Netgear M4300-24X (XSM4324CS) - Stackable Switches with Full PoE+ Provisioning 管理者ガイド

The first networks were small enough for the end stations to communicate directly. As 
networks grew, Layer 2 bridging was used to segregate traffic, a technology that worked well 
for unicast traffic, but had problems coping with large quantities of multicast packets. The 
next major development was routing, where packets were examined and redirected at 
Layer 3. End stations needed to know how to reach their nearest router, and the routers had 
to interpret the network topology so that they could forward traffic. Although bridges tended to 
be faster than routers, using routers allowed the network to be partitioned into logical 
subnetworks, which restricted multicast traffic and also facilitated the development of security 
mechanisms.

An end station specifies the destination station’s Layer 3 address in the packet’s IP header, 

but sends the packet to the MAC address of a router. When the Layer 3 router receives the 
packet, it will minimally:

Look up the Layer 3 address in its address table to determine the outbound port.

Update the Layer 3 header. 

Re-create the Layer 2 header. 

The router’s IP address is often statically configured in the end station, although the switch 
supports protocols such as DHCP that allow the address to be assigned dynamically. 
Likewise, you can assign some of the entries in the routing tables used by the router 
statically, but protocols such as RIP and OSPF allow the tables to be created and updated 
dynamically as the network configuration changes.

The switch always supports Layer 2 bridging, but Layer 3 routing must be explicitly enabled, 
first for the switch as a whole, and then for each port that is to be part of the routed network. 

The configuration commands used in the example in this section enable IP routing on ports 
1/0/2,1/0/3, and 1/0/5. The router ID will be set to the switch’s management IP address, or to 
that of any active router interface if the management address is not configured. 

After the routing configuration commands have been issued, the following functions will be 

active:

IP forwarding, responsible for forwarding received IP packets.

ARP mapping, responsible for maintaining the ARP Table used to correlate IP and MAC 

addresses. The table contains both static entries and entries dynamically updated based 
on information in received ARP frames.

Routing Table Object, responsible for maintaining the common routing table used by all 
registered routing protocols. 

You can then activate RIP or OSPF, used by routers to exchange route information, on top of 
IP Routing. RIP is more often used in smaller networks, while OSPF was designed for larger 
and more complex topologies.