Routing

The OSPF network type loopback is enabled by default for loopback interfaces, it can also only be used on loopback interfaces. The network type of loopback will always advertise the host address of the loopback into the network, and not the network itself, even if not configured with a 255.255.255.255 subnet. Changing the loopback to…

The network circuit that only has two devices on it that can communicate is considered a point to point circuit. Point to point circuits do not require the use of ARP or broadcast traffic. OSPF sets the network type of point to point by default for serial, GRE, and point-to-point frame relay interfaces. The Hello…

Ethernet is a broadcast network type for OSPF by default. Broadcast networks can be multi-access with the possibility of there being more than two OSPF enabled routers on the subnet, so the election of a DR/BDR is required. The Hello timer on a broadcast network is 10 seconds, and defined in RFC 2328

The default network type in an OSPF network is set by the physical media used. It can be overridden in interface configuration mode with the command ip ospf network Type Description DR/BDR field used Timers Broadcast Default in Ethernet networks Yes Hello: 10Wait: 40Dead: 40 Non-broadcast Used in Frame Relay main or multipoint subinterfaces Yes…

The OSPF hello packet has a secondary function: it can detect when a neighbour is unreachable or in a poor state. OSPF has a second timer called the dead interval timer, which by default is 4 times the length of the hello timer. When a hello packet is received, the dead timer resets. If a…

Interface costs are part of the the shortest path first calculation that OSPF performs – the metric that helps OSPF decide what route to take is based on how much it will cost to reach the destination. OSPF can assign a dynamic interface cost based on a formula with a reference bandwidth of 100Mbps. Cost…

OSPF can advertise a default router on a router into the rest of the OSPF domain. The command default-information originate in the router configuration mode will advertise the default route. The command on its own will only advertise a default route if the local router has a default route of its own in the routing…

The command show ip ospf neighbour provides a table of information regarding OSPF neighbours. Neighbour ID – The Router ID / RID of the neighbouring router PRI – The priority of the neighbours interface for DR/BDR elections State – The state of the neighbour, in DR / BDR or DROTHER Dead Time – Time remaining…

After configuring OSPF, it is good practice to verify interfaces are correctly utilising OSPF. The command show ip ospf interface is good for this. The command can provide the following information: Interfaces with OSPF enabled The OSPF process ID associated with an interface The area that the interface is associated with The IP address and…

Enabling an interface with OSPF is the quickest method to advertise its network to other OSPF enabled routers. It is not only the network that is advertised to other routers that is enabled – the interface now listens and takes part in the OSPF process itself. An unauthorised OSPF router can be plugged into that…