Home > Articles > Networking

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

BGP Peering

What are the mechanics of one BGP speaker peering with another speaker? What substrate protocols does BGP use to transport routing information? This section describes various aspects of BGP peering.

NOTE

While BGP is most often run on routers, which are also responsible for forwarding traffic, in some cases other devices may run BGP as well. Whether to simply gather information about the routing tables being carried in BGP, or to carry routing information between routers. Since this is the case, we will sometimes refer to devices which are running BGP, rather than routers specifically. A device which is running BGP is called a BGP speaker, and two BGP speakers that form a BGP connection for the purpose of exchanging routing information are called BGP peers or neighbors.

BGP Transport

How does BGP carry information about reachable destinations between the devices (routers) running BGP? How is the information encoded when it's transported between peers?

Transporting Data Between Peers

A Transmission Control Protocol (TCP) transport connection is set up between a pair of BGP speakers at the beginning of the peering session, and is maintained throughout the peering session. Using TCP to transport BGP information allows BGP to delegate error control, reliable transport, sequencing, retransmission, and peer aliveness issues to TCP itself, and focus instead on properly processing the routing information exchanged with its peers.

When a BGP speaker first initializes, it uses a local ephemeral TCP port, or random port number greater than 1024, and attempts to contact each configured BGP speaker on TCP port 179 (the well known BGP port). The speaker initiating the session performs an active open, while the peer performs a passive open. It's possible for two speakers to attempt to connect to one another at the same time; this is known as a connection collision. When two speakers collide, each speaker compares the local router ID to the router ID of the colliding neighbor. The BGP speaker with the higher router ID value drops the session on which it is passive, and the BGP speaker with the lower router ID value drops the session on which it is active (i.e., only the session initiated by the BGP speaker with the larger router ID value is preserved).

BGP Routes and Formatting Data

A BGP route is defined as a unit of information that pairs a set of destinations with the attributes of a path to those destinations. The set of destinations is referred to by BGP as the Network Layer Reachability Information (NLRI), and is a set of systems whose IP addresses are represented by one IP prefix.

BGP uses update messages to advertise new routing information, withdraw previously advertised routes, or both. New routing information includes a set a BGP attributes and one or more prefixes with which those attributes are associated. While multiple routes with a common set of attributes can be advertised in a single BGP update message, new routes with different attributes must be advertised in separate update messages.

There are two mechanisms to withdraw routing information in BGP: To withdraw routes explicitly one or more prefixes that are no longer reachable (unfeasible) are included in the withdrawn routes field of an update message (the update message may contain one or more new routes as well). No additional information, such as associated path attributes (e.g., AS Path) is necessary for the routes being withdrawn. Alternatively, because a BGP speaker only advertises a single best route for each reachable destination, a BGP update message that contains a prefix that has already been advertised by the peer, but with a new set of path attributes, serves an implicit withdraw for earlier advertisements of that prefix.

A BGP update message is made up of a series of type-length-vectors (TLVs). Attributes carried within the BGP message provide information about one or more prefixes that follow; attributes are described in the BGP Attributes section later in this chapter.

BGP data, as it's transported between peers, is formatted as shown in Figure 1-5.

Figure 5Figure 1-5: Encoding Information in a BGP Packet

As previously noted, one interesting aspect of this packet format is that while only a single set of attributes may be carried in each update message, many prefixes sharing that common set of attributes may be carried in a single update. This leads to the concept of update packing, which simply means placing two or more prefixes with the same attributes in a single BGP update message.

Interior and Exterior Peering

Beyond the mechanics of building peering relationships, and transporting data between two BGP speakers, there are two types of peering relationships within BGP: interior peering and exterior peering. BGP sessions between peers within a single autonomous system are referred to as interior BGP, or iBGP, sessions, while BGP running between peers in different autonomous system are referred to as exterior BGP, or eBGP sessions.

There are four primary differences between iBGP and eBGP peering relationships:

  • Routes learned from an iBGP peer are not (normally) advertised to other iBGP peers. This prevents routing loops within the autonomous system, as discussed in the section BGP Path Vector Implementation, above.

  • The attributes of paths learned from iBGP peers are not (normally) changed to impact the path selected to reach some outside network. The best path chosen throughout the autonomous system must be consistent to prevent routing loops within the network.

