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CISCO INTERNETWORK DESIGN EXAM OBJECTIVES COVERED IN THIS CHAPTER:
For many years, Novells IPX protocol commanded a significant share of the networking market. However, like AppleTalk, Novells IPX protocol is being replaced with TCP/IP in most modern networks.
As with AppleTalk, IPX was designed to simplify administrative functions and avoid some of the manual, complex tasks that were required by administrators and designers. For example, IPX does not incorporate the concept of subnets, which negates the need for calculating subnet masks or prelimiting the number of hosts that will be supported by the network. This is both a positive and a negativeadministrators need to configure the network address only once and all workstations will automatically learn this information. However, this automation adds to the total overhead.
This chapter will address many of the common issues that arise when designing IPX networks, and it will also provide some direction to creating a scalable design.
As noted at the beginning of this chapter, Novells IPX protocol was designed to simplify the configuration of the network. While this chapter will document some of the penalties that came from these features, it is important for designers to be aware of how these features differ from IP and how they may benefit from using IPX. Table 6.1 compares the IP and IPX protocols.
|Automatic addressing||Automatic address assignment requires DHCP.||Automatic address assignment is built in. IPX routers assign a four-byte network number that is added to the MAC (Media Access Control) address to create a unique address.|
|Automatic naming||Resource names require WINS (Windows Internet Naming Service) or DNS (Domain Name System).||Server names and other resources are communicated via the SAP (Service Advertising Protocol) process. This feature is built in.|
|Route summarization||Available.||With NLSP (NetWare Link Services Protocol), IPX routes can be summarized.|
|Internet connectivity||IP is the protocol of the Internet; therefore, IP workstations can connect directly to the Internet.||IPX traffic cannot traverse the Internet, and IPX-only workstations require a gateway.|
|Subnet masks||The IP protocol is designed around the concept of subnet masks.||IPX does not include a subnet mask.|
|Scalability||Scales with minor effort.||Can scale to hundreds of networks, but typically requires filters and other techniques.|
In modern network design, it is increasingly unlikely for designers to select IPX because of Novells support for IP and the growth of the Internet. Many designers prefer to use a single network protocol when possible, and the most-supported protocol is IP. However, legacy networks may incorporate large installations of IPX, and there are still applications that may warrant its deployment.
This chapter will focus on the IPX protocol on Novell servers, but it is important to note that Novell also supports NFS (Network File System) for Unix systems and AFP (AppleTalk File Protocol) for Apple systems on the server. This is in addition to the native NCP (NetWare Core Protocol) running on IPX. Novell also supports gateway services for mainframes with its SAA (Systems Application Architecture) gateway product.
Cisco and Novell recommend that individual IPX networks contain no more than 500 nodes. This limitation results from the broadcast traffic inherent in IPX designs. In practice, this value is fairly highmost IPX environments experience degradation at the 200-to-300-node level.
|In production networks, do not use the broadcast percentage to evaluate the health of the network. Broadcasts-per-second values provide a clearer indication of how the broadcasts are affecting the users.|
Also, note that Cisco routers typically require the configuration of an IPX internal network number for NLSP and other services within the Novell environment. As with other network numbers, the internal network number must be unique within the internetwork.
Novell IPX employs a routing protocol similar to IP RIP, which is transmitted every 60 seconds (as opposed to every 30 seconds) and may contain up to 50 different network entries per update packet. The network diameter is still limited to 15 hops when using IPX RIP, the same as with IP RIP.
|While there are many similarities between IP RIP and IPX RIP, please note that they are different routing protocols.|
In order to reduce the possibility of routing loops, IPX RIP must use split horizonsimilar to the requirement with AppleTalk RTMP. In addition, IPX RIP employs a lost-route algorithm that helps prevent routing loops. This function also locates new routes upon failure.
IPX RIP Metrics
Unlike IP RIP, IPX RIP includes two mechanisms for determining the best route. In addition to a hop counter, IPX RIP incorporates delay into the protocol. By default, all LAN technologies are assessed a cost of one tick, or 1/18 of a second. WAN technologies, regardless of their actual bandwidth, are assessed by default a cost of six ticks (this value can be changed). Cisco routers augment these metrics by using the local interface delay to break ties in both hop count and ticks. However, Cisco supports multiple concurrent IPX paths, which the designer enables with the ipx maximum-paths command.
By default, Cisco routers support a single IPX route through the network. The ipx maximum-paths command allows the designer to permit up to four route entries. By establishing more than one IPX path, the designer can incorporate faster convergence and load balancing into the design.
It is important to note that there are differences between IP switching and IPX switching. These differences will also factor into a designers implementation.
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