Cisco Internetwork Operating System (Cisco IOSTM) Software Release 11.1 contains several enhancements designed to make it easier to build and manage large Internetwork Packet Exchange (IPX) internetworks. One of these enhancements, NetWare Link Services Protocol (NLSP) route aggregation, increases the flexibility of the NLSP in large networks. NLSP route aggregation from Cisco makes it possible for network managers to more effectively segment their large IPX internetworks, more efficiently manage the exchange of routing and service information within the network, and reduce routing overhead and network resource utilization.
This white paper outlines the operation of Cisco NLSP route aggregation and describes its implementation, benefits, and features. Cisco designed its NLSP route aggregation implementation with the goal of compatibility with the Novell NLSP specification, revision 1.1. (For an overview of the NLSP route aggregation feature, refer to Cisco Product Bulletin #434, NLSP Route Aggregation Support in Cisco IOS 11.1 Software.)
Cisco IOS Software Release 11.1 provides the industry's first implementation of NLSP route aggregation---a set of features that provide IPX network builders with the following capabilities, all of which allow building larger NLSP networks than has been previously possible:
A side benefit of route aggregation is improved network performance. The NLSP Route Aggregation feature provides the ability to summarize routes. Route summaries do not require the router to recalculate the forwarding table unless the aggregated route is created or removed, thus reducing the processor load. In addition, because routes are summarized, less system memory is required to store routing information.
Revision 1.0 of the NLSP specification, published by Novell in early 1995 and first implemented by Cisco in Cisco IOS Software Version 10.3 in March 1995, allowed large IPX networks to be segregated into distinct NLSP areas only by using IPX RIP. (See Figure 1.)
Figure 1. : Typical Configuration of Large IPX Networks Using Previous Versions of NLSP
The flexibility of Cisco NLSP Route Aggregation provides unparalleled options to builders of large IPX networks. With Cisco's NLSP Route Aggregation feature, network managers can divide large IPX networks into multiple NLSP areas without being restricted to using only RIP to connect these areas. Instead, areas can be interconnected using NLSP, Cisco's Enhanced IGRP, or RIP. (See Figure 2.)
Figure 2. : Connecting IPX Networks Using NLSP Route Aggregation
NLSP takes a step-by-step approach to learning about the routers and services on the network; this section summarizes the process.
Learning about Adjacent Routers
NLSP routers on the same network must exchange Hello packets to learn about adjacent routers before a link can be established. Once this information is known, it is added to an adjacency database.
The adjacency database stores information accumulated about a router's immediate neighbors and the status of the attached links. (See Figure 3.)
Figure 3. : NLSP 1.1 Overview
Creating a Snapshot of the Network Topology
Once the adjacency database is established, the next step is to create some representation of the network as a whole. First, portions of the adjacency database that describe a router's immediate neighbors are combined with similar information learned from all other routers. This information is shared in the form of link-state packets (LSPs). These newly detected LSPs are flooded to other routers in the area, then merged into the receiving router's link-state database. As a result, the link-state database is a collection of two types of LSP packets---packets derived from the router's own link-state database and LSP packets received from other routers.
Synchronization is an important function in NLSP. Every router within the local routing area must have an identical version of the link-state database. Short transitional periods can occur during which versions differ when a topology changes; part of the NLSP function is to make them converge again.
To efficiently convey the topology changes to many routers connected to one LAN segment and have each of them process the information to make routing decisions, NLSP provides the concept of a pseudonode that represents the LAN as a whole in the link-state database. Each router represents itself as being directly connected to the pseudonode. This process significantly reduces the size of the adjacencies in the router's link-state database.
The pseudonode does not physically exist, so a physical device (router) must represent the pseudonode for the link-state exchanges. This physical device is called a designated router, and it maintains an LSP database that includes the pseudonode's link to each router on the network. The designated router transmits packets on behalf of the pseudonode. It is elected using a priority value included in the Hello packets; the router with the highest priority value and address is elected. Each router that receives the packet adds an entry to its link-state database so that when the exchanges are finished, each router has an identical link-state database.
