Operational Considerations

Table 2-1 shows a matrix of operational issues to consider when selecting a routing protocol.

Table 2-1 Important Operational Considerations

OSPF

IS-IS

Protocols Supported

IP

IP, ISO, CLNP

Routing Hierarchies

Supported

Supported

IP Address Management

Required

Required

IP Encapsulation Support

Yes

No (OSI Layer 2)

Available Resources

Yes

Historically, all routed protocols have had their own independent routing protocols: AppleTalk uses Routing Table Maintenance Protocol (RTMP), Novell uses Internetwork Packet Exchange (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 multiprotocol environments. Therefore, they need to be able to manage a mix of routing protocols.

OSPF supports 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.

58 Chapter 2: Introduction to OSPF

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 supports other network protocols; this can be of great benefit in a multiprotocol network.

Routing Hierarchies

The key to building large networks is to introduce a logical hierarchy. Problems related to complexity and scale can be addressed with the proper use of hierarchy. Having a hierarchical network can provide you with many benefits—most notably, route summarization and reduced SPF calculation—thus giving your network a faster convergence time.

OSPF was the first major routing protocol to support hierarchical networking (areas) within a single routing domain (AS). OSPF supports two levels of hierarchy: a backbone area (Area 0) 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 because the boundaries of areas fall within a router. This results in router interfaces being bound to areas. Because a router has multiple interfaces, it can be in multiple areas as well. However, this design would require the router to run separate SPF calculations for each area; therefore, you should design the network appropriately.

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 IS-IS Level 2 router. OSPF, on the other hand, injects all the connectivity information about the other areas into each area. This enables every router in an OSPF area to choose the optimal area border router (ABR) for traffic that needs to go out of its area.

IS-IS area boundaries are segmented on a link between two routers; therefore, a router is typically only in one area. However, it is hard to deploy true hierarchical network architecture with IS-IS for several reasons, one of which is that IS-IS operates best with very large areas with at least one router in an area for it to operate properly. This is one of the factors that causes ISPs to deploy a single IS-IS area as their core, although many ISPs are now reconsidering OSPF.

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. The two significant reasons to summarize routes follows:

• Summarization localizes the effects of topology changes and thus contributes to network stability.

• Summarization reduces the amount of routing information that is carried by all routers.

These reasons simplify network administration and troubleshooting, in addition to reducing the resources that are consumed by the routing protocol (CPU, memory, and so on).

TIP Each area used by OSPF or integrated IS-IS should have a contiguous set of network or subnet numbers assigned to it. The area border routers should summarize that set of addresses with an address mask. Summarization provides substantial benefits for your network.

IP Encapsulation Support

OSPF is a TCP/IP-based protocol and fully supports IP. On the other hand, IS-IS is a native OSI protocol and must still be implemented using ISO addressing for the IS-IS aspects of its operation. IS-IS allows the forwarding of IP packets although not specifically designed to do so.

Available Resources

You must be pragmatic when selecting a routing protocol. Resources should be available to assist network engineers at all levels of competence to help them understand the idiosyncrasies of the protocol that you want to implement.

OSPF is much more widely implemented and understood through the use of books (like this one), which explain to you the importance and functions of OSPF while providing practical examples to enhance your understanding. The standards surrounding OSPF are extremely well written thanks to the OSPF Working Group in the Internet Engineering Task Force (IETF); however, the specification does not include a lot of explanation on the "how" and "why."

IS-IS is deployed in some places, but compared to OSPF, it is still considered a niche protocol. Some ISPs are running it, but almost no enterprise networks are using it as of this writing. The IS-IS specification uses ISO language and terms and not IP.

If formal classroom instruction is of interest to you, several dedicated OSPF courses are available; however, few are available on IS-IS.

60 Chapter 2: Introduction to OSPF

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