  • The AS Path is not manipulated when advertising a route to an iBGP peer; the local AS is added to the AS Path only when advertising a route to an eBGP peer.

  • The BGP next hop is normally not changed when advertising a route to an iBGP peer; it is always changed to the local peer termination IP address when a route is being advertised to an eBGP peer.

These last two points, the BGP next hop is normally changed when advertising a route to an eBGP peer, while it is left unchanged when advertising a route to an iBGP peer, and the addition of the local autonomous system in the AS Path, are illustrated using Figure 1-6.

Figure 6Figure 1-6: eBGP and iBGP peering

In Figure 1-6, the 10.1.1.0/24 prefix originates on router A with an empty AS Path list and a BGP next hop of router A. Router A then advertises this prefix to router B. Router B, when advertising the route to router C, adds AS65100 to the AS Path list and sets the BGP next hop to 10.1.3.1, because router C is an exterior peer (a peer outside the autonomous system). Router C then advertises the 10.1.1.0/24 prefix to router D without changing the AS Path or the BGP next hop, since router D is an interior peer (a peer within the same autonomous system). Router D will need a path to router B in order to consider this prefix reachable; generally, the BGP next hop reachability information is provided by advertising the link between B and C through an interior gateway protocol, or through iBGP originating the link as a prefix from C into AS65100.

All BGP peers are connected over a TCP transport session. As such, IP reachability must exist before a pair of BGP speakers can peer with one another. For iBGP sessions, reachability between speakers typically is provided using an interior gateway protocol. EBGP peers are normally directly connected over a single hop (across a single link), with no intervening routers, and therefore require no additional underlying routing information. There are mechanisms for connecting eBGP peers across multiple hops; these are covered in more detail in Multipath section of Chapter 7.

Converting an understanding of BGP into practical, running configurations, isn't always as easy at it seems, so we will often provide sample configurations for networks used as examples. These examples will be shown using Cisco IOS Software as the operating system. For the network in Figure 1-5, the following configurations, along with some explanation of the various parts of the configuration, are provided.

!
hostname router-a
!
router BGP 65100
! enables the BGP process and defines the local AS number
 network 10.1.1.0 mask 255.255.255.0
 ! the above line causes router-a to originate the 10.1.1.0/24
 ! prefix in BGP
 neighbor 10.1.2.2 remote-as 65100
 ! configures an iBGP session with router-b

!
hostname router-b
!
router bgp 65100
 ! The number following the router bgp command above is
 ! the local autonomous system number
 neighbor 10.1.2.1 remote-as 65100
 ! configures an iBGP session with router-a
 neighbor 10.1.3.2 remote-as 65200
 ! configures an eBGP session with router-c; note the AS
 ! number in this command does not match the AS number of
 ! the local router

!
hostname router-c
!
router bgp 65200
 neighbor 10.1.3.1 remote-as 65100
 ! configures an eBGP session with router-b; note the AS
 ! number in this command does not match the AS number of
 ! the local router
 neighbor 10.1.4.2 remote-as 65200
 ! configures an iBGP session with router-d
 network 10.1.3.0 mask 255.255.255.0
 ! configures this router to advertise the 10.1.3.0/24 
 ! prefix to router-d, so router-d will be able to reach the
 ! BGP nexthop towards 10.1.1.0/24; reachability could also
 ! be provided through an interior gateway protocol or static 
 ! routing

!
hostname router-d
!
router bgp 65200
 neighbor 10.1.4.1 remote-as 65200
 ! configures an iBGP session with router-c

With these configurations in place, router D should learn the 10.1.1.0/24 prefix from router C, and install it as a reachable destination within its routing table.

BGP Notifications

Throughout the duration of a BGP session between two BGP speakers it's possible that one of the two peers will send some data in error, or send malformed data, or data the other speaker doesn't understand. The easiest remedy in any of these situations is to simply shut the BGP session down, but a simple session shutdown doesn't provide any diagnostic information to the speaker that transmitted the information that triggered the peering session to shut down, and therefore no corrective action can be taken. To provide the information needed to take corrective action, BGP includes Notifications, which should be sent by the BGP speaker closing the session.

