3Com 4007, Руководство по внедрению, Страница: 356 / 684

356 C
HAPTER
16: IP R
OUTING
3Com Bridging and
Routing
The destination MAC address determines whether the module bridges or
routes a packet. Before a host system sends a packet to another host, the
host system compares its own network address to the network address of
the other host as follows:
■
If network addresses are on the same subnetwork, the packet is
bridged directly to the destination address of the host.
■
If network addresses are on different subnetworks, the packet must
be routed from one to the other. In this case, the host sends an ARP
request for its default gateway MAC address, then transmits the
packet using the MAC address of the default gateway.
Figure 30 illustrates bridging on a 3Com Multilayer Switching Module:
1 The packet enters the module.
2 The bridging layer examines the destination MAC address of the packet.
The destination MAC address does
not correspond to the MAC address
of one of the module ports that are configured for routing.
3 The bridging layer selects a segment (port) based on the destination MAC
address and forwards the packet to that segment.
Figure 30 3Com Bridging
1
Transmitting host
3
Destination host
2
1 2 3
Router interfaces
Bridging layer
Routing layer
Subnetworks 1 2 3
Router
Bridge