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When to Use
Advanced Router Configurations
You should first recognize that this chapter
covers advanced router configuration. If your
Linux-based router delivers so little traffic that it has no trouble keeping up
with the load, and if the router's configuration is simple and static so that
it doesn't need to reconfigure itself to work with other routers, then there's
no point in using the options described in this chapter. You can simply enable
router features by configuring two network cards and enabling routing between
them by typing the following command: # echo "1" > /proc/sys/net/ipv4/ip_forward
This command, in conjunction with an
appropriate routing table configuration for multiple networks, as described in
Network Configuration, is enough to turn a Linux computer into a router. This
configuration may work well enough for many networks.NOTE
align=left border=0> Configuring your Linux router with multiple
interfaces, an appropriate routing table, and IP forwarding is all you need
to do locally. The systems to which this
computer most directly communicates, though, must recognize it as a router in
order for this configuration to work. For instance, consider a Linux computer
that serves as a router for a dozen computers using a Symmetrical Digital
Subscriber Line (SDSL) broadband connection to the Internet. If the router
doesn't perform NAT, your ISP's router, to which one network interface of the
Linux router connects, must know that your Linux router is a router for your
network block. If this isn't the case, network packets will get out, but your
ISP's router won't know how to route return packets back in. For most router
configurations, therefore, you must communicate with whoever maintains the
other routers to which your router connects. You must also configure your
local systems to use your Linux router as a gateway.
One type of advanced router configuration
described in this chapter, in the "
sections, relates to modifications of the way Linux decides to route packets
based on some feature of the packets, such as their origins, destinations, or
protocol type. These options can influence how responsive the Internet appears
to be, because you might give priority to packets associated with interactive
protocols, or favor traffic from certain computers. Such configurations are
most often used on heavily loaded dedicated routers.The second type of advanced router
configuration described in this chapter is routing protocols. These are
protocols you can use to communicate with other routers. The servers that
implement these protocols allow your system to dynamically update its routing
table to reflect the best path for packets to travel so that the packets can
reach their destinations most quickly. Such protocols can enhance the
performance of a network that uses them, but they're most useful for a router
that has several network interfaces that reach the Internet. If the router has
just one link to the Internet, that link can't be changed in any meaningful
way, and so there's no point to running a routing protocol on the server,
because the protocol will have no effect on the router's routing table.
When to Use
Advanced Router Configurations
You should first recognize that this chapter
covers advanced router configuration. If your
Linux-based router delivers so little traffic that it has no trouble keeping up
with the load, and if the router's configuration is simple and static so that
it doesn't need to reconfigure itself to work with other routers, then there's
no point in using the options described in this chapter. You can simply enable
router features by configuring two network cards and enabling routing between
them by typing the following command: # echo "1" > /proc/sys/net/ipv4/ip_forward
This command, in conjunction with an
appropriate routing table configuration for multiple networks, as described in
Network Configuration, is enough to turn a Linux computer into a router. This
configuration may work well enough for many networks.NOTE
align=left border=0> Configuring your Linux router with multiple
interfaces, an appropriate routing table, and IP forwarding is all you need
to do locally. The systems to which this
computer most directly communicates, though, must recognize it as a router in
order for this configuration to work. For instance, consider a Linux computer
that serves as a router for a dozen computers using a Symmetrical Digital
Subscriber Line (SDSL) broadband connection to the Internet. If the router
doesn't perform NAT, your ISP's router, to which one network interface of the
Linux router connects, must know that your Linux router is a router for your
network block. If this isn't the case, network packets will get out, but your
ISP's router won't know how to route return packets back in. For most router
configurations, therefore, you must communicate with whoever maintains the
other routers to which your router connects. You must also configure your
local systems to use your Linux router as a gateway.
One type of advanced router configuration
described in this chapter, in the "
sections, relates to modifications of the way Linux decides to route packets
based on some feature of the packets, such as their origins, destinations, or
protocol type. These options can influence how responsive the Internet appears
to be, because you might give priority to packets associated with interactive
protocols, or favor traffic from certain computers. Such configurations are
most often used on heavily loaded dedicated routers.The second type of advanced router
configuration described in this chapter is routing protocols. These are
protocols you can use to communicate with other routers. The servers that
implement these protocols allow your system to dynamically update its routing
table to reflect the best path for packets to travel so that the packets can
reach their destinations most quickly. Such protocols can enhance the
performance of a network that uses them, but they're most useful for a router
that has several network interfaces that reach the Internet. If the router has
just one link to the Internet, that link can't be changed in any meaningful
way, and so there's no point to running a routing protocol on the server,
because the protocol will have no effect on the router's routing table.
