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CCNP ISCW Exam Guide: GRE Tunneling over Ipsec

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 24, 2008.


  1. "Do I Know This Already?" Quiz
  2. Foundation Topics
  3. Foundation Summary
  4. Q&A

Chapter Description

The growth of IPsec saw a rebirth in the use of GRE in IP networks. This chapter talks about the use of GRE in an IPsec environment and how that relates to the CCNP ISCW exam.

Exam Topic List

This chapter covers the following topics that you need to master for the CCNP ISCW exam:

  • GRE Characteristics—Describes how generic routing encapsulation (GRE) can be used to encapsulate virtually any routed or routing protocol through an IP network
  • GRE Header—Describes the GRE header that defines what is carried inside the GRE tunnel
  • Basic GRE Configuration—Describes how to define the tunnel source, destination, mode, and contents
  • Secure GRE Tunnels—Describes how GRE and IPsec complement each other across the network
  • Configure GRE over IPsec Using SDM—Describes how SDM wizards permit easy configuration of GRE over IPsec

Generic routing encapsulation (GRE) tunnels have been around for quite some time. GRE was first developed by Cisco as a means to carry other routed protocols across a predominantly IP network. Some network administrators tried to reduce the administrative overhead in the core of their networks by removing all protocols except IP as a transport. As such, non-IP protocols such as IPX and AppleTalk were tunneled through the IP core via GRE.

GRE adds a new GRE header to the existing packet. This concept is similar to IPsec tunnel mode. The original packet is carried through the IP network, and only the new outer header is used for forwarding. Once the GRE packet reaches the end of the GRE tunnel, the external header is removed, and the internal packet is again exposed.

Today, multiprotocol networks have mostly disappeared. It is difficult to find traces of the various protocols that used to be abundant throughout enterprise and core infrastructures. In a pure IP network, GRE was initially seen as a useless legacy protocol. But the growth of IPsec saw a rebirth in the use of GRE in IP networks. This chapter talks about the use of GRE in an IPsec environment.

"Do I Know This Already?" Quiz

The purpose of the "Do I Know This Already?" quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.

The 15-question quiz, derived from the major sections in the "Foundation Topics" portion of the chapter, helps you to determine how to spend your limited study time.

Table 14-1 outlines the major topics discussed in this chapter and the "Do I Know This Already?" quiz questions that correspond to those topics.

Table 14-1. "Do I Know This Already?" Foundation Topics Section-to-Question Mapping

Foundation Topics Section

Questions Covered in This Section


GRE Characteristics


GRE Header


Basic GRE Configuration


Secure GRE Tunnels


Configure GRE over IPsec Using SDM


Total Score

  1. What is the minimum amount of additional header that GRE adds to a packet?

    1. 16 bytes
    2. 20 bytes
    3. 24 bytes
    4. 36 bytes
    5. 48 bytes
  2. Which of the following are valid options in a GRE header (select all that apply)?

    1. GRE Header Length
    2. Checksum Present
    3. Key Present
    4. External Encryption
    5. Protocol
  3. What is the purpose of a GRE tunnel interface?

    1. It is always the tunnel source interface.
    2. It is always the tunnel destination interface.
    3. It is where the protocol that travels through the tunnel is configured.
    4. It is the interface that maps to the physical tunnel port.
    5. It is not used today.
  4. When IPSec transport mode is used, how many IP headers are found in the GRE over IPsec packet?

    1. One—the original IP header is replicated when needed.
    2. Two—the original IP header and the GRE IP header.
    3. Two—the original IP header and the IPsec IP header.
    4. Three—the original IP header, the GRE IP header, and the IPsec IP header.
    5. Four—the original IP header, the GRE IP header, the IPsec IP header, and the outer IP header.
  5. What feature does GRE introduce that cannot be accomplished with normal IPsec?

    1. GRE increases the packet size so that the minimum packet size is easily met.
    2. GRE adds robust encryption to protect the inner packet.
    3. GRE requires packet sequencing so that out-of-order packets can be reassembled correctly.
    4. GRE adds an additional IP header to further confuse packet-snooping devices.
    5. GRE permits dynamic routing between end sites.
  6. What are the basic components within the Secure GRE Wizard (select all that apply)?

    1. Router interface configuration
    2. GRE tunnel configuration
    3. IPsec parameters configuration
    4. Router authentication configuration
    5. Routing protocols configuration
  7. What is the IP address inside of the GRE tunnel used for?

    1. The GRE tunnel peering point.
    2. The IPsec tunnel peering point.
    3. The routing protocols peering point.
    4. The management interface of the router.
    5. There is no IP address inside of the GRE tunnel.
  8. Which option must be configured if a backup secure GRE tunnel is configured?

    1. Source interface
    2. Source IP address
    3. Destination interface
    4. Destination IP address
    5. Destination router name
  9. What methods are available for VPN authentication when used with a GRE tunnel (select all that apply)?

    1. Digital certificates
    2. Pre-shared keys
    3. Biometrics
    4. OTP
    5. KMA
  10. When creating/selecting an IKE proposal, what does the Priority number indicate?

    1. The Priority number is a sequence number.
    2. The Priority number determines the encryption algorithm.
    3. The Priority number helps determine the authentication method.
    4. The Priority number is related to the Diffie-Hellman group.
    5. The Priority number is necessary to select the hash algorithm.
  11. How are IPsec transform sets used in the Secure GRE Wizard?

    1. There must be a unique IPsec transform set for each VPN peer.
    2. There must be a unique IPsec transform set for each GRE tunnel.
    3. The two ends of a VPN must use the same IPsec transform set.
    4. The same IPsec transform set can be used for all VPN peers.
    5. Site-to-site IPsec VPN transform sets cannot be used for GRE over IPsec VPNs.
  12. Which dynamic routing protocols can be configured in the GRE over IPsec tunnel (select all that apply)?

    1. RIP
    2. OSPF
    3. EIGRP
    4. BGP
    5. Static
  13. Which routing options are appropriate when using both a primary and a backup GRE tunnel (select all that apply)?

    1. RIP
    2. OSPF
    3. EIGRP
    4. BGP
    5. Static
  14. When using OSPF in the GRE over IPsec tunnel, what OSPF parameters must match so that the two peers establish an OSPF adjacency (select all that apply)?

    1. IP address of the GRE tunnel interface
    2. Subnet of the GRE tunnel interface
    3. OSPF area of the GRE tunnel interface
    4. OSPF process ID of each router
    5. Number of networks configured in OSPF on each router
  15. In the Summary of the Configuration window, how can the displayed configuration be modified?

    1. Type changes directly into the scroll window and click the Apply button at the bottom of the window.
    2. Changes cannot be made from within any wizard.
    3. Click the Modify button to return to the configuration windows.
    4. Click the Back button to return to the configuration windows.
    5. Click the Next button to proceed to the Modify Configuration window.

The answers to the "Do I Know This Already?" quiz are found in Appendix A, "Answers to the 'Do I Know This Already?' Quizzes and Q&A Sections." The suggested choices for your next step are as follows:

  • 10 or fewer overall score—Read the entire chapter. This includes the "Foundation Topics," "Foundation Summary," and "Q&A" sections.
  • 11 or 13 overall score—Begin with the "Foundation Summary" section, and then go to the "Q&A" section.
  • 14 or more overall score—If you want more review on these topics, skip to the "Foundation Summary" section, and then go to the "Q&A" section. Otherwise, move to the next chapter.
2. Foundation Topics | Next Section

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