sets the subinterface to trunk and associates it with a specific VLAN.

(config)# interface TYPE NUMBER.SUBINTERFACE РІР‚вЂњ creates the subinterface and enters the subinterface command mode.

To configure trunking on a router, the following commands are used: For example, the command interface Gi0/0.1 creates a subinterface. The subinterface number begins with the period and it is usually the same as the VLAN the subinterface will be in. The physical interface is divided into logical interfaces (also known as subinterfaces), one for each VLAN.Р”Р„ subinterface is created with the interface TYPE NUMBER.SUBINTERFACE command. This scenario is called router on a stick (ROAS) and allows all VLANs to communicate through a single physical interface. To enable inter-VLAN communication, you can divide a single physical interface on a router into logical interfaces that will be configured as trunk interfaces.

Ospfv2 with router on a stick