3Com CoreBuilder 3500, Implementation Manual, Page: 261 / 592

Routing Overview 261
IP Routing Overview An IP router, unlike a bridge, operates at the network layer of the Open
Systems Interconnection (OSI) Reference Model. The network layer is also
referred to as Layer 3. An IP router routes packets by examining the
network layer address (IP address). Bridges use data link layer MAC
addresses to perform forwarding. See Figure 41.
Figure 41
OSI Reference Model and IP Routing
When an IP router sends a packet, it does not know the complete path to
a destination — only the next hop (the next device on the path to the
destination). Each hop involves three steps:
1 The IP routing algorithm computes the next hop IP address and the next
router interface, using routing table entries.
2 The Address Resolution Protocol (ARP) translates the next hop IP address
into a physical MAC address.
3 Using the physical MAC address, the router sends the packet out the
appropriate bridge port over the network to the next hop.
For more information about IP addresses and next hops, see “IP
Addresses” in this chapter.
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
MAC
IP
OSI Reference Model
ARP
RIP
ICMP
OSPF
IP Routing
Bridging