3com 7600 Benutzerhandbuch
12-12
C
HAPTER
12: ATM, LAN E
MULATION
,
AND
V
IRTUAL
LAN
S
Bridge Forwarding
Database
In order to forward data frames efficiently to both local LAN stations on
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 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:
is populated as in a standard bridge (see “Forwarding Database” on page
11-7). The ARP-cache is populated by the following methods:
Q
Address resolution - querying the LES
Q
Address 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.
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.
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.
abtthgde.book Page 12 Tuesday, June 23, 1998 10:29 AM