For Part 1 of this series on IPv6 Transition Mechanisms, click here.

If you loved the simplicity of the IPv6 manual tunnel we created in Part 1, you should love the Generic Routing Encapsulation (GRE) approach to tunneling IPv6 traffic through an IPv4 cloud just as much.

This simple approach takes the IPv6 packet and encapsulates it using the standard IPv4 GRE tunnel. But wait a minute? In Part 1 of this blog series, we had IPv4 encapsulate the IPv6 packet directly for tunneling across the v4 cloud. Why do we even need this additional method that adds GRE encapsulation to the process? Well, the answer is that this method is required within integrated IS-IS and IPv6 tunnel environments. If you plan on sending both IS-IS traffic and IPv6 traffic over the tunnel, you need the protocol field of the GRE header that allows identification of the passenger protocol.

From a CCIE lab perspective, you might say, "well, I will never need this tunnel type then! We are not running IS-IS in the CCIE R/S lab. Woohoo!" Hold on. Not so fast. The exam authors could easily phrase an IPv6 transition task in such a way that requires the above tunnel type without a single care of its relevance for your lab scenario.

The great news is, once you master the distinction between the Manual and the GRE/IPv4 tunnel types, the configuration is nearly identical. We will just be modifying the tunnel mode option. Here is our topology from Part 1, and the subsequent configurations and verifications:



R1(config)#ipv6 unicast-routing
R1(config)#interface loopback 0
R1(config-if)#ipv6 rip CCIERIP enable
R1(config)#interface tunnel 0
R1(config-if)#ipv6 address 2001:13::1/64
R1(config-if)#tunnel source fastethernet0/0
R1(config-if)#tunnel destination
R1(config-if)#tunnel mode gre ip
R1(config-if)#ipv6 rip CCIERIP enable


R3(config)#ipv6 unicast-routing
R3(config)#interface loopback 0
R3(config-if)#ipv6 rip CCIERIP enable
R3(config)#interface tunnel 0
R3(config-if)#ipv6 address 2001:13::3/64
R3(config-if)#tunnel source fastethernet0/0
R3(config-if)#tunnel destination
R3(config-if)#tunnel mode gre ip
R3(config-if)#ipv6 rip CCIERIP enable


R1#ping 2001:3::3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:3::3, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/13/16 ms

You should be aware that the example in the 12.4 Mainline Configuration Guide shows a configuration similar to the above, but incorrectly demonstrates the use of the tunnel mode gre ipv6 command. This command would be used to encapsulate v4 packets over an IPv6 infrastructure.

Thanks for reading, and for your Tier 2 and Tier 3 practice and assessments, be sure to investigate all of our R&S products!

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