Perle Systems P840 - Manual (Page 20)

ISDN Connection Management
20
P840 Session Participation (Spoofing)
While an ISDN call is up and connected, all traffic within the sessions will be considered interesting and will be
transferred to the partner P840 across the ISDN call. When the P840 determines that the ISDN call is to be
suspended, the P840 will consider keepalive and routing information packets to now be non-interesting and will
begin to generate and respond to keepalive and RIP packets.
When the P840 receives a keepalive packet from the LAN for one of the sessions, the P840 will not activate the
ISDN call and will not pass the keepalive packet to the remote LAN. The P840 will generate a response to the
keepalive packet and send it to the originator of the packet. In this way, the P840 will keep the ISDN call
suspended and will also keep the local side of the session active. The P840 at the remote site will also be
participating in the keepalive process with the remote side of the session.
Th is R ou ter w ill receive the keepa live
fra m es destined for th e C lient and generate
a response back to the S erver on behalf of the
Client w hile the IS D N call is suspended.
Th e S erver generates keepalive fram es
which are to be sent to th e C lient and
acknow ledged by the Client.
Th e C lient receives th e
keepa live fram es and
sends an a ckno w ledge
back to the S erver.
W h ile the ISD N call is
suspended, this Ro uter
will generate keepalive
fra m es and send them
to the C lien t a nd w ait for
an acknow ledem ent from
the C lient.
C lien t
S erver
S uspended
Figure 2 - 4 Session Keepalive Messages
While an ISDN call is suspended, if the P840 observes that one of the devices in the session stops sending
keepalive packets, the P840 will begin to generate keepalive packets and send them to the device in order to
determine the status of the device. The length of time the P840 waits before beginning to generate keepalive
packets is definable by the operator.
P840 routers incorporate a settling time for routing updates. This means that a P840 will wait after an initial
change in the network is reported before transmitting that change on to the remaining P840 routers connected
on the Wide Area Network.
The P840 will resume the suspended ISDN call in order to transmit routing messages to partner P840s. If the
ISDN call cannot be resumed, or has been closed, the routes will be aged out of the routing table.