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ISDN Router 26
Important Routing Concepts
TCP/IP Routing
The purpose of IP routing is to take the IP destination address and look
up the interface on which the packet should be forwarded. In the case of
the router, this can be either the LAN (Ethernet) or the WAN. Because
each remote entry has an IP route associated with it, this may involve
first bringing up the link to that destination in order to allow forwarding
to take place.
Ethernet interface
Each Ethernet interface needs to have defined an IP address and subnet
mask.
ISDN WAN interface
The IP address and mask can be defined statically (Static Seeding),
- or -
The IP address and mask can be assigned dynamically through the PPP
Protocol,
- or -
An IP address and mask may simply not be required (Unnumbered
mode).
The exact detail for how the WAN port needs to be configured depends
upon the requirements of the router at the remote end. Normally, the
required information is passed between the routers using the PPP
Protocol, thus no manual configuration is required.
When the local WAN interface has an IP address and mask defined, the
remote WAN interface also has an IP address and mask associated with
it. These interfaces are known as the
local (source) WAN IP address
and the remote WAN IP address.
Each remote router ISDN WAN link may have local and remote WAN IP
addresses and subnet masks depending on the method of IP addressing
used. The IP routing table in the target router can be ‘seeded’ with
addressing information for networks/stations beyond the remote router.