Home > Articles > Networking > Routing & Switching

This chapter is from the book

This chapter is from the book

Routing and Backbone Label Forwarding Design

All networks, whether they span whole continents or just a group of geographic regions, present design challenges that must be addressed by the network architects. Some issues are easier to tackle than others, and certain services present unique challenges. This section reviews how USCom decided to deploy its internal and external IP routing, and also how it decided to organize its Layer 3 MPLS VPN service.

We have established that USCom operates a national backbone infrastructure that spans the continental U.S. This network must support a number of different services, including Internet access and Layer 3 VPN service. During the initial Layer 3 VPN deployment, USCom decided to deploy MPLS technology to support the architecture specified in [2547bis]. This architecture provides a network-based VPN service. It was discussed in detail in Chapter 1, "Technology Primer: Layer 3 VPN, Multicast VPNs, IPv6, and Psuedowire."

Having deployed MPLS for this service, USCom also felt that it was the right technology to support fast rerouting (FRR) capability (which you'll read about in the "Network Recovery Design for Link Failures" section). Clearly, the network will need to support even more new services in the future, so USCom's selection of MPLS as its primary technology allows the company to support existing and future service requirements.

Label Distribution Protocol (LDP) is used within the backbone to allow label switching from one edge of the USCom network to the other. However, at this point in time, only the Layer 3 VPN traffic is label-switched, leaving the Internet traffic to be forwarded by normal IP forwarding procedures. The rationale behind the decision to separate VPN forwarding from standard IP forwarding was driven primarily by the desire to continue operating the Internet network in the exact same way as before Layer 3 VPN services were introduced. This avoided any changes in configuration, monitoring, troubleshooting, or any other operational procedures that were in place for Internet traffic. In addition to this, a number of technical challenges exist if Internet traffic is label-switched, including how the existing IP tools (such as NetFlow) might behave, and how network events such as denial of service (DoS) attacks can be tracked and resolved. Chapter 5, "Global Service Provider Design Study," shows how these issues can be overcome and the USCom plan to introduce these new technologies in the future.

From an internal routing perspective, USCom runs Intermediate System–to-Intermediate System (IS-IS) as its Interior Gateway Protocol (IGP), which carries the loopback interface addresses of the PE routers (IP and VPN PE routers) as well as internal link addresses. The number of internal routes is approximately 3000. USCom does not expect to have more than 1000 routers in the IS-IS routing domain within the next two years. Hence, the IS-IS network is a flat Level 2 network that avoids having to manage the complexity of multiple levels of hierarchy.

USCom measured that the flooding activity on the existing network was perfectly reasonable. The Shortest Path First (SPF) computation time was calculated on the order of 100 ms (usually closer to 60 ms), not including the routing table updates. If at some point in the future the number of IS-IS routers has to be drastically increased because of the activation of IS-IS on various edge devices such as the ADSL or Dial access routers, USCom might consider splitting the network into multiple levels (each POP would be the Level 1 hierarchy). This would be necessary to also preserve the network convergence times. (A detailed analysis of these aspects appears in [NET-RECOV].)

Separation of Internet and Layer 3 MPLS VPN Services

From a forwarding perspective, Layer 3 VPN traffic is separated from Internet traffic, where VPN traffic is label-switched across the USCom network and Internet traffic is IP-routed/forwarded. The PE routers serving VPN and Internet customers are also separate. This is primarily because the Internet service has been deployed for a number of years and USCom wanted to deploy the new Layer 3 MPLS VPN service as a separate project, without concern that it might affect the existing customer base.

The backbone network infrastructure is addressed from the 23.49.0.0/16 block. This includes all P routers, PE routers (whether Internet or Layer 3 VPN), and any other equipment within the USCom network. The P router and core-facing interfaces on the Internet and Layer 3 VPN PE routers take their addresses from the 23.49.0.0/21 range (providing IP addresses 23.49.0.1–23.49.7.254).

The Internet PE routers and IPv4 route reflectors (RRs) take their loopback interface addresses from the 23.49.8.0/22 range (providing IP addresses 23.49.8.1–23.49.11.254).

