Alcatel Carrier Internetworking Solutions Omni Switch/Router User Manual
Page 889
Frame Relay IP Routing
Page 29-17
If an additional Router and Subnet were added to the network and a new VC was added to
connect the new location, then much of the
WAN
routing load would fall on the
WSX
attached
to Subnet 1.1.1.x.
Adding A New Router Raises New Questions
The new
WSX
attached to Subnet 4.4.4.x connects to the
WAN
through the addition of a new
virtual circuit connecting directly to the
WSX
attached to Subnet 1.1.1.x. However, for the new
WSX
to route to Subnet 3.3.3.x it must go through the
WSX
router attached to Subnet 1.1.1.x.
This is okay for the initial routed path decision. But IP will try to find the most efficient route
between Subnet 4.4.4.x and 3.3.3.x. Unfortunately the most efficient route—which would be a
direct path between the two routers—is not possible because no
WAN
link exists between the
two.
Frame Relay routing allows the new Subnet, 4.4.4.x, and Subnet 3.3.3.x to route through the
WSX
router attached to Subnet 1.1.1.x. Normal IP would have a problem with this solution
because it does not allow “backtracking” through IP Subnets, which is exactly what must be
done in this case. Routed frames actually pass through the Frame Relay Network Subnet
2.2.2.x twice—once to get the
WSX
Router attached to Subnet 1.1.1.x and another time to get
to the Router attached to either Subnet 4.4.4.x or 3.3.3.x.
Standard routing uses a technique called “split horizon” that prevent loops through the same
Subnet from occurring. Frame Relay enhances split horizon to account for the nature of virtual
circuits. Loops through a
LAN
Subnet are inefficient, but Frame Relay routing makes allow-
ances to compensate for the fact that a
WAN
does not enjoy the same flexibility with router
connections as a LAN.
Note
Backtracking in InARP is allowed only through the IP
Subnet defined for the Frame Relay network.
The configuration of
WSX
routing services is described in Configuring a WAN Routing Service
R
WSX
Subnet 3.3.3.X
Subnet 2.2.2.X
New virtual circuit added for
routing to Subnet 4.4.4.x.
WSX
How will Subnets 4.4.4.x and
3.3.3.x route to each other?
Subnet 4.4.4.X
Subnet 1.1.1.X