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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



Network Layer Information



IPv4 Unicast



IPv4 Multicast



IPv4 Unicast with MPLS Label






IPv6 Unicast



IPv6 Unicast with MPLS Label






Virtual Private LAN Service (VPLS)

Virtual Private Wire Service (VPWS)



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

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