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PNNI in LANE Networks
Network designers can deploy PNNI as a Layer 2 routing protocol for bandwidth management,
traffic distribution, and path redundancy for LANE networks. PNNI is an ATM routing protocol used for routing call setups and is implemented in the ATM switches. Most LANE networks consist of
multiple ATM switches and typically employ the PNNI protocol.
Note Although PNNI is an advanced routing protocol and supports QoS-based routing, this
particular aspect of PNNI is not discussed in this chapter because most LANE networks are based
on the best-effort traffic category.
8-22
Cisco CCIE Fundamentals: Network Design
LANE Design Considerations
The LightStream 1010 ATM switch supports some PNNI-related features that can be useful in
scaling LANE networks:
• To load balance call setup requests across multiple paths between two end stations
• To load balance call setups across multiple parallel links
• To support link and path redundancy with fast convergence
• To provide excellent call setup performance across multiple hops using the background routing
feature
Figure 8-17 shows how the LightStream 1010 switch supports load balancing.
Figure 8-17
Load balancing calls across multiple paths and multiple links.
Call setups are load balanced
across multiple paths
LEC
LightStream
LEC
LEC
1010
ATM Core
Call setups are load balanced
across parallel links
LEC
LightStream
LEC
1010
LEC
ATM Core
As Figure 8-17 shows, load balancing of calls is enabled by default on the LightStream 1010 switch.
Background routing, however, is not enabled by default. Background routing can be thought of as
routing of call setups using a path from a precomputed route database. The background routing
process computes a list of all possible paths to all destinations across all the service categories (for example, constant bit rate [CBR], virtual bit rate-real time [VBR-RT], virtual bit rate and nonreal time [VBR-NRT] and available bit rate-unspecified bit rate [ABR-UBR]).
When a call is placed from Point A to Point B, PNNI picks a cached routed from the background
route table instead of computing a route on demand. This eases the CPU load and provides a faster rate of processing the call setups.
Background routing can be useful in networks that have a stable topology with respect to QoS. It is, however, not very effective in networks that have rapidly changing topologies (for example, Internet Service Providers [ISP] networks or carrier networks). Campus LANE networks can use this feature
effectively because all the SVCs in the network belong to the UBR or ABR category. To enable this feature, use the following command:
atm router pnni
node 1 level 56
bg-routes
Designing ATM Internetworks 8-23
LANE Implementation
The current implementation of PNNI on the LightStream 1010 switch is full, ATM Forum-PNNI
Version 1 compliant. The LightStream default PNNI image license supports a single level of
hierarchy, where multiple peer groups can be interconnected by IISP or by other switches that
support full PNNI hierarchy; extra PNNI image license will support multiple levels of routing
hierarchy.
The PNNI protocols have been designed to scale across all sizes of ATM networks, from small
campus networks of a handful of switches, to the possible global ATM Internet of millions of
switches. This level of scalability is greater than that of any existing routing protocol, and requires very significant complexity in the PNNI protocol. Specifically, such scalability mandates the support of multiple levels of routing hierarchy based upon the use of prefixes of the 20-byte ATM address space. The lowest level of the PNNI routing hierarchy consists of a single peer group within which all switches flood all reachability and QoS metrics to one another. This is analogous, for instance, to a single area in the OSPF protocol.
Subsequently, multiple peer groups at one level of the hierarchy are aggregated into higher-level peer groups, within which each lower-level peer group is represented by a single peer group leader, and so on iteratively up the PNNI hierarchy. Each level of the hierarchy is identified by a prefix of the ATM address space, implying that PNNI could theoretically contain over 100 levels of routing
hierarchy. However, a handful of levels would be adequate for any conceivable network. The price
to be paid for such scalability is the need for highly complex mechanisms for supporting and
bringing up the multiple levels of hierarchy and for electing the peer group leaders within each peer group at each level.
Scaling an ELAN—Spanning-Tree Protocol Issues
Spanning-Tree Protocol is implemented in Layer 2 switches/bridges to prevent temporary loops in
networks with redundant links. Because a LEC essentially bridges Ethernet/Token Ring traffic over an ATM backbone, the Spanning-Tree Bridge Protocol Data Units (BPDUs) are transmitted over the
entire ELAN. The ATM network appears as a shared Ethernet/Token Ring network to the
spanning-tree process at the edge of the Layer 2 switches.
The spanning-tree topology of a LANE-based network is substantially simpler than a pure
frame-switched network that employs the Spanning-Tree Protocol. It follows that spanning-tree