3Com CoreBuilder 9000, Руководство пользователя

Страница: 388 / 468

388 C
HAPTER
15: LAN E
MULATION
Address Resolution When a LEC needs to forward a unicast data frame whose LAN
destination is not found in its local Forwarding Data Base, it attempts to
locate a LEC that represents the LAN destination on the ATM network. To
do this, it turns to the LES in a process called
address resolution .
As mentioned previously, the LES is the central “directory” service for
verifying LEC ATM addresses on an ELAN. (See “LAN Emulation Server”
on page 376.) The LEC submits an
LE_ARP (LE Address Resolution
Protocol) Request to the LES. If a LEC ATM address that corresponds to
the submitted LAN destination is found in the LES table, it is transmitted
back to the requesting LEC as an
LE_ARP Response. In case the requested
address is not found at the LES, the LES forwards the query to other LECs
to search the non-registered addresses and then return a positive
response back to the requesting LEC. On receipt of an LE_ARP Response,
the LEC records the destination LEC ATM address together with the LAN
destination in its FDB ARP-cache. The way that the LES handles LE_ARP
requests can be configured by management. (See “Configuring LES
LE_ARP Policies” on page 388.)
Data-direct VCC On receipt of the destination LEC address, the transmitting LEC
establishes a data-direct VCC for direct communication across the ATM
network to the destination LEC if one does not already exist.
VCC to BUS In a similar way, the LEC uses address resolution during initialization to
ascertain the ATM address of the BUS. When the ATM address of the BUS
is known, the multicast-send and multicast-forward VCCs are established
between LEC and BUS and any unknown unicast, multicast, or broadcast
messages are sent on it.
Configuring LES
LE_ARP Policies
The way in which the LES handles LE_ARP requests from a LEC can be
configured by management. Each LES in the CoreBuilder switch can be
configured independently. Three such policies can be set:
LE_ARP Response Policy
ELAN Response Policy
LE_ARP Response Policy for Route Descriptors