The OSPF protocol supports multiple network types. In different network types, OSPF neighbor establishment and DR and BDR election methods are different.
The OSPF protocol supports five network types, namely:
Broadcast (
Point-to-Point)
Point-to-multipoint (Point-to-multipoint)
Point-to-multipoint
non-broadcast (Non-Broadcast Multiple Access) , NBMA).
First look at the working principle of these types of networks:
Broadcast network: supports broadcast and multicast. When the link layer protocol is Ethernet or FDDI, OSPF considers the network type to be broadcast.
Point-to-point: supports broadcast and multicast, and the link layer protocol is PPP or HDLC.
Point-to-multipoint: supports broadcast and multicast, which can be understood as a collection of multiple point-to-point networks. There is no link layer protocol, and it needs to be forcedly changed from other network types.
Point-to-multipoint non-broadcast: broadcast and multicast are not supported.
NBMA: Although this type of network can reach multiple destination nodes on one interface, the network itself does not support the broadcast function. When the link layer protocol is Frame Relay or X.25, OSPF assumes that the network type is NBMA by default.
One: the establishment of neighbors
The establishment of OSPF neighbors is achieved by the two parties sending hello packets to each other. The hello packets are sent by multicast, and the multicast address is 224.0.0.5. Therefore, whether the network type supports broadcasting is a judgment condition for whether the neighbor relationship can be automatically established.
Broadcast network: The broadcast network supports multicast, and neighbors can be established automatically.
Point-to-point: Point-to-point networks support broadcasting, so neighbors can be established automatically.
Point-to-multipoint: Point-to-multipoint networks can be regarded as multiple point-to-point networks, so neighbors can also be established automatically.
Point-to-multipoint non-broadcasting: This network type does not support broadcasting, so neighbors need to be specified manually.
NBMA: Non-broadcast multi-access network, which does not support broadcasting, so the neighbor relationship needs to be specified manually.
Two: DR and BDR election
OSPF elects DR and BDR to limit unnecessary LSA flooding. Therefore, the principle of judging whether to elect DR and BDR is whether there can be multiple OSPF interfaces on the network type.
Broadcast network: The broadcast network supports multiple OSPF routers in the same broadcast domain, so DR and BDR need to be elected.
NBMA: Non-broadcast multi-access network. The network can have multiple OSPF routers on one link, so DR and BDR need to be elected.
Point-to-point: There are only two routers on a link in a point-to-point network, so there is no need to elect DR and BDR.
Similarly, point-to-multipoint and point-to-multipoint non-broadcast networks do not need to elect DR and BDR.
Summary:
The establishment of OSPF neighbors by different network types and whether DR and BDR elections are required are shown in the following table: