11.3.1 OSPF interface-type

OSPF classifies the interfaces that connect routers into the following three types, in terms of how packets are sent and received.

Broadcast

A broadcast type network in which multiple neighboring routers are managed in an integrated manner and addressed by multicast packets
Non-broadcast (NBMA)

A broadcast type network in which multiple neighboring routers are managed in an integrated manner, but which has no broadcast or multicast capability
Point-point

An interface that directly connects to a single neighboring router. It operates as a point-to-point interface on a virtual link.

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(1) Multihomed network

The Switch can also run OSPF on the secondary address assigned to an interface. Note the following when using OSPF on more than one IP network between multihomed routers in such a configuration:

On non-NBMA interfaces, routing packets that specify the multicast address are sent to all multihomed routers, which places an undue load on the networks and routers. To reduce the amount of unnecessary network traffic, configure the interfaces as NBMA interfaces.

(2) Notes on configuring interfaces that use OSPF

OSPF sometimes transmits packets that are equal in length to the Maximum Transfer Unit (MTU) value assigned to the interface. If the packet is longer than the Maximum Receive Unit (MRU, typically equal to the MTU) set on the interface that receives it, a situation might arise in which routers are unable to communicate with each other.This would not occur for normal traffic. For this reason, we recommend that you make sure that the MTU of every network and every router that connects them is set to a value equal to or less than the MRU of all other interfaces when using OSPF.