Dynamic routing terminology

Dynamic routing is a complex subject. There are many routers on different networks and all can be configured differently. It become even more complicated when you add to this each routing protocol having slightly different names for similar features, and many configurable features for each protocol.

To better understand dynamic routing, here are some explanations of common dynamic routing terms.

• Aggregated routes and addresses
• Autonomous system (AS)
• Area border router (ABR)
• Neighbor routers
• Route maps
• Access lists
• Bi-directional forwarding detection (BFD)

For more details on a term as it applies to a dynamic routing protocol, see one of Border Gateway Protocol (BGP) on page 338, Routing Information Protocol (RIP) on page 300, or Open Shortest Path First (OSPF) on page 377.

 

Aggregated routes and addresses

Just as an aggregate interface combines multiple interfaces into one virtual interface, an aggregate route combines multiple routes into one. This reduces the amount of space those routes require in the routing tables of the routers along that route. The trade-off is a small amount of processing to aggregate and de-aggregate the routes at either end.

The benefit of this method is that you can combine many addresses into one, potentially reducing the routing table size immensely. The weakness of this method is if there are holes in the address range you are aggregating you need to decide if its better to break it into multiple ranges, or accept the possibility of failed routes to the missing addresses.

For information on aggregated routes in BGP, see Border Gateway Protocol (BGP) on page 338, and Border Gateway Protocol (BGP) on page 338.

 

To manually aggregate the range of IP addresses from 192.168.1.100 to 192.168.1.103

1. Convert the addresses to binary

192.168.1.100	=	11000000	10101000	00000001	01100100
192.168.1.101	=	11000000	10101000	00000001	01100101
192.168.1.102	=	11000000	10101000	00000001	01100110
192.168.1.103	=	11000000	10101000	00000001	01100111

 

2. Determine the maximum number of matching bits common to the addresses.

There are 30-bits in common, with only the last 2-bits being different.

 

3. Record the common part of the address.

11000000 10101000 00000001 0110010X = 192.168.1.100

 

4. For the netmask, assume all the bits in the netmask are 1 except those that are different which are 0.

11111111 11111111 11111111 11111100 = 255.255.255.252

 

5. Combine the common address bits and the netmask.

192.168.1.100/255.255.255.252

Alternately the IP mask may be written as a single number:

192.168.1.100/2

 

6. As required, set variables and attributes to declare the routes have been aggregated, and what router did the aggregating.

 

Autonomous system (AS)

An Autonomous System (AS) is one or more connected networks that use the same routing protocol, and appear to be a single unit to any externally connected networks. For example an ISP may have a number of customer networks connected to it, but to any networks connected externally to the ISP it appears as one system or AS. An AS may also be referred to as a routing domain.

It should be noted that while OSPF routing takes place within one AS, the only part of OSPF that deals with the AS is the AS border router (ASBR).

There are multiple types of AS defined by how they are connected to other ASes. A multihomed AS is connected to at least two other ASes and has the benefit of redundancy — if one of those ASes goes down, your AS can still reach the Internet through its other connection. A stub AS only has one connection, and can be useful in specific configurations where limited access is desirable.

Each AS has a number assigned to it, known as an ASN. In an internal network, you can assign any ASN you like (a private AS number), but for networks connected to the Internet (public AS) you need to have an officially registered ASN from Internet Assigned Numbers Authority (IANA). ASNs from 1 to 64,511 are designated for public use.

NAs of January 2010, AS numbers are 4 bytes long instead of the former 2 bytes.

RFC 4893 introduced 32-bit ASNs, which FortiGate units support for BGP and OSPF.

 

Do you need your own AS?

The main factors in deciding if you need your own AS or if you should be part of someone else’s are:

• exchanging external routing information
• many prefixes should exist in one AS as long as they use the same routing policy
• when you use a different routing protocol than your border gateway peers (for example your ISP uses BGP, and you use OSPF)
• connected to multiple other AS (multi-homed)

You should not create an AS for each prefix on your network. Neither should you be forced into an AS just so someone else can make AS-based policy decisions on your traffic.

There can be only one AS for any prefix on the Internet. This is to prevent routing issues.

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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.

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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

 

	BGP	RIP	OSPF / IS-IS
Interface	yes	yes	yes
Network	yes	yes	yes
AS	local and neighbor	no	yes
Neighbors	at least one	at least one	at least one
Version	no	yes	no
Router ID	no	no	yes

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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.

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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.

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

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

 

Feature	Static Routing	Dynamic Routing
Hardware sup- port	Supported by all routing hardware	May require special, more expensive routers
Router Memory Required	Minimal	Can require considerable memory for larger tables
Complexity	Simple	Complex

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

 

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!