Choosing a routing protocol

One of that hardest decisions in routing can be choosing which routing protocol to use on your network. It can be easy to decide when static routing will not meet your needs, but how can you tell which dynamic routing protocol is best for your network and situation?

Here is a brief look at the routing protocols including their strongest and weakest points. The steps to choosing your routing protocol are:

1. Answer questions about your network

2. Evaluate your chosen protocol

3. Implement your dynamic routing protocol

Answer questions about your network

Before you can decide what is best for your situation, you need to examine what the details of your situation are such as what you have for budget, equipment, and users.

The following questions will help you form a clear idea of your routing needs:

How many computers or devices are on your network?

It matters if you only have a few computers, or if you have many and if they are all at one location or not as well. All routing protocols can be run on any sized network, however it can be inefficient to run some on very small networks. However, routers and network hardware that support dynamic routing can be more expensive than more generic routers for static routing.

What applications typically run over the network?

Finding out what application your users are running will help you determine their needs and the needs of the network regarding bandwidth, quality of service, and other such issues.

What level of service do the users expect from the network?

Different network users have different expectations of the network. Its not critical for someone surfing the Internet to have 100% uptime, but it is required for a stock exchange network or a hospital.

Is there network expansion in your near future?

You may have a small network now, but if it will be growing quickly, you should plan for the expected size so you don’t have to chance technologies again down the road.

What routing protocols do your networks connect to?

This is most often how routing protocol decisions are made. You need to be able to communicate easily with your service provider and neighbors, so often people simply use what everyone else is using.

Is security a major concern?

Some routing protocols have levels of authentication and other security features built in. Others do not. If security is important to you, be aware of this.

What is your budget — both initial and maintenance?

More robust and feature laden routing protocols generally mean more resources are required to keep them working well. Also more secure configurations require still more resources. This includes both set up costs, as well as ongoing maintenance costs. Ignore these costs at the risk of having to drop the adoption of the new routing protocol mid-change.

Evaluate your chosen protocol

Once you have examined the features of the routing protocols listed above and chosen the one that best meets your needs, you can set up an evaluation or test install of that protocol.

The test install is generally set up in a sandbox configuration so it will not affect critical network traffic. The aim of the test install is to prove that it will work on a larger scale on your network. So be sure that the test install mirrors your larger network well enough for you to discover any problems. If its too simplistic, these problems may not appear.

If your chosen protocol does not meet your goals choose a different protocol and repeat the evaluation process until either a protocol meets your needs, or you change your criteria.

Implement your dynamic routing protocol

You have examined your needs, selected the best matching dynamic routing protocol, tested it, and now you are ready to implement it with confidence.

This guide will help you configure your FortiGate unit to support your chosen dynamic routing protocol. Refer to the various sections in this guide as needed during your implementation to help ensure a smooth transition. Examples for each protocol have been included to show proper configurations for different types of networks.

Comparison of dynamic routing protocols

Each dynamic routing protocol was designed to meet a specific routing need. Each protocol does some things well, and other things not so well. For this reason, choosing the right dynamic routing protocol for your situation is not an easy task.

Features of dynamic routing protocols

Each protocol is better suited for some situations over others.

Choosing the best dynamic routing protocol depends on the size of your network, speed of convergence required, the level of network maintenance resources available, what protocols the networks you connect to are using, and so on. For more information on these dynamic routing protocols, see Routing Information Protocol (RIP) on page 300, Border Gateway Protocol (BGP) on page 338, Open Shortest Path First (OSPF) on page 377, and Intermediate System to Intermediate System Protocol(IS-IS) on page 419.

Comparing RIP, BGP, and OSPF dynamic routing protocols

Protocol                           RIP                                   BGP                                 OSPF / IS-IS

Routing algorithm           Distance Vector, basic        Distance Vector, advanced

Link-state

Common uses Small non-complex net- works

Network backbone, ties multinational offices together

Common in large, com- plex enterprise networks

Strengths    Fast and simple to imple- ment

Near universal support

Good when no redund- ant paths

Graceful restart

BFD support

Only needed on border routers

Summarize routes

Fast convergence

Robust

Little management over- head

No hop count limitation

Scalable

Minimum configuration for dynamic routing

Dynamic routing protocols do not pay attention to routing updates from other sources, unless you specifically configure them to do so using CLI redistribute commands within each routing protocol.

The minimum configuration for any dynamic routing to function is to have dynamic routing configured on one interface on the FortiGate unit, and one other router configured as well. Some protocols require larger networks to function as designed.

Minimum configuration based on dynamic protocol

Interior versus exterior routing protocols

The names interior and exterior are very descriptive. Interior routing protocols are designed for use within a contained network of limited size, whereas exterior routing protocols are designed to link multiple networks together. They can be used in combination in order to simplify network administration. For example, a network can be built with only border routers of a network running the exterior routing protocol, while all the routers on the network run the interior protocol, which prevents them from connecting outside the network without passing through the border. Exterior routers in such a configuration must have both exterior and interior protocols, to communicate with the interior routers and outside the network.

Nearly all routing protocols are interior routing protocols. Only BGP is commonly used as an exterior routing protocol.

You may see interior gateway protocol (IGP) used to refer to interior routing protocols, and exterior gateway protocol (EGP) used to refer to interior routing protocols.

Distance vector versus link-state protocols

Every routing protocol determines the best route between two addresses using a different method. However, there are two main algorithms for determining the best route — Distance vector and Link-state.

