Ip Routing Overview - 3Com corebuilder 3500 Implementation Manual

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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

OSI Reference Model

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data link layer

Physical layer

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.

Routing Overview