5.17.1. Internal Rerouting Disabled 
This SIS is used on DX when a network is created such that there are
nodes with only 1 route out to a particular destination.In this situation
congestion is unavoidable, and so in order to stop the situation where
the call drops back and the node tries a different route, which will
inevitably lead to the same point of congestion, IRD is used. This is
administered on a network basis.
Figure 45. Network with Internal routes only
If a point of congestion is encountered in the network then IRD flag is sent backwards.
In Figure 47
there is a possible point of congestion between nodes 4 and 5. Without
the Internal rerouting Disable being set a call from Node 1 to Node 5
would route initially to node 4 via Node 2. A Disc message would be sent
back from Node 4 with the cause No Route to Dest.
The LCR in Node 1 would then choose the alternate route via Node 3
but with the same result a Disc with the same clearing code.
This can lead to a dynamic loading of the network To overcome this the IRD feature
is to be used.
In this case the clearing message is accompanied by a further flag which says no internal
rerouting is enabled
This is done in the route element leading to the congested route
ADD-LDAT:LROUTE=xx,LSVC=ALL,TGRP=xx,ODR=xx,LAUTH=xx, LATTR=INTBNECK;
When a LROUTE ends in a bottleneck of the net, all trunk groups to this LROUTE have
to be
marked with the attribute "INTBNECK".
In the following figure LCR searches for a new route. In the database it is verified
whether
the new route leads to a Foreign network or not. This is marked in the COT of the
trunk
CHANGE-COT:COTNO=xx,COTTYPE=COTADD,PAR=NTBO;
The system administrator knows the network boundary of the private network and can
set the
COT parameter "NTBO" where it's necessary, and if it not then the call is disconnected.
In the following figure the call would route out to the public network at sys 3 as
the route
is marked as a network boundary (this assumes the route is in the LDAT for this call).
On receipt of the DISC message Sys 3 would reroute only if the chosen route is marked
as a
network boundary i.e COT parameter NTBO is set