Juniper Networks JUNOS OS 10.4 - Manual (Page 27)

segments can be linked at the frame level, and these devices are called bridges. Bridges
examine arriving frames and decide whether to forward them on an interface. All bridges
today are called learning bridges because they can find out more about the network than
could older bridges that were less intelligent devices. Bridges learn much about the LAN
segments they connect to from protocols like those in the Spanning Tree Protocol (STP)
family.
The network layer (Layer 3) is the highest layer used by network nodes to forward traffic
as part of the data plane. On the Internet, the network layer is the IP layer and can run
either IPv4 or IPv6, which are independent implementations of the same functions. The
IP layer defines the structure and purpose of the packet, which is in turn the content of
the frame at Layer 2. As expected, LAN segments (which now form perfectly functional
networks on their own at the frame level) can be linked at the network layer, and in fact
that is one of the major functions of IP. Devices that link LANs at the network layer are
called routers, and IP routers are the network nodes of the Internet.
Related
Documentation
Ethernet Networking •
• Ethernet Terms and Acronyms on page 3
• Network Addressing at Layer 2 and Layer 3 on page 7
• Networking at Layer 2: Benefits of Ethernet Frames on page 9
• Networking at Layer 2: Challenges of Ethernet MAC Addresses on page 10
• Networking at Layer 2: Forwarding VLAN Tagged Frames on page 11
• Networking at Layer 2: Forwarding Dual-Tagged Frames on page 13
• Networking at Layer 2: Logical Interface Types on page 14
• A Metro Ethernet Network with MX Series Routers on page 15
• Layer 2 Networking Standards on page 17
Network Addressing at Layer 2 and Layer 3
The Internet is a global, public network with IP subnets connected by routers and
exchanging packets. Can a global, public network consist of Ethernet LANs connected
by bridges and exchanging frames? Yes, it can, but there are several differences that
must be addressed before Ethernet can function as effectively as IP in the metropolitan
area (Metro Ethernet), let alone globally. One of the key differences is the addresses
used by Layer 2 frames and Layer 3 packets.
Both Ethernet and IP use globally unique network addresses that can be used as the
basis for a truly global network. Ethernet MAC addresses come from the IEEE and IP
subnet addresses come from various Internet authorities. (IP also employs a naming
convention absent in Ethernet, but we'll ignore that in this discussion.) The key differences
in how these addresses are assigned make all the difference when it comes to the basic
functions of a bridge as opposed to a router.
7 Copyright © 2013, Juniper Networks, Inc.
Chapter 1: Overview of Ethernet Solutions