Alcatel Carrier Internetworking Solutions Switch/Router ユーザーズマニュアル

Page 18-9

There are essentially two requirements for the any to any switching transformation function to 
address the network header fields:

• Network Address to MAC Address Mapping

In every protocol there is a mechanism for mapping global network wide addresses to the 
MAC addresses required in the local broadcast domain.

• Frame Size Requirements of the Media

Different media have different minimum and maximum frame sizes leading to the issues 
of padding insertion/stripping and fragmentation/reassembly or maximum frame size 
negotiation protocols at the network level.

There are almost as many ways to map a global network level address to a local subnetwork 
MAC address as there are routing protocols. These may or may not be affected by any to any 
switching.

Some may construct MAC addresses algorithmically, for example, DECNET model. Some may 
involve table lookups with an additional protocol to build and maintain these tables, for 
example, the IP/ARP model. Others may involve some form of building the network address 
around the MAC address as in the IPX model.

In all cases these mechanisms are susceptible, without good design and forethought, to the 
problem of canonical versus non-canonical representation of addresses in the network header 
area.

To map a 32-bit IP network address into the MAC address of a locally connected station a 
router uses the Address Resolution Protocol (ARP) to build an ARP Table. The router broad-
casts a request containing the IP address in the body of the frame. The station with that IP 
address responds with its MAC address in the body  of an ARP reply frame. The router inserts 
these two addresses in its ARP table and can then use the MAC address received to transmit 
any frames addressed to that IP address.

Since a router can have interfaces to Ethernet ports (canonical MAC addresses) and FDDI and 
Token Ring (non-canonical MAC addresses), it is crucial that the router keeps track of what 
media type it receives on each port.

If IP ARP were defined such that all MAC addresses, when conveyed in the body of an ARP, 
were in canonical format, switching would be easy. A router, when taking an address from 
the ARP table and using it as the destination MAC address on an Ethernet port would use the 
address as is. If sending to FDDI or Token Ring it would bit swap the address to non-canoni-
cal format as required by the media.