Distance vector protocols

In distance vector protocols, routers are told about remote networks through neighboring routers. The distance part refers to the number of hops to the destination, and in more advanced routing protocols these hops can be weighted by factors such as available bandwidth and delay. The vector part determines which router is the next step along the path for this route. This information is passed along from neighboring routers with routing update packets that keep the routing tables up to date. Using this method, an outage along a route is reported back along to the start of that route, ideally before the outage is encountered.

On distance vector protocols, RFC 1058 which defines RIP v1 states the following:

Distance vector algorithms are based on the exchange of only a small amount of information. Each entity (gateway or host) that participates in the routing protocol is assumed to keep information about all of the destinations within the system. Generally, information about all entities connected to one network is summarized by a single entry, which describes the route to all destinations on that network.

There are four main weaknesses inherent in the distance vector method. Firstly, the routing information is not discovered by the router itself, but is instead reported information that must be relied on to be accurate and up-to- date. The second weakness is that it can take a while for the information to make its way to all the routers who need the information — in other words it can have slow convergence. The third weakness is the amount of overhead involved in passing these updates all the time. The number of updates between routers in a larger network can significantly reduce the available bandwidth. The fourth weakness is that distance vector protocols can end up with routing-loops. Routing loops are when packets are routed for ever around a network, and often occur with slow convergence. The bandwidth required by these infinite loops will slow your network to a halt.

There are methods of preventing these loops however, so this weakness is not as serious as it may first appear.

Link-state protocols

Link-state protocols are also known as shortest path first protocols. Where distance vector uses information passed along that may or may not be current and accurate, in link-state protocols each router passes along only information about networks and devices directly connected to it. This results in a more accurate picture of the network topology around your router, allowing it to make better routing decisions. This information is passed between routers using link-state advertisements (LSAs). To reduce the overhead, LSAs are only sent out when information changes, compared to distance vector sending updates at regular intervals even if no information has changed. The more accurate network picture in link-state protocols greatly speed up convergence and avoid problems such as routing-loops.

Classful versus classless routing protocols

Classful or classless routing refers to how the routing protocol handes the IP addresses. In classful addresses there is the specific address, and the host address of the server that address is connected to. Classless addresses use a combination of IP address and netmask.

Classless Inter-Domain Routing (CIDR) was introduced in 1993 (originally with RFC 1519 and most recently with RFC 4632) to keep routing tables from getting too large. With Classful routing, each IP address requires its own entry in the routing table. With Classless routing, a series of addresses can be combined into one entry potentially saving vast amounts of space in routing tables.

Current routing protocols that support classless routing out of necessity include RIPv2, BGP, IS-IS, and OSPF. Older protocols such as RIPv1 do not support CIDR addresses.

Dynamic routing protocols

A dynamic routing protocol is an agreed-on method of routing that the sender, receiver, and all routers along the path (route) support. Typically the routing protocol involves a process running on all computers and routers along that route to enable each router to handle routes in the same way as the others. The routing protocol determines how the routing tables are populated along that route, how the data is formatted for transmission, and what information about a route is included with that route. For example RIP, and BGP use distance vector algorithms, where OSPF uses a shortest path first algorithm. Each routing protocol has different strengths and weaknesses — one protocol may have fast convergence, while another may be very reliable, and a third is very popular for certain businesses like Internet Service Providers (ISPs).

Dynamic routing protocols are different from each other in a number of ways, such as:

• Classful versus classless routing protocols
• Interior versus exterior routing protocols
• Distance vector versus link-state protocols

Comparing static and dynamic routing

A common term used to describe dynamic routing is convergence. Convergence is the ability to work around network problems and outages — for the routing to come together despite obstacles. For example, if the main router between two end points goes down, convergence is the ability to find a way around that failed router and reach the destination. Static routing has zero convergence beyond trying the next route in its limited local routing table — if a network administrator doesn’t fix a routing problem manually, it may never be fixed, resulting in a downed network. Dynamic routing solves this problem by involving routers along the route in the decision-making about the optimal route, and using the routing tables of these routers for potential routes around the outage. In general, dynamic routing has better scalability, robustness, and convergence. However, the cost of these added benefits include more complexity and some overhead: the routing protocol uses some bandwidth for its own administration.

Comparing static and dynamic routing

Overhead                  None                                                    Varying amounts of bandwidth used for routing protocol updates

Scalability                Limited to small networks                    Very scalable, better for larger networks

Robustness              None – if a route fails it has to be fixed manually

Robust – traffic routed around failures auto- matically

Convergence           None                                                    Varies from good to excellent

 Dynamic Routing Overview

This section provides an overview of dynamic routing, and how it compares to static routing. For details on various dynamic routing protocols, see the following chapters for detailed information.

The following topics are included in this section: What is dynamic routing?

Comparison of dynamic routing protocols

Choosing a routing protocol Dynamic routing terminology IPv6 in dynamic routing

What is dynamic routing?

Dynamic routing uses a dynamic routing protocol to automatically select the best route to put into the routing table. So instead of manually entering static routes in the routing table, dynamic routing automatically receives routing updates, and dynamically decides which routes are best to go into the routing table. Its this intelligent and hands-off approach that makes dynamic routing so useful.

Dynamic routing protocols vary in many ways and this is reflected in the various administrative distances assigned to routes learned from dynamic routing. These variations take into account differences in reliability, speed of convergence, and other similar factors. For more information on these administrative distances, see Advanced Static Routing on page 256.

This section includes:

• Comparing static and dynamic routing
• Dynamic routing protocols
• Minimum configuration for dynamic routing