By setting network types for OSPF interfaces and adjusting the OSPF attributes, you can flexibly build OSPF networks.
In Table 1, OSPF classifies networks into four types based on the types of link layer protocols.
Network Type |
Characteristic |
Default Configuration |
|---|---|---|
Broadcast |
On a broadcast network, Hello packets, LSU packets, and LSAck packets are transmitted using multicast; DD packets and LSR packets are transmitted using unicast. |
If the link layer protocol is Ethernet or Fiber Distributed Data Interface (FDDI), OSPF regards the network as a broadcast network by default. |
Non-broadcast multiple access (NBMA) |
On an NBMA network, Hello packets, DD packets, LSR packets, LSU packets, and LSAck packets are transmitted using unicast. An NBMA network must be fully meshed. That is, any two switches on an NBMA network must be directly reachable. |
If the link layer protocol is ATM, OSPF regards the network as an NBMA network by default. |
Point-to-point (P2P) |
On a P2P network, Hello packets, DD packets, LSR packets, LSU packets, and LSAck packets are transmitted using multicast. |
If the link layer protocol is PPP, HDLC, or Link Access Procedure Balanced (LAPB), OSPF regards the network as a P2P network by default. |
Point-to-multipoint (P2MP) |
On a P2MP network, Hello packets are transmitted using multicast; DD packets, LSR packets, LSU packets, and LSAck packets are transmitted using unicast. |
OSPF does not regard a network as a P2MP network by default regardless of its link layer protocol. A P2MP network must be forcibly changed from another network type. |
As described in Table 1, the difference between OSPF configurations on four types of networks lies in the packet transmission mode.