3Com CoreBuilder 7000, Operation Manual

Page: 159 / 264

LAN Emulation Redundancy 159
same secondary LES to replace a failed active primary LES and the
topology databases remain the same. The split ELAN is an exception; it is
discussed in the next section.
If neither primary nor secondary LES can be made active, an error
message is issued to a LEC whenever it tries to join the network.
Note that when a primary LES for which no secondary LES is defined fails,
the LECS continues attempting to assign a requesting LEC to the ELAN
until the LEC stops requesting.
Recovery from Split ELAN
A split ELAN is one in which the secondary LES becomes active along with
the first active LES; the two LESes act to divide the LECs of the ELAN into
two groups, preventing communication between them.
Such a split ELAN can occur, for example, when the location of a link
failure in the network is such that the ATM connection between an active
LES to one LECS is terminated while the connection to another LECS is
not. In addition, the same link failure isolates one LECS from the other. In
this situation, the first LECS decides that the primary active LES has failed
and replaces it with the secondary LES while the second LECS maintains
the primary LES as active.
When the link is repaired, the network recognizes the split ELAN and
recovery occurs automatically as follows: Each LECS now begins receiving
ACTIVE_LES messages from both primary and secondary LESes of the split
ELAN; this tells the LECS that a split ELAN exists. Both LECS act
simultaneously to re-establish the primary LES in the ELAN topology
database as the sole active LES of the ELAN. A message is sent to the
secondary LES instructing it to disconnect all LECs attached to it. When
those LECs turn to the LECS to be readmitted to the ELAN they are
directed to the primary LES and the ELAN is reunited.
Selecting LAN
Emulation
Redundancy Mode
Two types of LANE redundancy are available in the CoreBuilder 7000 ATM
switch: LECS-based LANE redundancy as described previously, or the
NMS-based LANE redundancy which was the standard prior to version 4.
You can select the type of redundancy through LMA (see “Enable/Disable
LE Servers Redundancy” on page 264 in the Management Guide). If you
select LECS-based LANE redundancy the LECS of the switch must be
enabled. The factory default is the NMS-based LANE redundancy.