Understanding Selecting Network Protocols | OSPF Network Design Solutions

Table of Contents

Technical Considerations

The matrix as shown in Table 3-2 provides a list of the more important features to consider when weighing the technical issues concerning the selection of a routing protocol.

**Table 3-2** Important technical considerations
	RIP	IS-IS	OSPF

Fast convergence	Yes	Yes	Yes
Routing updates	Fast, whole table	Fast, change only	Fast, change only
VLSM support and CIDR	Yes (v2)	Yes	Yes
Load sharing	No	Yes, equal cost	Yes, equal cost
Metric range	0-15	0-65,535	0-65,535
Static metric pieces	Number of hops	Sum of bandwidth	Sum of bandwidth
Dynamic metric pieces	None	None	None

Fast Convergence

All routing protocols have three important characteristics when dealing with the issue of convergence:

1. Detecting that a change has occurred

2. Adapting to that change

3. Updating the network topology to reflect the change

RIP, IS-IS, and OSPF detect certain types of changes instantly. In general, any change that can be detected by a physical change (such as loss of carrier) will be detected immediately by any routing protocol.

In addition, all three routing protocols have mechanisms to detect other failures (such as the loss of an adjacent router or the degradation of an interface to the point where it should no longer be used). All three protocols cause adjacent routers to exchange information periodically.

After the routing protocol has detected the topology change, it needs to adjust the routing tables to accommodate the new topology. RIP generally updates its routing tables every 30 seconds (this is the default time), although during the normal update process, RIP sends out its entire routing table. OSPF and Integrated IS-IS have two mechanisms for updating routing tables. If the topology change were within the area, all of the existing routes affected by the change would be discarded and a new routing table would be generated. In general, OSPF and Integrated IS-IS converge in less than two seconds. The amount of CPU required to do the recompilation is strongly affected by the number of routes and the amount of redundancy in the network.

Routing Updates

All routing protocols exchange routing information dynamically. The three most important and interesting questions concerning the operation of routing updates are as follows:

• When are they sent? All three routing protocols exchange periodic hellos and full topology information when a router starts up and periodically thereafter, depending how they are configured. RIP will flood the full topology table every 30 seconds. OSPF will flood the full topology table every 1,800 seconds. Integrated IS-IS will flood the full topology table every 15 minutes

• What is in them? Within an area, OSPF and Integrated IS-IS exchange changed link state information. Between areas, OSPF and Integrated IS-IS exchange changed routes.

• Where are they sent? Changed information in a RIP network is broadcast out to all of its neighbors after it has finished updating its topology. Changed information in OSPF and Integrated IS-IS propagates throughout the area in which the change occurred. If route summarization is not done, change information might also propagate to the backbone and into other areas.

Variable-Length Subnet Masks

RIPv2, OSPF, and Integrated IS-IS all include support for variable-length subnet masks (VLSM). VLSM is required to support route summarization. In addition, VLSM also enables network administrators to use their address space more effectively.

Load Sharing

Today’s networks are commonly designed with redundant paths. This has two positive benefits: re-routing in case of failure and load sharing. All routing protocols supported by Cisco provide load sharing across up to four equal-cost paths.

Metrics

The quality of route selection is essentially controlled by the value of the metrics placed upon the various routes. There are two components of importance in how a routing protocol uses metrics: the range of the values the metric can express and how the metric is computed.

RIP uses a very simple hop count metric that can be expressed with values between 0 and 16. The computation is also very straightforward in that the cost determined for the metric is a matter of how many hops the destination is from the source router.

OSPF uses a flat metric with a range of 16 bits. This results in OSPF having a metric range that is from zero to 65,535. By default, OSPF metrics are assigned as an inverse of the bandwidth available on an interface—normalized to give FDDI a metric of 1. OSPF computes the cost of a path by summing the metrics for each hop on that path.

Integrated IS-IS uses a flat metric. The metric range is 0-1,023. By default, all Integrated IS-IS metrics are 10. Network administrators need to configure non-default values. Integrated IS-IS computes the cost of a path by summing the metrics for each hop on that path.

Scaling

RIP has many problems associated with scaling into the larger network sizes. For example, if a network has more than 15 hops, RIP will run into problems due to its design limitations.

As for IS-IS, ISO 10589 states that 100 routers per area and 400 L2 routers should be possible. The biggest scaling issue now seems to be the overhead of the flooding in large meshed networks—for example, flat ATM clouds with many routers attached that form a full mesh.

OSPF, on the other hand, scales very well no matter how large the network is. In order to make the network operate optimally, however, you should implement physical and logical areas, as needed.

Business Considerations

Table 3-3 documents some of the more important features to consider when weighing the business issues concerning the selection of a routing protocol.

**Table 3-3** Important business considerations for routing protocol selection
	RIP	Integrated IS-IS	OSPF

Standard-based	Yes	Yes	Yes
Multi-vendor environments	Yes	Yes	Yes
Proven technology	Yes	Yes	Yes

Table of Contents