higher value, the forward delay parameter can be used...

LANE Redundancy
Although LANE allows network designers to connect their legacy LANs to an ATM network, LANE
Version 1.0 does not define mechanisms for building redundancy and fault tolerance into the LANE
services. Consequently, this makes the LANE services a single point of failure. Moreover, router
redundancy and path/link redundancy are also issues that the network designer needs to consider.
Network designers can use the following techniques to build fault-tolerant and resilient LANE
networks:
• Simple Server Replication Protocol (SSRP) for LANE Services redundancy that works with
Cisco and any third-party LECs.
• Hot Standby Router Protocol (HSRP) over LANE provides redundancy for the default router
configured at IP end stations.
8-24
Cisco CCIE Fundamentals: Network Design
LANE Redundancy
• Dual PHY LANE card on the Catalyst 5000 switch, or multiple ATM uplinks on the Catalyst
3000 switch.
• Spanning-Tree Protocol on the Ethernet-ATM switches.
The following subsections examine these various mechanisms and highlights design rules and issues to consider while implementing redundant LANE networks. It begins with a discussion on SSRP that
was developed to provide redundant LANE services.
Although many vendors have implemented redundant LANE services of some fashion, they violate
the LANE 1.0 specification and therefore are not interoperable with other third-party
implementations. SSRP, however, does not violate the LANE 1.0 specification and is interoperable
with third-party LEC implementations, which is important when implementing an interoperable
ATM network.
The discussion on SSRP is followed by a description of HSRP over LANE, which provides a
mechanism for building router redundancy. Following this is a discussion on the Spanning-Tree
Protocol and other product-specific features that can be used to build link and path redundancy into edge devices.
Issues in a LANE 1.0 Network
The main issue with a LANE 1.0 network is that only one set of LANE service components can be
accessed by a LEC at any given time. This results in the following limitations:
• Only a single LECS supports all ELANs.
• There can be only one LES/BUS pair per ELAN.
A failure in any of these service components has the following impact on network operation:
• LECS failure—A failed LECS impacts all the ELANs under its control because it provides access control for all the ELANs under its control. Although the existing ELANs would continue to
work normally (assuming only Cisco LECs), no new LEC can join any ELAN under the control
of that LECS. Also, any LEC that needs to rejoin its ELAN or change its membership to another
ELAN cannot because the LES cannot verify any LEC trying to join an ELAN.
• LES/BUS failure—The LES/BUS pair is needed to maintain an operational ELAN. The LES
provides the LE_ARP service for ATM-MAC address mappings and the BUS provides broadcast
and unknown services for a given ELAN. Therefore, a failure of either the LES or the BUS
immediately affects normal communication on the ELAN. However, a LES/BUS failure impacts
only the ELAN served by that pair.
It is clear that these issues can be limiting to networks where resiliency and robustness is a
requirement and might even be a deciding factor in your design of whether to implement
LANE-based ATM networks. In addition, there are other design considerations such as the
placement of the LANE service components within an ATM network that can have implications on
the overall robustness of the LANE environment.
Resiliency in LANE 1.0 Networks
Increasing the resiliency of a LANE-based network essentially includes delivering increased
robustness in the LANE service components such as the LECS, LES, and BUS. Such robustness is
provided by SSRP through a primary-secondary combination of the LANE services. For LECS
redundancy, one primary LECS is backed up by multiple secondary LECSs. LES/BUS redundancy
is also handled in a similar fashion where one primary LES/BUS pair is backed up by multiple
secondaries. Note that the LES/BUS functions are always co-located in a Cisco implementation and
the pair is handled as one unit with respect to redundancy.
Designing ATM Internetworks 8-25
LANE Implementation
LECS Redundancy
In the LANE 1.0 specification, the first step for a LEC during initialization is to connect with the LECS to obtain the LES ATM address for the ELAN it wants to join. In order for the LEC to connect to the LECS, multiple mechanisms are defined. The first mechanism that a LEC should use is to
query the ATM switch it is attached to for the LECS address. This address discovery process is done using the ILMI protocol on VPI, VCI - 0, 16.
The following is an example of the configuration command to add a LECS address to a LightStream
1010 switch:
atm lecs-address <LECS NSAP address> <index>
With SSRP, multiple LECS addresses are configured into the ATM switches. An LEC, which
requests the LECS address from the ATM switch, gets the entire table of LECS addresses in
response. The behavior of the LEC should be to attempt to connect to the highest ranking LECS
address. If this fails, it should try the next one in the list and so on until it connects to the LECS.
Whereas the LEC always tries to connect to the highest ranking LECS available, SSRP ensures that