3Com manual Bridges in the 7600 Card

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 To populate the LES directory, LECs are normally required to register the Registration in the MAC addresses of LAN stations they represent with the LES. However, the

7600 Card 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.