Using ISATAP for Intra-Site IPv6 Connectivity Over IPv4 Core

300-420 By islever Editorial Last updated

Determine IPv6 migration strategies Describe SD-Access Architecture (underlay, overlay, control and data plane, automation, wireless, and security)

Short answer

A The architect must use ISATAP to provide connectivity between the mail servers, as it is the designated intra-site tunneling protocol for carrying IPv6 traffic over an IPv4-only core within a single administrative domain.

Question

Exhibit: Using ISATAP for Intra-Site IPv6 Connectivity Over IPv4 Core
Refer to the exhibit. Which method must an architect use to provide connectivity between the mail servers?
  1. ISATAP Correct Answer
  2. 6to4
  3. 6rd
  4. IPv4 compatible

Key Takeaways

The main trap is confusing ISATAP (intra-site) with 6to4 (inter-site); the question tests your ability to identify whether the topology represents a single administrative domain requiring NBMA-like behavior rather than distinct site-to-site links.

This page explains why ISATAP is the correct method for connecting IPv6 mail servers across an IPv4-only core in a single site scenario for the 300-420 exam.

Common mistake: Candidates often choose 6to4 because they assume 'mail servers' implies separate geographic sites. However, 6to4 requires public IPv4 addresses and is intended for inter-site connections, whereas ISATAP is designed for internal, single-site IPv6 deployment over IPv4 infrastructure.

Expert Analysis

The exhibit depicts two IPv6 mail servers connected via dual-stack edge routers across an IPv4-only core network. The critical distinction between the options lies in the scope of the tunneling mechanism: intra-site versus inter-site.

ISATAP (Intra-Site Automatic Tunnel Addressing Protocol) is designed specifically for intra-site communication. It treats the existing IPv4 infrastructure as a Non-Broadcast Multi-Access (NBMA) link, allowing IPv6 nodes to communicate within the same administrative domain without requiring manual tunnel configuration or distinct site prefixes. As noted in community discussions, ISATAP is the standard Cisco-recommended approach for connecting IPv6 islands within a single enterprise site over an IPv4 backbone. The diagram implies a single administrative context (mail servers), making ISATAP the appropriate choice for seamless, automated connectivity.

Conversely, 6to4 is an inter-site transition mechanism. It relies on a specific 2002::/16 prefix derived from the border router’s public IPv4 address to establish point-to-multipoint tunnels between geographically dispersed sites. While some voters argue that mail servers imply different sites, the exhibit shows them under a unified routing structure typical of a single campus design where ISATAP simplifies management by avoiding the need for global IPv4 addresses and static 6to4 configurations. 6to4 is generally used when connecting separate, distinct sites, not internal segments of the same site.

6rd (Rapid Deployment) is typically used by ISPs to deliver IPv6 to customers over an IPv4 broadband network, involving stateless delegation, which does not fit this enterprise server connectivity scenario. IPv4-compatible addresses (::/96) are deprecated legacy mechanisms and are not recommended for modern designs.

Therefore, because the topology represents an intra-site IPv6 overlay on an IPv4 underlay, ISATAP is the correct architectural decision.

Official Reference

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/configuration/xe-16-12/ir-xe-16-12-book/ip6-isatap-xe.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When analyzing IPv6 transition technologies, always check the scope indicator: 'Intra-' vs 'Inter-'. For 300-420 questions showing multiple IPv6 nodes behind a single IPv4 cloud or within one campus, prioritize ISATAP for its ease of use and lack of manual tunnel configuration. Reserve 6to4 and Teredo for scenarios explicitly describing separate, distant sites or public internet traversal.

Frequently Asked Questions

Why is ISATAP preferred over 6to4 for this scenario?

ISATAP is designed for intra-site communication, treating the IPv4 network as an NBMA link. It does not require public IPv4 addresses or manual tunnel setup, making it ideal for connecting internal servers like mail hosts within a single enterprise site.

Can 6to4 be used for connecting two mail servers in the same building?

Technically yes, but it is architecturally incorrect for intra-site designs. 6to4 is an inter-site mechanism relying on the 2002::/16 prefix and public IPv4 addresses, adding unnecessary complexity compared to ISATAP for local connectivity.