The link-state database provides the router's view of the network topology. After this information is available, the next step is to determine the best route to a particular network. The NLSP decision process selects the best route based on a computation using Dijkstra's algorithm, which determines the shortest distance (best metric) from the local node to every other node. Each router has a unique version of the forwarding database---basically a table that maps a destination network into a next-hop address. The decision process rebuilds the forwarding database only when the router detects a change in the link-state database. With Cisco NLSP Route Aggregation, a single router can support multiple NLSP areas.
Building Large IPX Networks Using Cisco NLSP Route Aggregation
Because Cisco NLSP Route Aggregation provides the ability for a router to directly interconnect multiple areas, the NLSP 1.1 router must be able to manage sets of adjacency and link-state databases for each area to which it is attached. However, it is important to note that a single forwarding database is created based on data collected in the other databases. (See Figure 4.)
A router can be configured with an NLSP instance for each NLSP area in which it is a member. A router can support up to 28 different NLSP areas unless Enhanced IGRP is used.When Enhanced IGRP is configured, the number of possible NLSP areas is reduced by the number of Enhanced IGRP autonomous systems present.
Figure 4. : Cisco NLSP Route Aggregation
Route aggregation was designed to minimize routing traffic in large network backbones with several NLSP areas attached. In Figure 5, traffic in the backbone is minimized by having routers R1, R3, and R5 each report address summaries for the attached areas. Routing traffic between areas is reduced because addresses for each area can be summarized at the backbone and routed to other areas in an efficient form. Simply put, route aggregation is a method of reporting many IPX network numbers using a small amount of information.
Route Aggregation Address Structure
The IPX network is divided into areas by assigning area addresses that uniquely identify each NLSP area. An area address is configured on each router to define the areas to which the router belongs.
Up to three area addresses are allowed per NLSP area on the router. Adjacencies are formed only between routers that share at least one common area address. Each configured area address is identified by two 32-bit quantities that specify a network address and a mask. The mask indicates how much of the area number identifies the area and how much identifies individual networks in the area. For example, the area address pair 12345600 FFFFFF00 describes an area composed of 256 networks in the range 12345600 to 123456FF.
To derive the maximum benefit from NLSP Route Aggregation, it is important that IPX network addresses be assigned in a structured, hierarchical manner.
One of two metrics is used to determine the value of paths to destinations included in an aggregated route or in an explicit route, depending on whether the destination is within the area or in another area.
When the router is configured to send an address summary into an NLSP area, by default, the ticks are equal to 1 and the area count is equal to 6. A tick is a time measurement in which 18.21 ticks equals 1 second. The area count is the number of areas through which the information is allowed to propagate. The tick value of an address summary increases and the area count decreases as the address summary is passed from area to area.
Together, the tick and cost metrics allow the initial sending router to control the spread of information and ensure that if a summary ceases to exist, the routing mechanism will eventually purge the summary. As the aggregated route circulates to different areas, each router that connects the areas increases the tick value. If a router that interconnects multiple areas receives several route summaries that have equal ticks for a destination, the router selects the aggregated route with the least cost value to that destination. (Note that only routers that support NLSP Version 1.1 route aggregation can participate in this process, and they are referred to as NLSP 1.1 routers. See the section "Generating Aggregated Routes" later in this paper for more information about NLSP 1.1 routers.)
Figure 5. : Typical Topology for Defining Aggregated Routes
When an NLSP 1.1 router receives an explicit route to a destination and an aggregated route that subsumes the same destination's address, the router selects the explicit route as the best path. In addition, if the router receives two aggregated routes that subsume the same destination address, the router selects the aggregated route that is more explicit. For example, if the router receives an aggregated route of a0000000 f0000000 and then receives an aggregated route of aaaa0000 ffff0000, it chooses the aaaa0000 ffff0000 aggregated route as having the best path if the destination is aaaa0123. It chooses a0000000 f0000000 if the destination is a1234567. Novell refers to this as the longest match.
A route summary defines a set of explicit routes that the router uses to generate an aggregated route. (See Figure 6.)
Route summaries can be used within and between NLSP 1.1 areas to transmit routing information. However, because IPX RIP, NLSP 1.0, and Enhanced IGRP routers cannot interpret route summaries, the default route must be used to reach any destination for which no explicit route is known. The default route is a reserved network (-2) that is used to route packets to any destination for which a more explicit route is not known. It is assumed that eventually the packet will reach a router with more complete routing information such that the packet will either be delivered to its destination or dropped. Default routes for IPX are new; Cisco IOS software introduced support for default routes in Software Version 10.3. Default and aggregated routes change the way in which SAP functions in an IPX network. For more information, refer to the section "Compatibility Issues" following in this paper.