Notifications consist of three parts:

  • A notification code

  • A notification subcode

  • A variable length data field

The notification code indicates what type of error occurred:

  • An error occurred in a message header, error code 1

  • An error occurred in the Open message, error code 2

  • An error occurred in an Update message, error code 3

  • The hold timer expired, error code 4

  • An error occurred in the finite state machine, error code 5

  • Cease, error code 6

The subcode provides more information about the error. For instance, where in the Open message the error was. The BGP speaker transmitting the Notification can fill in the data field with information such as the actual part of the Open message causing the error. While the data field is variable in length, there is no length field in the Notification code format. This is because the length of the data field is implied by the length of the complete message.

Message Header Errors

Message header errors generally indicate problems in the packet format. Since TCP is a reliable transport service, message header errors should be very rare, although it is possible for an implementation of BGP to malform a packet, causing this type of error. Three subcodes are defined in the base BGP specification:

  • Connection not synchronized

  • Bad message length

  • Bas message type

Open Message Errors

Notifications transmitted while two BGP peers are opening a session are generally the result of misconfiguration, rather than packet level errors or problems in a BGP implementation.

  • Unsupported version number, which means the BGP peer has transmitted a BGP version this speaker does not support

  • Bad peer autonomous system; the peer has claimed an autonomous system number which isn't valid

  • Bad BGP Identifier; the peer has transmitted a BGP router ID which is invalid

  • Unsupported optional parameter; the peer has indicated it wants to use some optional parameter the receiver doesn't support

  • Authentication failure; the peer is sending packets which are encrypted or authenticated in some way, but the authentication check is failing

  • Unacceptable hold time

Update Message Errors

As BGP peers exchange updates, a number of errors can occur which make it impossible for one speaker to process an update transmitted by the other speaker. These include:

  • Malformed attribute list; the list of attributes included in the update packet has some error which makes it unreadable by the receiver

  • Unrecognized well-known attribute; the sender is including an attribute the receiver must be able to process, but does not recognize

  • Missing well-known attribute; the sender is not including a required well known attribute

  • Attribute flags error; the flags included with an attribute are not formed correctly (generally flags carry various options which apply to the attribute)

  • Attribute length error; an attribute is either too long or too short

  • Invalid Origin; the origin code attribute is set to an invalid value

  • Invalid Next Hop; the Next Hop attribute is set to an invalid value

  • Optional attribute error; an optional attribute is malformed

  • Invalid network field; a prefix included in the update is invalid

  • Malformed AS Path; the AS Path included in the update is invalid

Cease

The Cease code indicates to the receiver that the peer for some reason has chosen to close the BGP connection. The Cease Notification is not sent if a fatal error occurs, but rather, provides a graceful mechanism to shutdown a BGP connection.

BGP Capabilities

There are various extensions to BGP which in order to function correctly, require both BGP speakers in a session to support; how does a BGP speaker know when another BGP speaker it's peering with supports these extensions to BGP? Through BGP capabilities, which are negotiated when a BGP session is started.

NOTE

The ability for one BGP speaker to advertise capabilities to a peer BGP speaker is described in RFC3392, Capabilities Advertisement with BGP-4. draft-ietf-idr-dynamic-cap describes a way in which these capabilities can be advertised dynamically not only on session startup, but after a session is establised.

When first initiating a session, a BGP speaker sends an Open message describing various parameters, including a set of capability codes, one for each optional capability it supports. Capability codes are defined for things such as:

  • Route refresh, capability code 0 and 2

  • Multiprotocol extensions, capability code 1

  • Cooperative route filtering, capability code 3

  • Dynamic capability exchange, capability code 6

  • Graceful restart, capability code 64

  • Four octet autonomous system numbers

  • The applicability and value of these and other BGP capabilities and extensions with be discussed in later sections.

  • If a BGP speaker receives a capability code it does not support when enabling a peering with another BGP speaker, it will send a Notification message to its peer, which shuts the session down, with a notification subcode indicating that the peer requested a capability the local BGP speaker doesn't support. The receiving peer can either break off communications altogether on receipt of a notification code indicating an unsupported capability, or it can attempt to peer again without that capability enabled.

The BGP Peering Process

There are a lot of elements to the BGP peering process; when a BGP speaker begins a session with a new peer, it must determine if it is peering with an external neighbor or an internal neighbor, it must negotiate capabilities, and do a number of other things. The BGP session state machine in Figure 1-7 illustrates the process in an attempt to bring all these different actions together in one place.

Figure 7Figure 1-7: The BGP Peering State Machine





  • + 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