The Layer 3 MPLS VPN PE routers and VPNv4 RRs (used for the MPLS VPN service) take their loopback interface addresses from the 23.49.16.0/22 range (providing IP addresses 23.49.16.1–23.49.19.254). This block is large enough to address 1022 devices. If the service increases above this amount, the 23.49.20.0/22 range is made available.

Each Layer 3 MPLS VPN PE router has a loopback interface configured; it is used as the source address for all Multiprotocol BGP (MP-BGP) peering sessions. Likewise, each Internet access PE router has a loopback interface assigned; it is used as the source address for all IPv4 BGP-4 peering sessions.

USCom also evaluated using one of the private IP address blocks from the [PRIVATE] range for its internal infrastructure. The use of private addresses provides some protection from the Internet because it is not a routable address space. Therefore, the internal USCom network would theoretically be hidden from the outside. However, locally attached customers could still access the network—for example, by sending traffic via a default route to USCom. Therefore, the advantages of using private address space are mitigated. Also, a future acquisition of another company might present some integration challenges, so the use of private addresses for the design was rejected.

Because the Internet PE routers and Layer 3 MPLS VPN PE routers are separate, and because USCom chose to forward only VPN traffic through label switching, forwarding separation needs to occur at the LDP level. The default behavior of the LDP protocol when executing in frame-based mode is to create and distribute label bindings for every IGP learned or local (static or connected) prefix. This is unnecessary in the USCom network because only the VPN traffic is to be label-switched, and all Internet traffic is to be routed and will never need any of the allocated label space. Therefore, only the MPLS VPN PE router loopback interface addresses (255 currently) require label bindings, because they are the only destinations to which traffic is forwarded through label switching. Example 3-1 shows how LDP filtering is achieved.

Example 3-1 Filtering Label Binding for PE Router Loopback Interfaces

no tag-switching advertise-tags
tag-switching advertise-tags for ldp-pe-filter
!
ip access-list standard ldp-pe-filter
 ! Main IP VPN PE-router loopbacks
 permit 23.49.16.0 0.0.3.255
 ! Reserved IP VPN PE-router loopback block
 permit 23.49.20.0 0.0.3.255

In the future, USCom may also decide to label-switch its Internet traffic. This may be achieved by either removing the LDP filtering (the configuration of which is shown in Example 3-1) or updating the LDP filter to include the Internet PE router loopback interface addresses.

Internet Service Route Reflection Deployment

The USCom RR design for Internet service is fairly typical. It follows the network's physical topology (for loop avoidance), as shown in Figure 3-7. (Only core POPs with external peering points are shown in the figure even though the design is relevant to all Level 1 POPs.) Each Level 1 POP has two Internet RRs (the backbone P routers). All Internet PE routers peer locally and are clients of these devices. All Level 1 POP RRs are fully meshed at the BGP-4 level. The aggregation P routers are also clients of these RRs.

Figure 7

Figure 3-7 Placement of IPv4 Route Reflectors for Internet Service

A second level of RR hierarchy is deployed between the Level 1 and Level 2 POPs. Each Level 2 POP has two RRs (which again are the exiting backbone P routers); these are clients of their nearest Level 1 POP RRs. Every Internet PE router within a Level 2 POP is a client of the local Level 2 RRs. Each Level 3 POP Internet PE router and backbone P router peers with its nearest Level 2 POP RRs (once again following the network's physical topology).

Figure 3-8 shows how the IPv4 BGP peerings are arranged between different levels of POPs and the placement of the RRs within those POPs. Note that this figure provides the typical topology, although in some cases the Level 2 POP RRs may peer with different Level 1 POPs. (In other words, one RR peers with a different Level 1 POP than the other RR within the Level 2 POP.) This depends on the RR's geographic location in the overall topology.

Figure 8

Figure 3-8 IPv4 POP-to-POP BGP Route Reflection

The global IPv4 BGP table currently contains approximately 155,000 Internet routes.

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