couldolder bridges that were lessintelligent devices. Bridges learn much about the LAN
segmentsthey connect to from protocolslike those in the Spanning TreeProtocol (STP)
family.
Thenetwork layer (Layer3) is the highest layer used by network nodes to forwardtraffic
aspart of the data plane. On the Internet, the network layeris the IP layer and can run
eitherIPv4 or IPv6, which are independent implementations of the same functions. The
IPlayer defines the structure and purpose of the packet, which is in turn the contentof
theframe at Layer 2. As expected,LAN segments (which now form perfectly functional
networkson their own at the frame level)can be linked at the network layer,and in fact
thatis one of the major functions of IP. Devices thatlink LANs at the network layer are
calledrouters, and IP routers are the networknodes of the Internet.
Related
Documentation
MXSeries Ethernet Services Routers SolutionsPage•
•EthernetTerms and Acronymson page 3
•NetworkAddressing at Layer 2 and Layer3 on page 7
•Networkingat Layer 2: Benefits of Ethernet Frames on page9
•Networkingat Layer 2: Challenges of EthernetMAC Addresses on page 10
•Networkingat Layer 2: Forwarding VLAN TaggedFrames on page 11
•Networkingat Layer 2: Forwarding Dual-TaggedFrames on page 13
•Networkingat Layer 2: Logical InterfaceTypes on page 14
•AMetro Ethernet Network with MX Series Routers on page15
•Layer2 Networking Standards on page 17
NetworkAddressing at Layer 2 and Layer 3
TheInternet is a global, public network with IP subnets connectedby routers and
exchangingpackets. Can a global, public networkconsist of Ethernet LANs connected
bybridges and exchanging frames? Yes,it can, but there are several differencesthat
mustbe addressed before Ethernet can function as effectivelyas IP in the metropolitan
area(Metro Ethernet), let alone globally.One of the key differences is the addresses
usedby Layer 2 frames and Layer 3 packets.
BothEthernet and IP use globally unique network addresses thatcan be used as the
basisfor a truly global network. Ethernet MACaddresses come from the IEEE and IP
subnetaddresses come from various Internet authorities. (IP also employsa naming
conventionabsentin Ethernet, but we'll ignore that in this discussion.) The key differences
inhow these addresses are assigned make all the differencewhen it comes to the basic
functionsof a bridge as opposed to a router.
7Copyright© 2010, Juniper Networks,Inc.
Chapter1: Overview of Ethernet Solutions