By default, the LSI will use the MAC address burned into the associated ATM
adapter as the source MAC address of frames transmitted over the LANE shortcut
VCCs. It is possible, though unlikely, that this could confuse some end-station
protocol stack implementations, since the MAC address will not match that of the
router that the end-station uses as a gateway to transmit packets to the associated
IP address.
For this to happen, the end-station would have to learn router MAC addresses from
unicast IP frames which is not normal (IP-to-MAC address mappings are normally
learned from ARP packets). If this were to happen, the end-station might use the
learned MAC address as the destination MAC address of frames that it transmits to
the associated IP destination instead of using the MAC address of the router. Such
frames would either be dropped or forwarded over the LANE shortcut VCC.
Forwarding would only occur if the LEC learns MAC-to-ATM address binding from
received frames (which is an optional implementation choice).
In either case, these frames will not reach the destination since the LSI discards
frames received over a LANE shortcut VCC. Furthermore, the LSI releases the
LANE shortcut VCC and no further shortcuts will be established to the associated
ATM address.Traffic for destinations associated with that ATM address will follow
the routed path thereafter. Note that ELS messages and console display for LANE
shortcuts aid in identifying these destinations.
The LSI can be configured not to use the universally administered MAC address as
the source MAC address. With this option, you have two choices for the source
MAC address:
1. Youcan use the MAC address of the last-hop router, provided in the NHRP
resolution reply packet, as the source MAC address.
Using the last-hop router’s MAC address as the source MAC address solves the
problem of end-station protocol stack confusion but introduces another potential
problem. It may confuse LECs that learn MAC-to-ATM address binding from
received frames, and therefore should not be used with LECs that perform this
type of learning. For example, the LEC in IBM’s 8281 ATM-LANbridge performs
this type of learning.
2. Youcan configure the source MAC address.
The source MAC address can be configured to avoid the problem of duplicate
MAC addresses seen on an ELAN because of inter-ELAN shortcuts. The MAC
address should be configured for this LSI network when there are any
disallowed LANE shortcut entries. See “LANE Shortcuts” on page372 for details
on displaying disallowed LANE shortcut entries.
These configuration options are provided to maximize flexibility in achieving
compatibility with the largest possible set of destinations in a given installation. See
“Configuring the LANE Shortcuts Interface (LSI)” on page357 for further
information and “Change” on page363 for a description of the change command.
Configuration Parameters
This section describes some of the NHRP related configuration parameters and
their recommended usage. See “NHRP Configuration Commands” on page359 for
command syntax, command parameters, valid values and default values.
Using NHRP
Chapter11. Using NHRP 353