[Chapter 7] 7.5 Exterior Routing Protocols

[Chapter 7] 7.5 Exterior Routing Protocols



a neighbor.

Once a neighbor is acquired, routing information is requested via a poll. The neighbor responds by

sending a packet of reachability information called an update. The local system includes the routes

from the update into its local routing table. If the neighbor fails to respond to three consecutive polls,

the system assumes that the neighbor is down and removes the neighbor's routes from its table. If the

system receives a poll from its EGP neighbor, it responds with its own update packet.

Unlike the interior protocols discussed above, EGP does not attempt to choose the "best" route. EGP

updates contain distance-vector information, but EGP does not evaluate this information. The routing

metrics from different autonomous systems are not directly comparable. Each AS may use different

criteria for developing these values. Therefore, EGP leaves the choice of a "best" route to someone

else.

When EGP was designed, the network relied upon a group of trusted core gateways to process and

distribute the routes received from all of the autonomous systems. These core gateways were expected

to have the information necessary to choose the best external routes. EGP reachability information

was passed into the core gateways, where the information was combined and passed back out to the

autonomous systems.

A routing structure that depends on a centrally controlled group of gateways does not scale well and is

therefore inadequate for the rapidly growing Internet. As the number of autonomous systems and

networks connected to the Internet grew, it became difficult for the core gateways to keep up with the

expanding workload. This is one reason why the Internet moved to a more distributed architecture that

places a share of the burden of processing routes on each autonomous system. Another reason is that

no central authority controls the commercialized Internet. The Internet is composed of many equal

networks. In a distributed architecture, the autonomous systems require routing protocols, both

interior and exterior, that can make intelligent routing choices. Because of this, EGP is no longer

popular.



7.5.2 Border Gateway Protocol

Border Gateway Protocol (BGP) is the leading exterior routing protocol of the Internet. It is based on

the OSI InterDomain Routing Protocol (IDRP). BGP supports policy-based routing, which uses nontechnical reasons (for example, political, organizational, or security considerations) to make routing

decisions. Thus BGP enhances an autonomous system's ability to choose between routes and to

implement routing policies without relying on a central routing authority. This feature is important in

the absence of core gateways to perform these tasks.

Routing policies are not part of the BGP protocol. Policies are provided externally as configuration

information. As described in Chapter 2, the National Science Foundation provides Routing Arbiters

(RAs) at the Network Access Points (NAPs) where large Internet Service Providers (ISPs)

interconnect. The RAs can be queried for routing policy information. Most ISPs also develop private

policies based on the bilateral agreements they have with other ISPs. BGP can be used to implement

these policies by controlling the routes it announces to others and the routes it accepts from others. In

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[Chapter 7] 7.5 Exterior Routing Protocols



the gated section of this chapter we discuss the import command and the export command, which

control what routes are accepted (import) and what routes are announced (export). The network

administrator enforces the routing policy through configuring the router.

BGP is implemented on top of TCP, which provides BGP with a reliable delivery service. BGP uses

well-known TCP port 179. It acquires its neighbors through the standard TCP three-way handshake.

BGP neighbors are called peers. Once connected, BGP peers exchange OPEN messages to negotiate

session parameters, such as the version of BGP that is to be used.

The UPDATE message lists the destinations that can be reached through a specific path and the

attributes of the path. BGP is a path vector protocol. It is called a path vector protocol because it

provides the entire end-to-end path of a route in the form of a sequence of autonomous system

numbers. Having the complete AS path eliminates the possibility of routing loops and count-toinfinity problems. A BGP UPDATE contains a single path vector and all of the destinations reachable

through that path. Multiple UPDATE packets may be sent to build a routing table.

BGP peers send each other complete routing table updates when the connection is first established.

After that, only changes are sent. If there are no changes, just a small (19-byte) KEEPALIVE message

is sent to indicate that the peer and the link are still operational. BGP is very efficient in its use of

network bandwidth and system resources.

By far the most important thing to remember about exterior protocols is that most systems never run

them. Exterior protocols are only required when an AS must exchange routing information with

another AS. Most routers within an AS run an interior protocol such as OSPF. Only those gateways

that connect the AS to another AS need to run an exterior routing protocol. Your network is probably

an independent part of an AS run by someone else. Internet Service Providers are good examples of

autonomous systems made up of many independent networks. Unless you provide a similar level of

service, you probably don't need to run an exterior routing protocol.



7.5.3 Choosing a Routing Protocol

Although there are many routing protocols, choosing one is usually easy. Most of the interior routing

protocols mentioned above were developed to handle the special routing problems of very large

networks. Some of the protocols have only been used by large national and regional networks. For

local area networks, RIP is still the most common choice. For larger networks, OSPF is the choice.

If you must run an exterior routing protocol, the protocol that you use is often not a matter of choice.

For two autonomous systems to exchange routing information, they must use the same exterior

protocol. If the other AS is already in operation, its administrators have probably decided which

protocol to use, and you will be expected to conform to their choice. Most often this choice is BGP.

