Cabletron Systems CSX100 series User Manual - Important Routing Concepts

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.