Route summarization is disabled by default to avoid the generation of aggregated routes in an area running mixed versions of NLSP or other protocols.
Relationship between Area Addresses and Route Summaries
When route summarization is enabled while running multiple instances of NLSP, the router performs route summarization by default based on the area address configured in each NLSP area. Explicit routes that match the area address in a given area are not redistributed individually into neighboring NLSP areas; instead, the router redistributes a single aggregated route that is equivalent to the area address into neighboring areas.
Redistributing Route Information
Cisco NLSP route aggregation features the ability to redistribute route information between NLSP areas. This feature allows network managers to connect a combination of Enhanced IGRP, NLSP, and IPX RIP/SAP routing areas into a single, comprehensive IPX internetwork.
Regardless of the NLSP version running on the internetwork, the Cisco IOS Software Release 11.1, by default, redistributes routes between multiple NLSP areas and between NLSP and RIP, and Enhanced IGRP and RIP. Unless otherwise specified, all routes are redistributed as individual, explicit routes. However, if IPX RIP/SAP, Version 1.0 NLSP, or Enhanced IGRP are to be used in conjunction with NLSP route aggregation, connecting the areas must be done carefully. The Cisco IOS software offers the redistribute nlsp command to redistribute NLSP 1.0 into the NLSP 1.1 area. It also offers a way to define an access list to redistribute an aggregated route instead of learned explicit routes using the access-list command.
Figure 6. : Route Summaries
Although NLSP does not permit filtering of routes and services within NLSP areas, the Cisco NLSP route aggregation feature makes it possible to redistribute routes and services between NLSP areas using filters. The Cisco IOS software provides filters for both input and output directions with the distribute-list in, distribute-list out, distribute-sap-list in, and distribute-sap-list out commands.
Filtering works independently of route summarization; however, since filtering occurs before the aggregation algorithm is applied, filtering could indirectly affect route summarization. It is possible to filter all explicit routes that could generate aggregated routes, making the router unable to generate aggregated routes even when it has been explicitly configured.
A few facts that should be remembered when generating aggregated routes using the Cisco NLSP route aggregation feature: First, the area address for each NLSP area is used as a basis for generating aggregated routes. Second, NLSP 1.0 routers do not recognize aggregated routes. NLSP 1.0 routers support only a single, level 1 area. Further, they cannot interpret the route summary field in an LSP, and so cannot learn about destinations that are included in the address summary options. If an NLSP 1.1 router detects that the next-hop router on the path to an aggregated destination is an NLSP 1.0 router, it drops the packet to prevent routing loops. Route aggregation between NLSP 1.0 and 1.1 routers must be specifically defined.
It is also necessary to specify custom route summaries between Enhanced IGRP and NLSP 1.1 routers; the summaries go only one way, from Enhanced IGRP to NLSP 1.1, and they must use an access list.
Router summaries between RIP routers and NLSP 1.1 routers work in a similar way and also require a custom configuration that directs the summaries one way, from RIP to NLSP.
Full two-way sharing of summaries is supported between NLSP 1.1 routers. The Cisco IOS software provides the ipx router nlsp, area-address, route-aggregation, and ipx nlsp commands to generate aggregated routes.
Interaction with Enhanced IGRP Areas
Numerous Cisco customers have chosen Enhanced IGRP as their IPX routing protocol. Enhanced IGRP can be used as a routing protocol in IPX, IP, and AppleTalk networks. For an overview of Enhanced IGRP functionality, see Cisco Technical Tips #3, "Introduction to Enhanced IGRP (EIGRP)." The Cisco NLSP route aggregation feature supports Cisco Enhanced IGRP for network designers who can take advantage of this protocol.
