Juniper Networks 10.4 Networkingand Internetworking with Bridges and Routers

•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

Networkingand Internetworking with Bridges and Routers

Traditionally,differenthardware, software, and protocolshave been used on LANs and

onnetworks that cover wider areas(national or global). A LAN switch is different than

arouter, an Ethernet frame is differentthan an IP packet, and the methods used to find

destinationMACaddresses are different than those used to find destinationIP addresses.

Thisis becauseLANs based on Ethernet were intended for differentnetwork environments

thannetworks based on IP.The Internet protocol suite (TCP/IP)was intended as an

internetworkingmethodto connect local customernetworks. The local customer network

thata serviceprovider's IP routers connected wasusually based on some form of Ethernet.

Thisis why Ethernet and IP fit so welltogether: Ethernet defines the LAN, and the Internet

protocolsdefine how these LANs are connected.

Morespecifically,Ethernet LANs and IP networks occupy different layersof the Internet’s

TCP/IPprotocolsuite.Between sender and receiver,networks deal with the bottom three

layersof the model: the physical layer(Layer 1), the data link or MAC layer (Layer2), and

thenetwork layer (Layer 3).

NOTE: Theselayers are alsofound in the Open Systems Interconnect

ReferenceModel (OSI-RM); however,in this chapter they are applied to the

TCP/IPprotocolsuite.

Alldigital networks ultimatelydeal with zeroes and ones, and the physical layer defines

bitrepresentationon the media. Physical layer standardsalso define mechanical aspects

ofthe network, such as electricalcharacteristics or connectorshapes, functional aspects

suchas bit sequence and organization, and soon. The physical layer only “spits bits” and

hasvery little of the intelligencerequired to implement a completenetwork. Devices that

connectLAN segments at the physical layerare called hubs, and all bits that appear on

oneport of the hub are also sent out on the other ports. This also means that bad bits

thatappear on one LAN segment are propagated toall other LAN segments.

Abovethe physical layer,the data link layer defines the first-order bit structure,or frame,

forthe network type. Also loosely calledthe MAC layer (technically,the MAC layer is a

sublayerrequired only on LANs), Layer2 sends and receives frames. Frames are the last

thingsthat bits were before they leftthe sender and the first things that bits become

whenthey arrive on an interface. Because frameshave a defined structure, unlike bits,

framescan be used for errordetection, control plane activities (not all frames must carry

userdata: some frames are used by the network tocontrol the link), and so forth. LAN

segmentscan be linked at the frame level, and thesedevices are called bridges. Bridges

examinearriving frames and decide whether to forwardthem on an interface. All bridges

todayare called learningbridges because they can find out more about the network than

Junos10.4 MX Series Ethernet ServicesRouters Solutions Guide