Alcatel Carrier Internetworking Solutions Switch/Router ユーザーズマニュアル

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.

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.

Backtracking in InARP is allowed only through the IP 
Subnet defined for the Frame Relay network.

The configuration of 

WSX

New virtual circuit added for
routing to Subnet 4.4.4.x.

How will Subnets 4.4.4.x and
3.3.3.x route to each other?