Home > Articles > Cisco Certification > CCNP > CCNP ISCW Exam Guide: GRE Tunneling over Ipsec

CCNP ISCW Exam Guide: GRE Tunneling over Ipsec

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

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.

Foundation Summary

The generic characteristics of a GRE tunnel are as follows:

  • A GRE tunnel is similar to an IPsec tunnel because the original packet is wrapped inside an outer shell.
  • GRE is stateless and offers no flow control mechanisms.
  • GRE adds at least 24 bytes of overhead, including the new 20-byte IP header.
  • GRE is multiprotocol and can tunnel any OSI Layer 3 protocol.
  • GRE permits routing protocols to travel through the tunnel.
  • GRE was needed to carry IP multicast traffic until 12.4(4)T.
  • GRE has relatively weak security features.

Table 14-3 describes the GRE header options.

Table 14-3. GRE Options

GRE Header Bit




Checksum Present

Adds a 4-byte checksum field to the GRE header after the protocol field if this bit is set to 1.


Key Present

Adds a 4-byte encryption key to the GRE header after the checksum field if this bit is set to 1.


Sequence Number Present

Adds a 4-byte sequence number to the GRE header after the key field if this bit is set to 1.


GRE Version

0 indicates basic GRE, while 1 is used for PPTP.

The basic configuration components of a GRE tunnel include

  • A tunnel source (an interface or IP address local to this router)
  • A tunnel destination (an IP address of a remote router)
  • A tunnel mode (GRE/IP is the default)
  • Tunnel traffic (data that travels through the tunnel, and is encapsulated by the GRE header)

GRE over IPsec uses the GRE tunnel to carry dynamic IP routing protocols, and uses IPsec to enforce confidentiality and integrity.

GRE over IPsec using tunnel mode has a total of three IP headers in the packet. GRE over IPsec using transport mode has only two IP headers in the packet.

Most GRE over IPsec implementations use a hub-and-spoke design to limit the number of IPsec tunnels required to secure the entire network.

The Secure GRE Wizard is accessed as follows:

  1. Step 1 Click the Configure button at the top of the window.
  2. Step 2 Click the VPN button in the Tasks bar on the left.
  3. Step 3 Click the Site-to-Site VPN option at the top of the menu.
  4. Step 4 Click the Create Site to Site VPN tab in the window.
  5. Step 5 Click the Create a secure GRE tunnel (GRE over IPSec) radio button.
  6. Step 6 Click the Launch the selected task button at the bottom of the window.

The basic steps of the Secure GRE Wizard include

  1. Step 1 Create the GRE tunnel.
  2. Step 2 Create a backup GRE tunnel (optional).
  3. Step 3 Select the IPsec VPN authentication method.
  4. Step 4 Select the IPsec VPN IKE proposals.
  5. Step 5 Select the IPsec VPN transform sets.
  6. Step 6 Select the routing method for the GRE over IPsec tunnel.
  7. Step 7 Validate the GRE over IPsec configuration.

The GRE Tunnel Information window is where the GRE tunnel is configured in SDM. Configuration includes

  • Tunnel source (local interface or IP address)
  • Tunnel destination (remote IP address)
  • Interior tunnel IP address and subnet mask
  • Optional MTU path discovery to know if fragmentation must be performed on this router due to the larger packet size created by GRE over IPsec

The Backup GRE Tunnel Information window is where the backup GRE tunnel is configured in SDM. The backup GRE tunnel uses the same source as the primary GRE tunnel. Configuration includes

  • Enable the backup tunnel
  • Tunnel destination (remote IP address)
  • Interior tunnel IP address and subnet mask

The IPsec VPN configuration has three phases, all of which are identical to those found in the site-to-site IPsec VPN configuration process:

  1. VPN authentication
  2. IKE proposals
  3. IPsec transform sets

There are four routing options supported within the GRE tunnel:

  • OSPF
  • RIP
  • Static routing

Static routing can configure only one subnet and is not appropriate for sites with multiple subnets or for sites using two GRE tunnels.

RIP cannot be configured if a backup GRE tunnel is configured.

Both OSPF and EIGRP use inverse masks when adding subnets to the routing protocol.

Be sure to include the internal IP subnet of the GRE tunnel in the routing protocol configuration so that the configured protocol will use the GRE tunnel interface.

The Configuration Summary window allows you to view the configuration just created with the wizard. You can return to the wizard by clicking the <Back button to make changes, or you can finish the wizard by clicking the Finish button.

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020