TCP/IP Tutorial and Technical Overview - IBM Redbooks

Basic InATMARP operates essentially the same as ARP, with the exception that
InATMARP does not broadcast requests. This is because the hardware address
of the destination station is already known. A requesting station simply formats a
request by inserting its source hardware and IP address and the known target
hardware address. It then zero fills the target protocol address field and sends it
directly to the target station. For every InATMARP request, the receiving station
formats a reply using the source address from the request as the target address
of the reply. Both sides update their ARP tables. The hardware type value for
ATM is 19 decimal and the EtherType field is set to 0x806, which indicates ARP,
according to RFC 3232.
Address resolution in a PVC environment
In a PVC environment, each station uses the InATMARP protocol to determine
the IP addresses of all other connected stations. The resolution is done for those
PVCs that are configured for LLC/SNAP encapsulation. It is the responsibility of
each IP station supporting PVCs to revalidate ARP table entries as part of the
aging process.
Address resolution in an SVC environment
SVCs require support for ATMARP in the non-broadcast environment of ATM. To
meet this need, a single ATMARP server must be located within the Logical IP
Subnetwork (LIS) (see “The Logical IP Subnetwork (LIS)” on page 53). This
server has authoritative responsibility for resolving the ATMARP requests of all IP
members within the LIS. For an explanation of ATM terms, refer to 2.10.2,
“Classical IP over ATM” on page 50.
The server itself does not actively establish connections. It depends on the
clients in the LIS to initiate the ATMARP registration procedure. An individual
client connects to the ATMARP server using a point-to-point VC. The server,
upon the completion of an ATM call/connection of a new VC specifying
LLC/SNAP encapsulation, will transmit an InATMARP request to determine the
IP address of the client. The InATMARP reply from the client contains the
information necessary for the ATMARP server to build its ATMARP table cache.
This table consists of:
► IP address
► ATM address
► Time stamp
► Associated VC
This information is used to generate replies to the ATMARP requests it receives.
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