3com 7600 Benutzerhandbuch

Data Transmission over an ELAN 12-15

When a LEC has established through address resolution that a certain 
LAN destination corresponds to a certain LEC ATM address, and a 
data-direct VCC exists to that ATM address, then a data frame addressed 
to that LAN destination is forwarded via that VCC.

When a unicast frame to be transmitted by a LEC has an unknown 
address, a data-direct VCC is not yet available. In this situation, there are 
two possible actions that the LEC can perform, either separately or in 
combination: 

Q

The unknown unicast frame can be transmitted over the 
multicast-send VCC to the BUS. The BUS forwards the frame to all 
LECs and their attached bridges then flood the frame to all bridge 
ports to ensure that the frame is received at its destination. 

Q

Address resolution can be performed continuously on the unknown 
address until the address becomes known and a VCC can be set up. 

The 7600 Card allows these two actions to be performed in combination 
according to management selection. The unknown unicast flooding mode 
allows the user to specify one of the following possibilities for 
transmitting unknown unicast frames by a bridge:

The number of unknown frames sent to the BUS in the time period 
maximum-unknown-frame-time

 

is limited by the LEC parameter 

maximum-unknown-frame-count. 

Table 12-5   Unknown Unicast Flooding Mode

No flooding

No unicast frames are sent to the BUS.

Flooding until ARP 
Request 

Unicast frames are sent to the BUS until the LE_ARP 
request is sent.

Flooding until ARP 
Response

Unicast frames are sent to the BUS until the LE_ARP 
response is received.

abtthgde.book  Page 15  Tuesday, June 23, 1998  10:29 AM