The Cisco Enhanced IGRP implementation for Novell IPX networks provides numerous capabilities that help network managers build large, robust Novell IPX networks. Enhanced IGRP was designed to work with fast link-state algorithms, including NLSP. A link-state protocol actively tests the status of its link to each of its neighbors rather than exchanging distances with its neighbors. It sends this status information to its neighbors, which then propagate it through the autonomous systems. Each router listens for the information and builds a complete routing table based on the information it receives. link-state algorithms build these tables very quickly. In many cases, convergence to the new routes takes less than five seconds. Cisco's Enhanced IGRP Diffusing Update Algorithm (DUAL) provides convergence to changed network topologies at speeds equivalent to those achieved by link-state protocols such as NLSP.
In addition to the fast rerouting capability, Enhanced IGRP for Novell IPX networks supports incremental SAP updates. In other words, SAP updates are transmitted only when routing topology changes occur. Routers running IPX RIP transmit these updates every 60 seconds. RIP also has a network-diameter limit of 15 hops; Enhanced IGRP expands this limit to 224 hops. Finally, Enhanced IGRP provides an optimal path selection process for Novell IPX networks. This process uses a combination of delay, bandwidth, reliability, and load metrics, which allows the router to calculate not only the shortest distance but also the fastest and most reliable path to its destination. RIP uses only ticks and hop counts to make its path selection decisions.
NLSP-to-Enhanced IGRP Route Redistribution
NLSP and Enhanced IGRP support the traditional IPX network by supporting RIP by default. Route redistribution is disabled between instances of NLSP Version 1.1 and Enhanced IGRP to minimize the possibility of routing loops in certain topologies. To add an Enhanced IGRP area, you must explicitly instruct the router to redistribute the route entry from an NLSP 1.1 area to an Enhanced IGRP area and vice versa. (See Figure 7.)
The Cisco IOS software provides the redistribute eigrp command to redistribute Enhanced IGRP into the NLSP 1.1 area. It also provides the ipx router eigrp command to enable Enhanced IGRP, the network command to specify the networks to be enabled for Enhanced IGRP, and the redistribute nlsp command to redistribute NLSP 1.1 into Enhanced IGRP.
Two rules about SAPs and Get Nearest Server (GNS) requests might affect the operation of IPX networks: The IPX Router Specification, Version 1.0, states that a router must have an explicit route to a destination service before a service is accepted. The Novell NetWare Link Services Protocol Specification, Revision 1.1, updates this rule. The new rule states that information about a service is accepted by a router as long as a packet can be forwarded to the service via an explicit route, an aggregated route, or the default route.
The rule for responding to a GNS request is to compare the route ticks of the service entry in the SAP table. In case of a tie, hops are used as the tie breaker. Note that the hop count for a summary route is always considered to be greater than the hop count for a specific route.
The Cisco implementation of NLSP route aggregation was developed in parallel with the Novell NLSP specification, Revision 1.1. The goal of the Cisco implementation is to conform to this specification. To that end, Cisco developers use interim testing tools provided by Novell to ensure interoperability. The Cisco implementation of NLSP will undergo official testing with the NLSP conformance tester, developed by Novell, when the tester is updated to test this capability.
Cisco's implementation of Novell NLSP route aggregation extends the choices for connecting areas from the standard IPX RIP to NLSP and Cisco's Enhanced IGRP. Benefits of Cisco NLSP route aggregation include more efficient network operation because of reduced routing traffic and reduced use of system resources.
Cisco designs its product with the goal of allowing customers to build networks according to their business needs, not the limitations of their vendors. The Cisco NLSP route aggregation feature makes building and managing large Novell IPX networks easier than ever before.
Figure 7. : Redistribution and Summarization between NLSP 1.1 and Enhanced IGRP Areas
Cisco Systems, Inc. Cisco Internetworking, "Integrating Enhanced IGRP into Existing Networks," UniverCD Vol. 2, No. 8, Revision AO.
Cisco Systems, Inc. Cisco IOS Course Update Release 11.1, Chapter 2, "NLSP Route Aggregation," 1995.
Cisco Systems, Inc. Cisco Product Bulletin #434, NLSP Route Aggregation Support in Cisco IOS 11.1 Software, 1995.
Cisco Systems, Inc. Cisco Technical Tips #3, Introduction to Enhanced IGRP (EIGRP)
Novell, Inc. NetWare Link Services Protocol Specification, Revision 1.1, 1995.Novell Authorized Education Centers. Internetworking with NetWare MultiProtocol Router, Course 740, Chapter 4, "Routing with NLSP," Revision 1.02.