Networking At Layer 2: Challenges Of Ethernet Mac Addresses - Juniper JUNOS OS 10.4 Manual

Junos OS 12.1 MX Series 3D Universal Edge Routers Solutions Guide
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Networking at Layer 2: Challenges of Ethernet MAC Addresses

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10
NOTE: Networking at the frame level says nothing about the presence or
absence of IP addresses at the packet level. Almost all ports, links, and devices
on a network of LAN switches still have IP addresses, just as do all the source
and destination hosts. There are many reasons for the continued need for IP,
not the least of which is the need to manage the network. A device or link
without an IP address is usually invisible to most management applications.
Also, utilities such as remote access for diagnostics, file transfer of
configurations and software, and so on cannot run without IP addresses as
well as MAC addresses.
Ethernet Networking
Ethernet Terms and Acronyms on page 3
Networking and Internetworking with Bridges and Routers on page 6
Network Addressing at Layer 2 and Layer 3 on page 7
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
If a networked Layer 2 device such as a bridge or LAN switch could contain a list of all
known MAC addresses, then the network node could function in much the same way as
a router, forwarding frames instead of packets hop-by-hop through the network from
source LAN to destination LAN. However, the MAC address is much larger than the IPv4
address currently used on the Internet backbone (48 bits compared to the 32 bits of
IPv4).
This poses problems. Also, because the MAC address has no "network organization" like
the IPv4 or IPv6 address, an Layer 2 network node must potentially store every conceivable
MAC address in memory for next-hop table lookups. Instead of tables of about 125,000
entries, every Layer 2 network node would have to store millions of entries (for example,
24 bits, the potential NIC production from one Ethernet vendor, would require a table of
more than 16 million entries).
Ethernet Networking
Ethernet Terms and Acronyms on page 3
Networking and Internetworking with Bridges and Routers on page 6
Network Addressing at Layer 2 and Layer 3 on page 7
Copyright © 2012, Juniper Networks, Inc.