Juniper Networks 10.4 Networkingat Layer 2: Challenges of Ethernet MAC Addresses

NOTE: Networkingat the frame levelsays nothing about the presence or

absenceof IP addressesat the packet level. Almostall ports, links, and devices

ona network of LAN switchesstill have IP addresses, just as do all the source

anddestination hosts. There are many reasonsfor the continued need for IP,

notthe least of which is the need to manage the network.A device or link

withoutan IP address is usually invisible to most managementapplications.

Also,utilities such as remote access fordiagnostics, file transfer of

configurationsand software, and so on cannot run without IP addressesas

wellas MAC addresses.

Documentation

MXSeries Ethernet Services Routers SolutionsPage•

•EthernetTerms and Acronymson page 3

•Networkingand Internetworking with Bridges and Routerson page 6

•NetworkAddressing at Layer 2 and Layer3 on page 7

•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

Networkingat Layer 2: Challenges of Ethernet MAC Addresses

Ifa networked Layer 2 device such asa bridge or LAN switch could contain a list of all

knownMAC addresses, then the network node couldfunction in much the same way as

arouter, forwarding framesinstead of packets hop-by-hop through the network from

sourceLAN to destination LAN. However,the MAC address is much larger than the IPv4

addresscurrently used on the Internet backbone (48bits compared to the 32 bits of

IPv4).

Thisposes problems. Also, because the MACaddress has no “network organization” like

theIPv4 or IPv6 address, an Layer2 network node must potentially storeevery conceivable

MACaddress in memory for next-hop table lookups.Instead of tables of about 125,000

entries,every Layer 2 network node wouldhave to store millions of entries (for example,

24bits, the potential NIC production from one Ethernet vendor,would require a table of

morethan 16 million entries).

Documentation

MXSeries Ethernet Services Routers SolutionsPage•

•EthernetTerms and Acronymson page 3

•Networkingand Internetworking with Bridges and Routerson page 6

•NetworkAddressing at Layer 2 and Layer3 on page 7

Junos10.4 MX Series Ethernet ServicesRouters Solutions Guide