13.7 Change Network Routing and Advertising Protocols

The development of LANs can be traced to the 1970s, a period during which internetworking for many organizations consisted of linking together two networks in the same building. Protocols developed during that time period fall primarily into a category of protocols referred to as distance vector algorithms and have certain inefficiencies associated with their use that can adversely affect the level of performance when used with large networks. In this concluding section, we examine how changing your routing and advertising protocols can enhance the efficiency of your network. However, prior to doing so, let us discuss the difference between routing and advertising protocols so we have an understanding as to why they consume bandwidth and, under certain network topologies, can adversely affect communications between interconnected networks.

13.7.1 Routing Protocol

A routing protocol transfers information between network devices that provides them with the information necessary to route frames through a network. In a NetWare environment, the Routing Information Protocol (RIP) is an example of a routing protocol that facilitates the exchange of IPX packets.

The RIP can be classified as a distance vector algorithm in that it exchanges routing table information through periodic broadcasts to make network routers and servers aware of the current topology of the network. Distance vector algorithms use information stored and retrieved based on the distance in terms of the number of hops between networks, with each router considered to represent one hop.

The RIP used by Novell was based on the distance vector algorithm developed by Xerox and was originally designed for connecting small LANs together. Novell modified RIP to include a cost metric based on the original IBM PC's timer tick of about 1/18th of a second. When a RIP request is transmitted, the round-trip acknowledgment time is counted in terms of timer ticks and used as a decision criteria when two or more routes have the same hop count.

13.7.2 Advertising Protocol

In comparison to a routing protocol that seeks to find an optimum path through a network, an advertising protocol makes other devices aware of the presence of the device using the advertising protocol. One common example of an advertising protocol is NetWare's Service Advertisement Protocol (SAP).

In a NetWare environment, different types of servers advertise their presence by broadcasting a SAP every 60 seconds. While the bytes contained in a SAP packet have a minimal effect on network bandwidth, even when a network has 10, 20, or more servers, when two networks are interconnected by a wide area network, the effect of transmitting SAP packets becomes more pronounced and can considerably affect internetwork communications.

To illustrate how SAP packets can adversely affect inter-LAN communications that occur via WAN transmission facilities, consider Figure 13.7, which shows four LANs interconnected via three WAN circuits. In Figure 13.7, the arrows indicate the flow of SAP packets between networks, while squares with the letter S and a subscript numeric are used to identify a specific server. The table in the lower left portion of Figure 13.7 indicates the SAP flow by circuit. As indicated, the six-server internetwork will result in 12 SAP packets being transmitted every minute, or a total of 720 per hour, every hour .

Figure 13.7: Examining the Effect of Service Advertising Protocol Broadcasts on WAN Circuits

While the effect of a SAP on network users is negligible when transmission occurs on the LAN at an operating rate expressed in Mbps, when forwarding occurs across WAN circuits, the effect becomes more pronounced. For example, a 64-Kbps WAN represents approximately one 156th of the bandwidth of a 10-Mbps Ethernet LAN. Because NetWare's Service Advertisement Protocol is implemented as a distance vector algorithm, each server maintains a table of servers it knows and the routes to those servers. Thus, as an internetwork increases, the tables maintained by each server increases , resulting in longer data exchanges every 60 seconds or whenever there is a change in network topology.

13.7.3 NLSP

Recognizing the problems associated with distance vector protocols, several internetwork protocols were developed that result in the exchange of information by lengthening the interval between routing updates. In a NetWare environment, Novell developed a routing protocol called NetWare Link Services Protocol (NLSP) that significantly reduces bandwidth usage during router and server exchanges. To accomplish this, NLSP employs three databases: adjacency , link state, and forwarding.

The adjacency database keeps track of a router's immediate neighbors, to include servers and routers on the same LAN segment and the operational status of directly connected circuits. When a circuit becomes operational, the router periodically transmits "Hello" packets and listens for messages from its directly attached neighbors, with responses recorded in its adjacency databases.

Once neighbors are noted for each LAN segment, the NLSP selects a designated router. Here, the designated router represents all routers that provide traffic to a common circuit. Thus, a designated router can reduce traffic caused when two or more routers provide traffic onto a common circuit. In addition, the designated router maintains a link state database that includes portions of adjacency databases. Thus, the link state database can include information about other routers, which reduces the number of tables that must be exchanged. In addition, a link state database is only exchanged when changes to the database occur, further reducing overhead traffic.

The third database, the forwarding database, contains circuit costs based on an algorithm that identifies the shortest path between network nodes. Thus, this database that is created from information in the link state database governs how frames are forwarded.

In addition to significantly reducing the overhead required to perform routing on relatively low-capacity WAN circuits, the NLSP has several additional advantages. Those advantages include the use of IPX header compression, which further reduces overhead, as well as reduced router processing. The latter results from a decrease in routing table updates, which provides more CPU time for frame processing.

Because the NLSP eliminates both SAP and RIP traffic, it can provide additional bandwidth for low-speed WAN links that might otherwise require a costly upgrade. Due to its relative efficiency in comparison to RIP and SAP, those who operate NetWare should consider the use of this relatively new protocol.