The type of equipment affects the choice of protocols. Routers support a wide range of protocols,

though individual vendors may have a preferred protocol. Hosts don't usually run routing protocols at

all, and most UNIX systems are delivered with only RIP. Allowing host systems to participate in

dynamic routing could limit your choices. gated, however, gives you the option to run many different

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[Chapter 7] 7.5 Exterior Routing Protocols



routing protocols on a UNIX system. While the performance of hardware designed specifically to be a

router is generally better, gated gives you the option of using a UNIX system as a router.

In the following sections we discuss the Gateway Routing Daemon (gated) software that combines

interior and exterior routing protocols into one software package. We look at examples of running

RIP, RIPv2, OSPF, and BGP with gated.



Previous: 7.4 Interior

Routing Protocols

7.4 Interior Routing Protocols



TCP/IP Network

Administration

Book Index



Next: 7.6 Gateway Routing

Daemon

7.6 Gateway Routing Daemon



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[Chapter 7] 7.4 Interior Routing Protocols



Previous: 7.3 Building a

Static Routing Table



Chapter 7

Configuring Routing



Next: 7.5 Exterior Routing

Protocols



7.4 Interior Routing Protocols

Routing protocols are divided into two general groups: interior and exterior protocols. An interior

protocol is a routing protocol used inside - interior to - an independent network system. In TCP/IP

terminology, these independent network systems are called autonomous systems. [9] Within an

autonomous system (AS), routing information is exchanged using an interior protocol chosen by the

autonomous system's administration.

[9] Autonomous systems are described in Chapter 2, Delivering the Data.

All interior routing protocols perform the same basic functions. They determine the "best" route to

each destination, and they distribute routing information among the systems on a network. How they

perform these functions, in particular, how they decide which routes are best, is what makes routing

protocols different from each other. There are several interior protocols:

●



●



●



●



The Routing Information Protocol (RIP) is the interior protocol most commonly used on UNIX

systems. RIP is included as part of the UNIX software delivered with most systems. It is

adequate for local area networks and is simple to configure. RIP selects the route with the

lowest "hop count" (metric) as the best route. The RIP hop count represents the number of

gateways through which data must pass to reach its destination. RIP assumes that the best route

is the one that uses the fewest gateways. This approach to route choice is called a distancevector algorithm.

Hello is a protocol that uses delay as the deciding factor when choosing the best route. Delay is

the length of time it takes a datagram to make the round trip between its source and

destination. A Hello packet contains a time stamp indicating when it was sent. When the

packet arrives at its destination, the receiving system subtracts the time stamp from the current

time, to estimate how long it took the packet to arrive. Hello is not widely used. It was the

interior protocol of the original 56 kbps NSFNET backbone and has had very little use

otherwise.

Intermediate System to Intermediate System (IS-IS) is an interior routing protocol from the OSI

protocol suite. It is a Shortest Path First (SPF) link-state protocol. It was the interior routing

protocol used on the T1 NSFNET backbone, and it is still used by some large service

providers.

Open Shortest Path First (OSPF) is another link-state protocol developed for TCP/IP. It is

suitable for very large networks and provides several advantages over RIP.



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[Chapter 7] 7.4 Interior Routing Protocols



Of these protocols, we will discuss RIP and OSPF in detail. OSPF is widely used on routers. RIP is

widely used on UNIX systems. We will start the discussion with RIP.



7.4.1 Routing Information Protocol

As delivered with most UNIX systems, Routing Information Protocol (RIP) is run by the routing

daemon routed (pronounced "route" "d"). When routed starts, it issues a request for routing updates

and then listens for responses to its request. When a system configured to supply RIP information

hears the request, it responds with an update packet based on the information in its routing table. The

update packet contains the destination addresses from the routing table and the routing metric

associated with each destination. Update packets are issued in response to requests, as well as

periodically to keep routing information accurate.

To build the routing table, routed uses the information in the update packets. If the routing update

contains a route to a destination that does not exist in the local routing table, the new route is added. If

the update describes a route whose destination is already in the local table, the new route is used only

if it has a lower cost. The cost of a route is determined by adding the cost of reaching the gateway that

sent the update to the metric contained in the RIP update packet. If the total metric is less than the

metric of the current route, the new route is used.

RIP also deletes routes from the routing table. It accomplishes this in two ways. First, if the gateway

to a destination says the cost of the route is greater than 15, the route is deleted. Second, RIP assumes

that a gateway that doesn't send updates is dead. All routes through a gateway are deleted if no

updates are received from that gateway for a specified time period. In general, RIP issues routing

updates every 30 seconds. In many implementations, if a gateway does not issue routing updates for

180 seconds, all routes through that gateway are deleted from the routing table.

7.4.1.1 Running RIP with routed

To run RIP using the routing daemon (routed), [10] enter the following command:

[10] On some systems the routing daemon is in.routed.

# routed

The routed statement is often used without any command-line arguments, but you may want to use

the -q option. The -q option prevents routed from advertising routes. It just listens to the routes

advertised by other systems. If your computer is not a gateway, you should probably use the -q option.

In the section on static routing we commented out the routed statement found in a startup file. If that

statement is in your startup file, no other action is required to run RIP; just boot your system and RIP

will run. Otherwise, add the routed command to your startup.



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