3Com 7600 manual 12-12CHAPTER 12 ATM, LAN EMULATION, and Virtual Lans

12-12CHAPTER 12: ATM, LAN EMULATION, AND VIRTUAL LANS

Bridge Forwarding In order to forward data frames efficiently to both local LAN stations on Database the transparent bridge as well as to remote LAN stations represented by other LECs on the ATM network, a combined Forwarding Database (FDB)

is established and maintained in the bridge. This FDB combines two tables. The first table is the FDB of the transparent bridge itself: a list of local LAN station MAC addresses together with the numbers of the bridge port to which they are attached (see “Forwarding Database” on page 11-7). The second table, called the LE ARP-cache, contains a list of remote LAN station MAC addresses together with the ATM addresses of the remote LECs which represent them. When a data frame is received at the bridge it searches its FDB for the destination MAC address. If the address is found to be local, the frame is forwarded to the appropriate bridge port. If the address is remote, the frame is sent over ATM to the destination LEC.

The part of the FDB of the bridge corresponding to the transparent bridge is populated as in a standard bridge (see “Forwarding Database” on page

11-7). The ARP-cache is populated by the following methods:

QAddress resolution - querying the LES

QAddress learning - from received data packets

Address Resolution When the bridge needs to forward a unicast data frame whose destination MAC address is not found in its FDB it attempts to locate a LEC that represents the destination MAC address on the ATM network. To do this, it turns to the LES in a process called address resolution.

As mentioned above, the LES is the central “directory” service for verifying LEC ATM addresses on an ELAN (see “LAN Emulation Server” on page 12-6). The LEC submits an LE_ARP (LE Address Resolution Protocol) Request to the LES. If an LEC ATM address corresponding to the submitted MAC address 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 will forward 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 bridge records the destination LEC ATM address together with the destination MAC address in its FDB ARP-cache.