3com 5500-ei pwr Instrução De Instalação

 

4-10 

OSPF divides networks into four types by link layer protocols: 

z 

Broadcast: If Ethernet or FDDI is adopted, OSPF defaults the network type to broadcast. In a 

broadcast network, protocol packets are sent in multicast (224.0.0.5 and 224.0.0.6) by default. 

z 

Non-broadcast multi-access (NBMA): If Frame Relay, ATM, or X.25 is adopted, OSPF defaults the 

network type to NBMA. In an NBMA network, protocol packets are sent in unicast. 

z 

Point-to-multipoint (P2MP): OSPF will not default the network type of any link layer protocol to 

P2MP. A P2MP network must be compulsorily changed from another network type. The common 

practice is to change an NBMA network into a P2MP network. In a P2MP network, protocol packets 

are sent to the multicast address (224.0.0.5) by default, but protocol packets can be sent in the 

unicast address as needed. 

z 

Point-to-point (P2P): If PPP or HDLC is adopted, OSPF defaults the network type to P2P. In a P2P 

network, protocol packets are sent in multicast (224.0.0.5). 

An NBMA network is a non-broadcast and multi-accessible network. ATM and frame relay networks are 

typical NBMA networks. 

Some special configurations need to be done on an NBMA network. In an NBMA network, an OSPF 

router cannot discover an adjacent router by broadcasting Hello packets. Therefore, you must manually 

specify an IP address for the adjacent router and whether the adjacent router has the right to vote for a 

DR. 

An NBMA network must be fully connected. That is, any two routers in the network must be directly 

reachable to each other through a virtual circuit. If two routers in the network are not directly reachable 

to each other, you must configure the corresponding interface type to P2MP. If a router in the network 

has only one neighbor, you can change the corresponding interface type to P2P. 

The differences between NBMA and P2MP are as follows: 

z 

An NBMA network is fully connected, non-broadcast, and multi-accessible, whereas a P2MP 

network is not necessarily fully connected. 

z 

DR and BDR are required to be elected on an NBMA network but not on a P2MP network. 

z 

NBMA is a default network type. A P2MP network, however, must be compulsorily changed from 

another network type. The more common practice is to change an NBMA network into a P2MP 

network. 

z 

Since NBMA interfaces send packets to unicast addresses, you need to configure neighbors 

manually. By default, P2MP sends protocol packets to multicast addresses. 

In a broadcast network or an NBMA network, routing information needs to be transmitted between any 

two routers. If there are n routers in the network, n × (n-1)/2 adjacencies need to be established. In this 

case, the route changes on any router will result in multiple transmissions, which waste bandwidth. To