4.3. Planning Routing
In Chapter 2, "Delivering the Data", we
learned that hosts communicate
directly only with other computers connected to the same network.
Gateways are needed to communicate with systems on other networks. If
the hosts on your network need to communicate with computers on other
networks, a route through a gateway must be defined. There are two
ways to do this:
Many networks use a combination of both static and dynamic routing.
Some systems on the network use static routing tables while others
run routing protocols and have dynamic tables. While it is often
appropriate for hosts to use static routing tables, gateways usually
run routing protocols.
The network administrator is responsible for deciding what type of
routing to use and for choosing the default gateway for each host.
ake these decisions before you start to configure your system.
Here are a few guidelines to help you plan routing. If you have:
- A network with no gateways to other TCP/IP networks
-
No special routing configuration is required in this case. The
gateways referred to in this discussion are IP routers that
interconnect TCP/IP networks. If you are not interconnecting TCP/IP
networks, you do not need an IP router. Neither a default gateway nor
a routing protocol needs to be specified.
- A network with a single gateway
-
If you have only one gateway, don't run any routing protocols.
Specify the single gateway as the default gateway in a static routing
table.
- A network with internal gateways to other subnets and a single gateway to the world
-
Here, there is a real choice. You can statically specify each subnet
route and make the gateway to the world your default route, or you
can run a routing protocol. Decide which you want to do based on the
effort involved in maintaining a static table versus the slight
overhead of running a routing protocol on your hosts and networks. If
you have more than a few hosts, running a routing protocol is
probably easiest.
- A network with multiple gateways to the world
-
If you have multiple gateways that can reach the same destination,
use a routing protocol. This allows the gateways to adapt to network
changes, giving you redundant access to the remote networks.
Figure 4-1 shows a subnetted network with five
gateways identified as A through
E. A central subnet (172.16.1.0) interconnects
five other subnets. One of the subnets has a gateway to an external
network. The network administrator would probably choose to run a
routing protocol on the central subnet (172.16.1.0) and perhaps on
subnet 172.16.12.0, which is attached to an external network. Dynamic
routing is appropriate on these subnets because they have multiple
gateways. Without dynamic routing, the administrator would need to
update every one of these gateways manually whenever any change
occurred in the network -- for example, whenever a new subnet was
added. A mistake during the manual update could disrupt network
service. Running a routing protocol on these two subnets is simpler
and more reliable.
Figure 4-1. Routing and subnets
On the other hand, the administrator would probably choose static
routing for the other subnets (172.16.3.0, 172.16.6.0, and
172.16.9.0). These subnets each use only one gateway to reach all
destinations. Changes external to the subnets, such as the addition
of a new subnet, do not change the fact that these three subnets
still have only one routing choice. Newly added networks are still
reached through the same gateway. The hosts on these subnets specify
the subnet's gateway as their default route. In other words,
the hosts on subnet 172.16.3.0 specify B as the
default gateway, while the hosts on subnet 172.16.9.0 specify
D as the default, no matter what happens on the
external networks.
Some routing decisions are thrust upon you by the external networks
to which you connect. In Figure 4-1, the local
network connects to an external network that requires that Border
Gateway Protocol (BGP) be used for routing. Therefore, gateway
E has to run BGP to exchange routes with the
external network.
4.3.2. Registering in a Routing Database
If you obtain
an official ASN, you must decide
whether you need to register in a routing database. If you got your
ASN because you're multi-homed, you should register with a
routing database. Section 2.3, "Internet Routing Architecture" explains that routing
databases are used to validate routing in the new Internet because
there is no longer a central core that can be relied on to determine
"best" routes. When you obtain an official ASN, you
become part of the structure of co-equal routing domains. You assume
responsibility for a small portion of the routing burden and you
declare that responsibility by registering in a routing database.
There are several different databases that make up the
Internet Routing
Registry (IRR). In addition to the Routing Arbiter Database (RADB) mentioned
in Chapter 2, "Delivering the Data", RIPE, ANS, Bell Canada, and Cable
& Wireless all maintain databases. RIPE serves customers in the
RIPE region. ANS, Bell Canada, and Cable & Wireless register only
their paying customers. RADB is available to anyone.
To register in the RADB, first register a maintainer object.
aintainer objects identify the person who will be responsible for
maintaining your database entries. Provide the required information,
and pay the $200 fee. You must then register the autonomous system as
an AS object. Finally, you create a Route object for each route your
system will advertise. See
http://www.radb.net for detailed information about
registering these database objects.
All of the items discussed so far (addressing, subnetting, and
routing) are required to configure the basic physical network on top
of which the applications and services run. Now we begin planning the
services that make the network useful and usable.
| | | 4.2. Basic Information | | 4.4. Planning Naming Service |
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