Caution

Make certain that all of the LECS database are identical.

The LECS redundancy mechanism is implemented by control connections between the LECSs. The LECS uses ILMI to acquire the complete list of LECSs servers from the ATM switch (just as an LEC does). Each LECS then builds an SVC to every LECS listed below itself on the list. The one LECS with no inbound connections is the primary LECS. However, if the primary fails, its connection to the second LECS closes, causing the second LECS to no longer have any inbound connections. For example, imagine three LECS devices A, B, and C. A is listed first on the LS1010 and C is listed last. After all three switches acquire the complete list of LECSs via ILMI, the A device opens a connection to B and C, and B opens a connection to C. Because B has one inbound connection and C has two inbound connections, both of these devices are backup LECSs. Because it has no inbound connections, A becomes the primary LECS. However, if A fails, the connection to B fails causing B to become the primary. If B fails, its connection to C drops and C becomes the primary.

The LES/BUS redundancy mechanism also utilizes ILMI. Upon startup, all LES/BUSs use ILMI to locate the primary LECS. Every LES/BUS then builds a connection to this single LECS. The Configuration Server then evaluates all of the inbound connections to determine which connection is from the LES/BUS listed first in the LECS database.

That LES/BUS' NSAP is then provided in LE_CONFIGURE_REPLY messages to LANE Clients.

Dual-PHY

Cisco offers Dual-PHY on all of the high-end Catalyst LANE Modules (such as the Catalyst 5000 and 6000). Although these cards still have only a single set of ATM and AAL logic, it provides dual physical layer paths for redundant connections. This allows the LANE card to connect to two different ATM switches (in case one link or switch fails), but only one link can be active at a time. You can configure which link is preferred with the atm preferred phy command.

Although this feature is of great value in large networks, it does introduce a subtle configuration change. Notice the configuration in Figure 9-27.

Figure 9-27 Dual-PHY Connection to Two ATM Switches

Figure 9-27 Dual-PHY Connection to Two ATM Switches

If A is the preferred port, the resulting NSAP is the following: 47.0091.8100.0000.1111.2222.3333.0000.0C12.3456.01

However, if the left-hand switch fails, the NSAP changes when the B port becomes active:

47.0091.8100.0000.AAAA.BBBB.CCCC.0000.0C12.3456.01

Although this change does not affect LANE Clients, it has a significant influence on LECS and LES/BUS configurations that use SSRP. In practice, this means that the LS1010 must list four LECS addresses to provide a single redundant LECS device. In addition, the LECS database must list four LES addresses to provide a single redundant LES/BUS.

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