| IPv6 Network - Cisco |
IPv6 Network - Cisco
Using a manually configured tunnel comes very natural to Cisco’s Internetwork Operating System (IOS), and it’s very easy to configure, as shown in Listing 3-25. However, this example only allows the router itself to use the tunnel, just as with the examples for Windows, FreeBSD, MacOS X, and Linux.
Configuring a Tunnel Interface and an IPv6 Default Route Under Cisco IOS
!
interface Tunnel0
ipv6 address 2001:DB8:31:1::2/64
ipv6 enable
tunnel source 192.0.2.1
tunnel destination 223.224.225.226
tunnel mode ipv6ip
!
ipv6 route ::/0 2001:DB8:31:1::1
!
The tunnel interface is created and put in the “up” state as soon as the interface tunnel0 command is given. (Deleting the interface again is done with no interface tunnel0.) IPv6 is enabled for the interface as soon as an IPv6 address is configured, so the ipv6 enable command isn’t really necessary here. When configuring the address, the router won’t accept the address without a prefix length, which is usually /64, as in the example. The tunnel source command also accepts an interface as its argument. In that case, the primary IPv4 address for that interface is used as the tunnel source. Because IOS supports a number of different tunneling mechanisms, it is necessary to specify IPv6 in IP encapsulation explicitly with the tunnel mode ipv6ip command.
The other tunneling mechanisms aren’t appropriate for IPv6, with the exception of Generic Route Encapsulation (GRE). GRE is a tunneling protocol developed by Cisco that allows the transport of many protocols other than just IPv6 over IP tunnels. GRE also offers additional features, such as having a checksum over the tunneled packets or enforcing that packets aren’t delivered out of order. However, GRE adds a few more housekeeping bytes on top of the new IPv4 header, so if the additional GRE features aren’t needed, simple IPv6 in IP is the best choice.
Note Both manually configured tunnels (RFC 2893) and all automatic tunneling mechanisms except Teredo set the “protocol” field in the IPv4 header to 41 (decimal) to indicate that the content of the IPv4 packet is an IPv6 packet. The protocol number for GRE is 47. Teredo uses UDP port 3544. Be sure, if you run a firewall or other filter, that your tunnel protocol is allowed through. For instance, Red Hat ES4 with the default firewall configuration filters out protocol 41 so that 6to4 or manual tunnels don’t work reliably.
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