Understanding Selecting Network Protocols | OSPF Network Design Solutions

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IS-IS Common Header Fields

Figure 3-4 shows the common header fields of the IS-IS packets:

Figure 3-4 IS-IS logical packet format.

The following list defines the common header fields of IS-IS packets as depicted in Figure 3-4.

• Protocol identifier. Identifies the IS-IS protocol, contains a constant (131).

• Header length. Contains the fixed header length. (The length is always equal to eight bytes, but is included so that IS-IS packets do not differ significantly from CLNP packets.)

• Version. Contains a value of 1 in the current IS-IS specification.

• ID length. Specifies the size of the ID portion of an NSAP address. If the field contains a value between 1 and 8 inclusive, the ID portion of an NSAP address is that number of bytes. If the field contains a value of zero, the ID portion of an NSAP address is 6 bytes. If the field contains a value of 255 (all ones), the ID portion of an NSAP address is zero bytes.

• Packet type. Specifies the type of IS-IS packet (hello, LSP, or SNP).

• Version. Repeated after the Packet Type field. I do not know anyone who knows why, but it is better than just a spacer field.

• Reserved. Ignored by the receiver, and is equal to 0.

• Maximum area addresses. Specifies the number of addresses permitted in this area.

Following the common header, each packet type has a different additional fixed portion.

Selecting a Routing Protocol

One of the most frequent questions that network designers and engineers ask about routing protocols is “Which routing protocol should I use?” This section will compare and contrast the three protocols discussed in the chapter: RIP, IS-IS, and OSPF; however, you need to consider three important issues before selecting a routing protocol:

• Operational considerations. Determine how easy it is to manage a network over time. These considerations include a protocol’s capability to adapt to changes, how to minimize disruptions to the network, and how easy is it to troubleshoot problems.

• Technical considerations. Assist in determining whether or not a given protocol is capable of supporting a particular set of network requirements.

• Business considerations. Defined as business priorities and policies that influence network design decisions. These types of considerations can originate from any area within a company and are sometimes the keystones to the success of the network.

Operational Considerations

The matrix shown in Table 3-1 provides a list of the more important features to consider when weighing the operational issues concerning the selection of a routing protocol.

**Table 3-1** Important operation considerations
	RIP	OSPF	IS-IS

Protocols Supported	IP	IP	IP, OS, CLNP
Routing Hierarchies	None	Supported	Supported
IP Address Management	Optional	Required	Required

Protocols Supported

Historically, all routed protocols have had their own independent routing protocols: AppleTalk uses RTMP; Novell uses IPX RIP; and IP uses RIP, IGRP, or OSPF. This is conceptually simple to understand, but it is often difficult to implement. Yet, it is necessary for network engineers to design and operate networks that support multi-protocol environments. Hence, they need to be able to manage a mix of routing protocols.

Both RIP and OSPF support only the TCP/IP protocol suite. Although TCP/IP is the most popular suite in use today, it is not the only one being used. The inability of a routing protocol to support other protocols can be a detriment to legacy networks or networks with unique routing needs.

When IS-IS was created, the protocol designers asked a significant question: why can’t one routing protocol handle multiple routed protocols? Consequently, Integrated IS-IS was enhanced to support both OSI CLNP and TCP/IP networks. In addition, Integrated IS-IS has the hooks in it to permit it to support other network protocols.

Using a single routing protocol simplifies the job of a network administrator considerably because she will need to learn the intricacies of only one protocol.

Logical Hierarchies

The key to building large networks is to introduce a logical hierarchy. Many problems of complexity and scale can be addressed with the proper use of hierarchy.

RIP does not support any type of logical hierarchy. This is a major drawback and makes it very unsuitable for today’s growing networks.

OSPF was the first major routing protocol to support hierarchical networking within a single routing domain. OSPF supports two levels of hierarchy: a backbone area and other connected areas. OSPF routers carry full topology information about the backbone and connectivity information about all of the areas. Within each area, OSPF routers exchange full topology information about that area.

Integrated IS-IS uses the same two levels of hierarchy as OSPF; however, the two protocols differ in the quantity of information that is carried inside each area. Within an area, Integrated IS-IS routers send all traffic that needs to go out of the area to the nearest area border router (ABR). OSPF, on the other hand, injects all of the connectivity information about the other areas into each area. This enables every router in an OSPF area to choose the optimal ABR for traffic that needs to go out of their area.

IP Address Management

The key to a successful hierarchical network structure is proper IP address management. If addresses are assigned appropriately, it is possible to summarize routing information. There are two significant reasons to summarize routes. Summarization localizes the effects of topology changes and thus contributes to network stability, and summarization cuts down on the amount of routing information that is carried by all routers. This simplifies network administration and troubleshooting, in addition to cutting down the resources consumed by the routing protocol.

Proper IP address management in RIP depends upon which version you are running. The optimal RIP solution in this area is version 2, which will allow for the use of various IP address management tools, such as VLSM and CIDR.

Each area used by OSPF and Integrated IS-IS should have a contiguous set of maskable network or subnet numbers assigned to it. The area border routers should summarize that set of addresses with an address mask.

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