• Routed protocols supported - Which routed protocol does the routing protocol support?
• Category of routing protocol - Does the protocol fall under the distance vector, link-state, or hybrid category of routing protocol.
• Interior or exterior routing protocol - Is the protocol used by routers within the same autonomous system or is it used by routers between autonomous systems.
• Bandwidth used - What is the overhead involved with using the routing protocol?
• Administrative distance - What is the trustworthiness of the source of the routing information?
• Information exchanged between routing peers - What type of information does the routing protocol pass between routing peers?

Routed Protocols Supported.  A specific protocol can typically be used only with one routed protocol.  The following table displays the routing protocols available with the three most popular routed protocols.

Interior routing protocols, such as RIP, IGRP and EIGRP, are used by routers within the same autonomous system.  Exterior routing protocols, such as BGP - Border Gateway Protocol, are used between autonomous systems.

Interior routing protocols are designed to find the best path through the network.  Although exterior routing protocols also find the best path through the network, they provide additional functionality. 

Small to medium sized businesses usually use only interior routing protocols.  The businesses then rely on an ISP to connect them to the larger internet.

Bandwidth Overhead.  Distance vector protocols, being periodic in nature, use a defined amount of bandwidth for sending their routing updates.  Teare's book has a table that characterizes the bandwidth used by a variety of routing protocols.

Administrative Distance.  It is probably easiest to think of administrative distance as a numeric measure of the trustworthiness of routes and routing information.  In some network designs, more than one routing protocol is configured.  Teare's book also has a table illustrating typical administrative distance measures for different sorts of router interactions.

Information Exchanged by Peers.  The major types of information is exchanged in different routing protocols is in the bulleted list below.

• Send periodic updates
• Have a separate hello mechanism
• Exchange information about links
• Exchange information about routes

Scalability Issues.  The scalability constraints for routing protocols can be characterized by the following list.

• Limits on metrics
• Convergence
• Resource requirements

Some protocols have metric limitations that make them incompatible with large networks.  Some of the standard metric limitations are in the following table.

Protocol

Maximum Hop Count

Routing protocol convergence occurs when routers arrive at consistent understandings of the network topology after a change takes place.  The time to convergence depends primarily on the following factors.

• Routing protocol update and timer
• Network diameter and complexity
• Frequency of routing protocol updates
• Features of the routing protocol

In general, EIGRP and the link state protocols converge more quickly than distance vector protocols.

Resource requirements include such things as CPU, memory and bandwidth in routers and the networks they run on.  The resource requirements depend on many factors including the following.

• How often routing updates are transmitted.
• How much data is transmitted.
  • the entire routing table or just changes
• How widely updates are distributed.
  • neighbors
  • bounded area
  • routers in the autonomous network
• How static and default routes are implemented
• Whether route summarization is supported and how it is implemented.