Although the automatic NSAP scheme based on MAC addresses and subinterface numbers does allow for very easy configuration of LANE across Cisco devices, it can lead to maintenance problems. Because the MAC addresses used for the NSAP scheme are tied to the chassis or Supervisor module, swapping cards can create a need for minor reconfigurations. Chassis that support redundant Supervisors such as the 55XX family derive their MAC address from chips located on the backplane. Each slot is assigned a block of addresses, causing problems if either the chassis is changed or the LANE module is moved to a different slot. Chassis that only support a single Supervisor engine (such as the Catalyst 5000) pull their MAC addresses from the Supervisor module itself. Again, each slot is assigned a block of addresses. However, in this case, problems do not result from a change in the chassis (after all, the backplane in these platforms is passive) but from a change in Supervisor modules or if the LANE module is moved to another slot.
Note that reconfiguration is only required if the device acting as the LECS or the LES is changed. If the LECS changes, you need to modify the NSAP address configured on every ATM switch (but, assuming ILMI or well-known LECS NSAPs are in use, this does not require a change on the LECs). If the LES changes, you need to modify the LES' NSAP listed in the LECS database.
The LANE 1.0 spec released in January 1995 did not provide for a server-to-server protocol (known as the LNNI [LANE Network-Network Interface] protocol). Because this limitation prohibited multiple servers from communicating and synchronizing information with each other, the ATM Forum had a simple solution: allow only a single instance of each type of server. The entire network was then dependent on a single LECS, and each ELAN was dependent on a single LES and BUS. Because this obviously creates multiple single points of failure, most ATM vendors have created their own redundancy protocols. Cisco's protocol is known as the Simple Server Redundancy Protocol (SSRP).
SSRP allows for an unlimited number of standby LECS and LES/BUS servers, although one standby of each type is most common. It allows the standby servers to take over if the primary fails, but it does not allow mu ltiple active servers. One of the primary benefits of SSRP is that it is interoperable with most third-party LECs.
An additional benefit of SSRP is that it is easy and intuitive to configure. To provide for multiple LECSs, you simply need to configure multiple atm lecs-address-default commands on every ATM switch. To provide for multiple LES/BUSs for every ELAN, just add multiple name ELAN_NAME server-atm-address NSAP commands to the LECS database. For example, the commands in Example 9-16 configure two LECSs on a LS1010 ATM switch.
Was this article helpful?