Home > Articles > Certification > Cisco Certification > CCNP Security / CCSP

Unicast Reverse Path Forwarding Concepts and Configuration

  • Print
  • + Share This
When connected to public networks, one common method to initiate an attack is to utilize IP source address spoofing. When using this method, the hacker attempts to send traffic into the network with a source address that is known or trusted by the target. If no protection exists, the organizational network will allow the traffic and potentially open themselves up to a number of different attack types. This article goes over the Unicast Reverse Path Forwarding (Unicast RPF) feature and how it can be configured to deal with these issues.
Like this article? We recommend

When connected to public networks, one common method to initiate an attack is to utilize IP source address spoofing. When using this method, the hacker attempts to send traffic into the network with a source address that is known or trusted by the target. If no protection exists, the organizational network will allow the traffic and potentially be open to a number of different attack types. This article explores the Unicast Reverse Path Forwarding (Unicast RPF) feature and how it can be configured to deal with these issues.

Unicast Reverse Path Forwarding Concepts

In the past, the best solution for these problems was to build a set of access lists that would manually be able to block traffic that was coming in from an external interface but sourced from an IP address that existed within the internal network. When dealing with only a small network, this configuration is typically not that big of a problem, as the list of IP addresses to guard against can be rather short and relatively easy to maintain. However, when dealing with a larger organization, the maintenance needed to keep up these access lists (ACLs) with the ongoing allocation of addresses within the organization is time-absorbing. The other problem with ACLs is that they limit the rate in which traffic could be forwarded, which can greatly affect network performance.

To deal with this in a way that solved these problems and required only a small amount of maintenance, the Unicast RPF feature was developed. When using the Unicast RPF feature, all traffic that comes into a configured interface is checked to ensure that the interface that would be used to route traffic back to the source address is the same interface that was used to receive the traffic. This feature performs the same function as the previously-configured ACLs did, with considerably less configuration and hassle. The one caveat of using Unicast RPF is that it only permits traffic whose return path (source interface and address) matches the best reverse path (symmetric routing), thus it does not work well when multiple connections (multi-homing) exist. To deal with this problem, a second version of Unicast RPF was later developed called “loose mode”; when using this mode only the address is matched against the FIB allowing the traffic to be received on any interface. This mode does not provide as much protection as the original “strict mode” but is more effective than having no protection.

The main prerequisite of Unicast RPF is that it relies on the Forwarding Information Base (FIB) that is generated by the Cisco Express Forwarding (CEF) feature; because of this, CEF must be enabled before Unicast RPF.

It is also a good idea to know in what order traffic is processed within a device; Figure 1 below shows this order.

Unicast Reverse Path Forwarding Configuration

The configuration of the Unicast RPF feature is very simple and is only required to be configured on the incoming interface. The steps below detail how the Unicast RPF feature is configured:

1

Enter global configuration mode.

router#configure terminal

2

Enable Cisco Express Forwarding (if it is not already enabled by default).

router(config)#ip cef [distributed]

3

Enter interface configuration mode for the inbound interface.

router(config)#interface interface-id

4

Enable the Unicast RPF feature (“strict mode”) on the interface. When an access list is specified, further customization is possible; access list permit statements allow traffic to be forwarded even if they fail the Unicast RPF check, access list deny statements will drop traffic matched that fail the Unicast RPF check.

router(config-if)#ip verify unicast reverse-path [access-list-number]

5

Exit configuration mode.

router(config-if)#end

The Unicast RPF “loose mode” configuration is rather similar but requires a different command; the steps to configure Unicast RPF “loose mode” are shown below:

1

Enter global configuration mode.

router#configure terminal

2

Enable Cisco Express Forwarding (if it is not already enabled by default).

router(config)#ip cef [distributed]

3

Enter interface configuration mode for the inbound interface.

router(config)#interface interface-id

4

Enable the Unicast RPF feature (“loose mode”) on the interface. When an access list is specified, further customization is possible; access list permit statements allow traffic to be forwarded even if they fail the Unicast RPF check, access list deny statements will drop traffic matched that fail the Unicast RPF check.

router(config-if)# ip verify unicast source reachable-via any [access-list-number]

5

Exit configuration mode.

router(config-if)#end

Unicast Reverse Path Forwarding Configuration Example

The functionality of this feature can be a little hard to understand, so I’ve included an example that utilizes the functionality of Unicast RPF. Figure 2 shows the topology of the example that we’ll use:

As shown in this example, the correct path from the 10.10.10.0 network to the 20.20.20.0 network is through R2 and R4. If Unicast RPF (“Strict mode”) was configured on R1’s F0/0 and F0/1 interfaces, traffic to and from the 10.10.10.0 and 20.20.20.0 network would pass fine, as long as it was received on the F0/0 interface. If an attacker attempted to send traffic to the 10.10.10.0 network through R3 using a source address of 20.20.20.10 without the Unicast RPF feature enabled, traffic could pass through and reach the destination. With the Unicast RPF feature enabled, the device (in this case R1) will check if the “best” return path is using the F0/1 interface where the traffic was received; when the “best” return path is shown to be through the F0/0 interface the Unicast RPF check will fail and the traffic will be dropped.

Using this same example, if the Unicast RPF (“Loose mode”) was configured, traffic would be allowed onto the 10.10.10.0 network as the 20.20.20.0 network is in the CEF FIB as a reachable network and the source interface would not be checked.

Summary

The best situation to use the Unicast RPF feature is when a site only has a single path out of the network; this way all of the functionality of the “strict mode” can be utilized. However, in many ISP networks, it is common for networks to have multiple paths out of the network. Before the “loose mode” was developed, the Unicast RPF feature was unusable; with its development, some of the functionality can be used in these situations. The Unicast RPF feature is unique from other features in that its configuration is very simple but the concept can be hard to understand and be implemented correctly without affecting valid traffic; many other features are easy to understand in concept but are hard to configure. Hopefully this article has been able to shed some light on this feature and how it can be used on modern networks.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

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

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about 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.

Surveys

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.

Newsletters

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

Security


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

Children


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

Marketing


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.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. 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.informit.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.

Links


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