Routing Updates

All routing protocols exchange routing information dynamically. The three most important 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 floods the full topology table every 30 seconds. OSPF floods the full topology table every 30 minutes. Integrated IS-IS floods the full topology table every 15 minutes to ensure synchronization.
• 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 to all of its neighbors after the network 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.

VLSM and CIDR Support

OSPF and integrated IS-IS include support for variable-length subnet masks (VLSMs) and classless interdomain routing (CIDR). VLSM is required to support route summarization. In addition, VLSM and CIDR also enable 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: rerouting in case of failure and load sharing. All routing protocols supported by Cisco provide load sharing across as many as six equal-cost paths. The default for OSPF is to use four equal-cost paths, but if you have more, you must configure OSPF to use them.

Metrics

The quality of route selection is essentially controlled by the value of the metrics placed upon the various routes. Two components are important in how a routing protocol uses metrics: the range of the values the metric can express and how the metric is computed. OSPF uses a flat metric with a range of 16 bits. This results in OSPF having a metric range that is from 0 to 65,535. By default, OSPF metrics are assigned as an inverse of the bandwidth available on an interface-normalized to give the Fiber Distributed Data Interface (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 to 1023. By default, all integrated IS-IS metrics are 10. Network administrators need to configure nondefault values. Integrated IS-IS computes the cost of a path by summing the metrics for each hop on that path.