For those of you that have been following the previous parts of this blog series (they are located in the subcategory of the CCIE R&S category to the left), get ready for a major paradigm shift. So far, we have been experimenting with transition techniques (tunnels) that have focused on connecting remote “island” networks of over an IPv4-only infrastructure. Now we are going to discuss a mechanism that was designed to help IPv4-only hosts communicate to other native devices.

Intra-Site Automatic Addressing Protocol (ISATAP) is most recently specified in RFC 5214. Notice the topology below that we will use to detail the workings of this transition approach. This internal network has RouterB in place that is not capable :-( . ISATAP provides a solution for the hosts behind this device! Dynamic tunneling will be done from these hosts to the ISATAP router (RouterA). Obviously, your job in the CCIE R&S Exam might be to configure or troubleshoot this important device.

IPv6ISATAP

Here is how ISATAP actually works. The networks DNS server is updated with a well-known name entry of “ISATAP” that resolves to the IPv4 address used in the on the ISATAP router (RouterA). HostA initializes and notes that it has been configured with ISATAP capabilities for . HostA then sends a request to the DNS server for the address associated with “ISATAP”. DNS responds with the IPv4 address of the ISATAP router. HostA tunnels a router discovery packet (using an -in-IPv4 encapsulation approach) and sends this packet to the ISATAP router. RouterA responds with a router advertisement that includes the prefix the host (HostA) should use. HostA takes this prefix and automatically constructs its own unique address. It uses a reserved identifier for ISATAP (0:5efe) and its own IPv4 address to do this. Now the host is fully able to communicate beyond its local network using and ISATAP.

One of the exciting things about the ISATAP soltuion is the fact that HostA will automatically transition to native communications once the network is upgraded (in our case, once RouterB is replaced or upgraded). The minute HostA begins receiving unsolicited, native router advertisements, it ignores its ISATAP capabilities.

The configuration of the ISATAP router is very simple. Here is an example:

RouterA:
configure terminal
!
interface  0
ip address 2001:80f0:4:300::/64 eui-64
no  nd suppress-ra
 source 172.16.1.20
 mode ipv6ip isatap

The prefix assigned to the interface is the prefix that will be assigned to hosts. Notice the no nd suppress-ra command is required to ensure that router advertisements are sent over the to hosts. By default, these messages are not used on interfaces.

After setting the mode, your interface should launch. To verify that your has been assigned the appropriate ISATAP address space, you can use show interface brief as follows:

RouterA#show  interface brief
FastEthernet0/0            [up/up]
FastEthernet0/1            [administratively down/down]
Tunnel0                    [up/up]
 FE80::5EFE:AC10:114
 2001:80F0:4:300:0:5EFE:AC10:114

Awesome! We will investigate another transition option in the next part of this series. Thanks for tuning in! If you want more training targeted at this subject, check out any workbook practice, while Tier 3 would be Poly-labs or Graded Mock Labs.

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