More 300-420 FAQ →

Community Votes

A
50%
B
42%
C
8%

50% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 14 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 14

  1. SpicyMochi 👍 6 Answered B
    B. 6to4 In this scenario, an architect should use the 6to4 method to provide connectivity between the mail servers. 6to4 is an IPv6 transition mechanism that allows IPv6 hosts to communicate with each other over an IPv4 network using a dual-stack router. The dual-stack routers automatically encapsulate IPv6 packets inside IPv4 packets for transmission over the IPv4 network and then decapsulate them when they reach the destination IPv6 network.
  2. Cleiton11 👍 3
    B. 6to4
  3. Lungful 👍 2 Answered B
    B is correct. 6to4 is for tunneling across a IPv4 network to connect two IPv6 networks while ISATAP is for connecting stand alone IPv4 devices with IPv6 devices and vice versa. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/configuration/xe-16-12/ir-xe-16-12-book/ip6-isatap-xe.html https://www.admin-magazine.com/Archive/2013/13/IPv6-tunnel-technologies/(offset)/6
  4. Seb82 👍 2
    if they would have put two PCs/clients, it would have been logic it's the same site, but they used mail servers. Logic says that the mail servers would be in two different sites, case in which 6to4 should be used.
  5. musclehamster 👍 2 Answered A
    All comes down to rather this is a single site or if the mail servers are at different sites. Same site - ISATAP Different sites - 6to4 Looks like the same site by the diagram. I am not sure though.
Show 9 more comments
  1. 314_pi 👍 2 Answered A
    A is correct https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/configuration/xe-16-12/ir-xe-16-12-book/ip6-isatap-xe.html
  2. NoHombre 👍 1 Answered A
    The figure shows two IPv6 islands (mail servers) separated by an IPv4-only core. The edge routers on both sides are dual-stack. Inside an enterprise, the standards-based way to carry IPv6 across an IPv4 intranet without touching the L3 core is ISATAP (Intra-Site Automatic Tunnel Addressing Protocol). ISATAP treats the IPv4 network as an NBMA IPv6 link, automatically building IPv6-over-IPv4 (proto 41) tunnels using ISATAP addresses (…:0:5EFE:w.x.y.z). It’s designed exactly for intra-site IPv6 connectivity across an IPv4 core.
  3. neiker45 👍 1 Answered B
    https://community.cisco.com/t5/networking-knowledge-base/ipv6-isatap-configuration-example/ta-p/3152483 "he main difference between automatic 6to4 tunnels and ISATAP tunnels is that the automatic 6to4 is Inter-site tunnel that allows IPv6 traffic between different sites where as ISATAP as the name specifies is for Intra-site which can be used for transporting IPv6 packets within a site, but not between sites."
  4. G_Singh12 👍 1 Answered B
    he exhibit explicitly displays two separate, isolated IPv6 routing "islands" trying to pass entire networks of traffic across an IPv4 backbone. 6to4 automatically builds a point-to-multipoint overlay connecting these dual-stack border routers. Whereas, ISATAP is designed exclusively for Intra-Site (Within a site) communication
  5. rickyarchi 👍 1 Answered C
    IPv6 6RD (Rapid Deployment) is an IPv6 tunneling technique, similar to 6to4 tunneling. It is stateless and encapsulates IPv6 packets into IPv4 packets. ISATAP and 6to4 are considered legacy nowadays.
  6. mgiuseppe86 👍 1 Answered A
    Based on https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/configuration/xe-16-12/ir-xe-16-12-book/ip6-isatap-xe.html which is basically the same diagram, I am going with A.
  7. J2J2J2J 👍 1 Answered A
    ISATAP is an automatic overlay tunneling mechanism that uses the underlying IPv4 network as a NBMA link layer for IPv6
  8. Clauster 👍 1 Answered A
    This is ISATAP 100% straight from OCG Book
  9. salmarin 👍 1 Answered B
    because the middle routers are dual stack

Reader comments & corrections

No reader corrections yet — spotted an error or have a note? Share it below.

Log in to comment, report an error, or add a note about this question.

POV Report Builds the Customer Platform During Discovery

S-grade

VPLS design for sites sharing one IP subnet

S-grade

Inter-VN Communication in Cisco SD-WAN Architecture

S-grade

Advertising a Default Route from Core to Aggregation to Hide Instability

S-grade

Practice All 300-420 Questions

Access 150 questions with complete answers and detailed explanations.

View Full 300-420 Practice Test →

← Back to 300-420 Study Guide