Bridges in the 7600 Card
12-13
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 (the 7600
Card actually inserts a pointer to this VCC directly into the FDB so that a
destination MAC address can be immediately associated with the right
VCC).
VCC to BUS
In a similar way, the bridge uses address resolution during initialization to
ascertain the ATM address of the BUS. Once known, the multicast-send
and multicast-forward VCCs are established between LEC and BUS and
any unknown unicast, multicast or broadcast messages are send on it.
Learning Addresses
The bridge learns addresses from data frames received over data-direct
VCCs in the same manner as the transparent bridge learns from data
frames received from LAN stations on local Ethernet segments (see
“Learning Station Addresses” on page 11-7). The bridge extracts the
source MAC address in each received data frame and compares it with
the entries in the FDB ARP-cache. If not found, it is inserted together with
the ATM address of the LEC that sent it (or, equivalently, the pointer to
the VCC on which it was received). Then, the next time a data frame
needs to be sent to that MAC address, the bridge picks up its VCC from
the FDB ARP-cache.
LES Address
Registration in the
7600 Card
To populate the LES directory, LECs are normally required to
register
the
MAC addresses of LAN stations they represent with the LES. However, the
situation is different for LAN stations which reside on a transparent
bridge, as is the case for LANs attached to the 7600 Card. In this case, the
LANs are considered to be “remote” from the LEC and the LEC is called a
proxy
for the bridged LANs. LES address registration does not apply to
proxy LECs. Instead, LE_ARP requests for MAC addresses on bridged
LANs are passed on by the LES to the proxy LEC for address resolution.
The requested MAC address is found in the bridge FDB and returned in
the LE_ARP response.
Unknown address
Note that in the initial phase of operation of the ELAN, a MAC address of
a bridged LAN station which has been requested from its proxy LEC may
not have been learned yet by the bridge so that the LE_ARP request will
be unsuccessful. As a result, a specific destination LEC address will not be
available to the transmitting LEC and, consequently, no data-direct VCC
will be established. A LAN station MAC address for which no destination
LEC address is available yet is called an
unknown
address.
abtthgde.book Page 13 Tuesday, June 23, 1998 10:29 AM
