11-8
C
HAPTER
11: E
THERNET
LAN
S
AND
B
RIDGES
IN
THE
7600 C
ARD
Static Addresses
The network manager may want to restrict a particular station to
communicate only through specific input and output bridge ports in
order to keep its traffic confined to certain portions of the network
topology. Or, he may want to maintain a station that transmits so
infrequently that it would be aged-out by the aging mechanism. For
these cases the learning method is not suitable. Instead, the FDB provides
for address entries called
static
addresses. These are entered or removed
manually by the operator (and are therefore also called
Management
addresses) or they can be supplied with the device. The
Read-only
address
is a special type of static address which is supplied with the device for
internal use and cannot be altered. An example of a Read-only address is
the address of a bridge port, called “
Self
”.
Defining
permanency of
static addresses
In the 7600 Card, static addresses with various degrees of permanency
can be defined as follows:
Q
Permanent - Entries that remain in the FDB when the device is reset
Q
Delete-on-Reset - Entries that are deleted when the device is reset
Q
Delete-on-Timeout - Static entry that is deleted after the aging time
Allowed ports for
static addresses
Input and output bridge ports assigned (
allowed
) to a static address are
entered or removed by the operator or are predefined. Static addresses
can be addresses of individual destination stations but they are more
frequently
multicast
addresses - addresses which include a group of
destination stations. A packet with a multicast address normally needs to
be transmitted on more than one bridge port in order to reach all the
destination stations. Consequently,
multiple
output bridge ports are often
allowed to a single static address. For data display, multiple allowed
output ports are coded into a single binary number. Normally, one input
(or
receiving
) port is allowed to a static address, but it is also possible to
designate
all
ports as allowed receiving ports.
Blocking ports for
static addresses
Instead of specifying the allowed input and output bridge ports for a
static address, it is sometimes more convenient to specify those input and
output bridge ports which are
not
allowed for that address (
blocked
ports). Then all ports other than the blocked ports can receive and
transmit for that address.
Spanning Tree
Protocol
When designing a bridged-LAN environment is it desirable to build
redundant bridge links between LANs: in case one link fails, another can
serve as a backup. However, when a LAN is attached to two or more
abtthgde.book Page 8 Tuesday, June 23, 1998 10:29 AM
