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410 Chapter 6: Routing
Figure 6-16
IP Tunneling Concepts
After the IP packet is created, routing logic is repeated by Router A, this time for the new IP
packet. Router A routes the IP packet based on the IP routing table, as does Router B and then
Router C. Routers B and C have no knowledge that there is an IPX packet inside the IP packet.
When the packet arrives at Router D, D notices that the destination address is one of its own
addresses, so it examines the data further. Upon finding the encapsulation protocol header
immediately after the IP header, the router knows that this is a tunneled packet, so Router D
strips off the encapsulation header, which leaves the IPX packet, in this case. The IPX packet
then is routed, which sends the packet out the Ethernet interface.
Three important terms are used to describe the three parts of the entity that is sent between the
two tunneling routers:
·
Passenger protocol--This is the protocol being encapsulated. In Figure 6-16, IPX is the
passenger protocol.
·
Encapsulation protocol--To identify the passenger protocol, an additional header is
used. You can think of this additional header as another place to include a field such as the
data link layer's type, DSAP, or protocol field. The IP header defines that one of these
encapsulating protocol headers follows IP, and the encapsulation protocol identifies the
type of Layer 3 passenger protocol that follows it.
·
Transport protocol--The transport protocol delivers the passenger protocol across the
network. IP is the only choice in the IOS.
For each packet of the encapsulated (passenger) protocol, there is the additional overhead of
applying the packet header of the encapsulating (transport) protocol. By adding more bytes of
Frame
Relay
Physical View
Client 1
Server 1
A
IPX Network 1
B
C
D
IPX Network 2
Logical View
Client 1
Server 1
Ethernet
IPX Packet
D
Ethernet
IPX Packet
Transport
(IP)
Encapsulation
Protocol
IPX Packet
(Passenger)
ch06.fm Page 410 Monday, March 20, 2000 5:11 PM