IP encapsulation
The IP packets are encapsulated in connectionless network protocol (CLNP) packets
which are routed through the OSI-only node which routes the CLNP packets as
normal, totally oblivious of the CLNP packets contents.
IP and DCC
A benefit of the tunneling approach, is that IP need not to be carried on the DCC. Only
the CLNP packets (possibly carrying IP) go on to the DCC.
IP routing
In the dual stack with tunnelling configuration, a second network layer entity (the IP
router) is added alongside the existing OSI network layer entity (CLNP). The CLNP
entity has the routing tables and the mechanisms to automatically populate them –
namely the IS-IS and ES-IS dynamic routing protocols.
This is not the case for the IP routing tables since there is no IP dynamic routing
protocol to populate the IP routing tables. IP routing tables have to be manually
provisioned by the operator.
DSwT routing table
A routing table, IP or OSI, is divided into 2 columns:
•
“Final destinations” of each network node,
•
for each final destination, the identity of the “next hop” which is the next network
node in the path towards the final destination.
The table below shows a simplified IP routing table for IP router in a DSwT stack. It
is divided into 2 main columns. These two parts of the routing table will be discussed
separately.
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