4000 Router Series Layer 3 - IP Services Configuration Guide

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ISATAP tunnels are mainly used for communication between IPv6 routers or between an IPv6 host

and an IPv6 router over an IPv4 network.

Figure 96 Principle of ISATAP tunneling

IPv4 over IPv4 tunneling

IPv4 over IPv4 tunneling (RFC 1853) enables isolated IPv4 networks to communicate. For example, an

IPv4 over IPv4 tunnel can connect isolated private IPv4 networks over a public IPv4 network.

Figure 97 IPv4 over IPv4 tunnel

Packets traveling through a tunnel undergo encapsulation and de-encapsulation, as shown in Figure 97.

• Encapsulation:

a. Device A receives an IP packet from an IPv4 host and submits it to the IP protocol stack.

b. The IPv4 protocol stack determines how to forward the packet according to the destination

address in the IP header. If the packet is destined for the IPv4 host connected to Device B,

Device A delivers the packet to the tunnel interface.

c. The tunnel interface adds a new IPv4 header to the IPv4 packet and submits it to the IP protocol

stack. In the new header, the source IP address specifies the tunnel source, and the destination

IP address specifies the tunnel destination. The IP protocol stack uses the destination IP address

of the new IP header to look up the routing table, and then sends the packet out.

• De-encapsulation:

d. After receiving the packet, Device B delivers it to the IP protocol stack.

e. If the protocol number is 4 (indicating an IPv4 packet is encapsulated within the packet), the IP

protocol stack delivers the packet to the tunnel module for de-encapsulation.

f. The tunnel module de-encapsulates the IP packet and sends it back to the IP protocol stack.

g. The protocol stack forwards the de-encapsulated packet.