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[Chapter 7] 7.6 Gateway Routing Daemon

[Chapter 7] 7.6 Gateway Routing Daemon

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[Chapter 7] 7.6 Gateway Routing Daemon

determining the best route, and combines that information to update the routing table. Before gated
was written, if a UNIX system ran multiple routing protocols each would write routes into the routing
table without knowledge of the other's action. The route found in the table was the last one written not necessarily the best route.
With multiple routing protocols and multiple network interfaces, it is possible for a system to receive
routes to the same destination from different protocols. gated compares these routes and attempts to
select the best one. However, the metrics used by different protocols are not directly comparable.
Each routing protocol has its own metric. It might be a hop count, the delay on the route, or an
arbitrary value set by the administrator. gated needs more than that protocol's metric to select the best
route. It uses its own value to prefer routes from one protocol or interface over another. This value is
called preference.
Preference values help gated combine routing information from several different sources into a single
routing table. Table 7.1 lists the sources from which gated receives routes, and the default preference
given to each source. Preference values range from 0 to 255, with the lowest number indicating the
most preferred route. From this table you can see that gated prefers a route learned from OSPF over
the same route learned from BGP.
Table 7.1: Default Preference Values
Route Type
Default Preference
direct route
0
OSPF
10
Internally generated default 20
ICMP redirect
30
static route
60
Hello protocol
90
RIP
100
OSPF ASE routes
150
BGP
170
EGP
200
Preference can be set in several different configuration statements. It can be used to prefer routes from
one network interface over another, from one protocol over another, or from one remote gateway over
another. Preference values are not transmitted or modified by the protocols. Preference is used only in
the configuration file. In the next section we'll look at the gated configuration file (/etc/gated.conf)
and the configuration commands it contains.

Previous: 7.5 Exterior
Routing Protocols
7.5 Exterior Routing Protocols

TCP/IP Network
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Book Index

Next: 7.7 Configuring gated

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7.7 Configuring gated

[Chapter 7] 7.6 Gateway Routing Daemon

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[Chapter 7] 7.5 Exterior Routing Protocols

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Routing Protocols

Chapter 7
Configuring Routing

Next: 7.6 Gateway Routing
Daemon

7.5 Exterior Routing Protocols
Exterior routing protocols are used to exchange routing information between autonomous systems.
The routing information passed between autonomous systems is called reachability information.
Reachability information is simply information about which networks can be reached through a
specific autonomous system.
RFC 1771 defines Border Gateway Protocol, the leading exterior routing protocol, and provides the
following description of the routing function of an autonomous system:
The classic definition of an Autonomous System is a set of routers under a single
technical administration, using an interior gateway protocol and common metrics to
route packets within the AS, and using an exterior gateway protocol to route packets to
other ASs.... The administration of an AS appears to other ASs to have a single coherent
interior routing plan and presents a consistent picture of what networks are reachable
through it. From the standpoint of exterior routing, an AS can be viewed as
monolithic...
Moving routing information into and out of these monoliths is the function of exterior routing
protocols. Exterior routing protocols are also called exterior gateway protocols. Don't confuse an
exterior gateway protocol with the Exterior Gateway Protocol (EGP). EGP is not a generic term; it is a
particular exterior routing protocol, and an old one at that.

7.5.1 Exterior Gateway Protocol
A gateway running EGP announces that it can reach networks that are part of its autonomous system.
It does not announce that it can reach networks outside its autonomous system. For example, the
exterior gateway for our imaginary autonomous system nuts-as can reach the entire Internet through
its external connection, but only one network is contained in its autonomous system. Therefore, it
would only announce one network (172.16.0.0) if it ran EGP.
Before sending routing information, the systems first exchange EGP Hello and I-Heard-You (I-H-U)
messages. These messages establish a dialog between two EGP gateways. Computers communicating
via EGP are called EGP neighbors, and the exchange of Hello and I-H-U messages is called acquiring

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[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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