Home > Articles > BGP Fundamentals

BGP Fundamentals

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jan 1, 2018.

Chapter Description

In This sample chapter from Troubleshooting BGP: A Practical Guide to Understanding and Troubleshooting BGP, the authors cover BGP Messages and Inter-Router Communication, Basic BGP Configuration for IOS, IOS XR, and NX-OS, IBGP Rules, EBGP Rules, and BGP Route Aggregation

The following topics are covered in this chapter:

  • BGP Messages and Inter-Router Communication

  • Basic BGP Configuration for IOS, IOS XR, and NX-OS

  • IBGP Rules

  • EBGP Rules

  • BGP Route Aggregation

A router’s primary function is to move packets from one network to a different network. A router learns about unattached networks through static configuration or through dynamic routing protocols that distribute network topology information between routers. Routers try to select the best loop-free path in a network based on the destination network. Link flaps, router crashes, and other unexpected events could impact the best path, so the routers must exchange information with each other so that the network topology updates during these types of events.

Routing protocols are classified as either an Interior Gateway Protocol (IGP) or an Exterior Gateway Protocol (EGP), which indicates whether the protocol is designed for exchanging routes within an organization or between organizations. In IGP protocols, all routers use a common logic within the routing domain to find the shortest path to reach a destination. EGP protocols may require a unique routing policy for every external organization that it exchanges routes.

Border Gateway Protocol

RFC 1654 defines Border Gateway Protocol (BGP) as an EGP standardized path-vector routing protocol that provides scalability, flexibility, and network stability. When BGP was created, the primary design consideration was for IPv4 inter-organization connectivity on public networks, such as the Internet, or private dedicated networks. BGP is the only protocol used to exchange networks on the Internet, which has more than 600,000 IPv4 routes and continues to grow. BGP does not advertise incremental updates or refresh network advertisements like OSPF or ISIS. BGP prefers stability within the network, because a link flap could result in route computation for thousands of routes.

From the perspective of BGP, an autonomous system (AS) is a collection of routers under a single organization’s control, using one or more IGPs, and common metrics to route packets within the AS. If multiple IGPs or metrics are used within an AS, the AS must appear consistent to external ASs in routing policy. An IGP is not required within an AS, and could use BGP as the only routing protocol in it, too.

Autonomous System Numbers

Organizations requiring connectivity to the Internet must obtain an Autonomous System Number (ASN). ASNs were originally 2 bytes (16 bit) providing 65,535 ASNs. Due to exhaustion, RFC 4893 expands the ASN field to accommodate 4 bytes (32 bit). This allows for 4,294,967,295 unique ASNs, providing quite a leap from the original 65,535 ASNs.

Two blocks of private ASNs are available for any organization to use as long as they are never exchanged publicly on the Internet. ASNs 64,512–65,535 are private ASNs within the 16-bit ASN range, and 4,200,000,000–4,294,967,294 are private ASNs within the extended 32-bit range.

The Internet Assigned Numbers Authority (IANA) is responsible for assigning all public ASNs to ensure that they are globally unique. IANA requires the following items when requesting a public ASN:

  • Proof of a publicly allocated network range

  • Proof that Internet connectivity is provided through multiple connections

  • Need for a unique route policy from your providers

In the event that an organization does not meet those guidelines, it should use the ASN provided by its service provider.

Path Attributes

BGP attaches path attributes (PA) associated with each network path. The PAs provide BGP with granularity and control of routing policies within BGP. The BGP prefix PAs are classified as follows:

  • Well-known mandatory

  • Well-known discretionary

  • Optional transitive

  • Optional nontransitive

Per RFC 4271, well-known attributes must be recognized by all BGP implementations. Well-known mandatory attributes must be included with every prefix advertisement, whereas well-known discretionary attributes may or may not be included with the prefix advertisement.

Optional attributes do not have to be recognized by all BGP implementations. Optional attributes can be set so that they are transitive and stay with the route advertisement from AS to AS. Other PAs are nontransitive and cannot be shared from AS to AS. In BGP, the Network Layer Reachability Information (NLRI) is the routing update that consists of the network prefix, prefix length, and any BGP PAs for that specific route.

Loop Prevention

BGP is a path vector routing protocol and does not contain a complete topology of the network-like link state routing protocols. BGP behaves similar to distance vector protocols to ensure a path is loop free.

The BGP attribute AS_PATH is a well-known mandatory attribute and includes a complete listing of all the ASNs that the prefix advertisement has traversed from its source AS. The AS_PATH is used as a loop prevention mechanism in the BGP protocol. If a BGP router receives a prefix advertisement with its AS listed in the AS_PATH, it discards the prefix because the router thinks the advertisement forms a loop.

Address Families

Originally, BGP was intended for routing of IPv4 prefixes between organizations, but RFC 2858 added Multi-Protocol BGP (MP-BGP) capability by adding extensions called address-family identifier (AFI). An address-family correlates to a specific network protocol, such as IPv4, IPv6, and the like, and additional granularity through a subsequent address-family identifier (SAFI), such as unicast and multicast. MBGP achieves this separation by using the BGP path attributes (PAs) MP_REACH_NLRI and MP_UNREACH_NLRI. These attributes are carried inside BGP update messages and are used to carry network reachability information for different address families.

Network engineers and vendors continue to add functionality and feature enhancements to BGP. BGP now provides a scalable control plane for signaling for overlay technologies like MPLS VPNs, IPsec Security Associations, and Virtual Extensible LAN (VXLAN). These overlays can provide Layer 3 connectivity via MPLS L3VPNs, or Layer 2 connectivity via MPLS L2VPNs (L2VPN), such as Virtual Private LAN Service (VPLS) or Ethernet VPNs (EVPNs).

Every address-family maintains a separate database and configuration for each protocol (address-family + subaddress family) in BGP. This allows for a routing policy in one address-family to be different from a routing policy in a different address family even though the router uses the same BGP session to the other router. BGP includes an AFI and a SAFI with every route advertisement to differentiate between the AFI and SAFI databases. Table 1-1 provides a small list of common AFI and SAFIs.

Table 1-1 Common BGP Address Families and Subaddress Families

AFI

SAFI

Network Layer Information

1

1

IPv4 Unicast

1

2

IPv4 Multicast

1

4

IPv4 Unicast with MPLS Label

1

128

MPLS L3VPN IPv4

2

1

IPv6 Unicast

2

4

IPv6 Unicast with MPLS Label

2

128

MPLS L3VPN IPv6

25

65

Virtual Private LAN Service (VPLS)

Virtual Private Wire Service (VPWS)

25

70

Ethernet VPN (EVPN)

BGP Sessions

A BGP session refers to the established adjacency between two BGP routers. BGP sessions are always point-to-point and are categorized into two types:

  • Internal BGP (IBGP): Sessions established with an IBGP router that are in the same AS or participate in the same BGP confederation. IBGP sessions are considered more secure, and some of BGP’s security measures are lowered in comparison to EBGP sessions. IBGP prefixes are assigned an administrative distance (AD) of 200 upon installing into the router’s routing information base (RIB).

  • External BPG (EBGP): Sessions established with a BGP router that are in a different AS. EBGP prefixes are assigned an AD of 20 upon installing into the router’s RIB.

2. Inter-Router Communication | Next Section

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.

Overview

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.

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@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.

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 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.

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