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Thomas Mangin edited this page Mar 6, 2026 · 10 revisions

Glossary

A-Z glossary of BGP and ExaBGP terminology

Comprehensive definitions of terms, acronyms, and concepts used in BGP routing and ExaBGP.

Table of Contents

A

ACK (Acknowledgment)

Feature in ExaBGP 4.x and 5.x (enabled by default) that sends command acknowledgments (done, error, shutdown) back to API processes after executing commands.

See also: API Overview - ACK Feature

ADD-PATH

RFC 7911 - BGP capability that allows advertising multiple paths for the same prefix without opening new BGP sessions. Useful for path diversity and multipath routing.

See also: ADD-PATH Capability

Address Family

A combination of AFI (Address Family Identifier) and SAFI (Subsequent Address Family Identifier) that defines the type of routing information being exchanged. Examples: IPv4 unicast, IPv6 unicast, VPNv4, FlowSpec, EVPN.

See also: Address Families

AFI (Address Family Identifier)

Top-level identifier for address families in BGP multiprotocol extensions (RFC 4760).

Common AFI values:

  • 1 - IPv4
  • 2 - IPv6
  • 25 - L2VPN
  • 16388 - BGP-LS

See also: Multiprotocol BGP

AGGREGATOR

BGP attribute (Type 7) - Identifies the AS and router that performed route aggregation. Consists of AS number and IP address.

See also: Attribute Reference - AGGREGATOR

AIGP (Accumulated IGP Metric)

RFC 7311 - BGP attribute (Type 26) that carries accumulated IGP metric for seamless MPLS and inter-domain routing.

See also: Attribute Reference

Anycast

Routing technique where the same IP address is announced from multiple locations. Traffic is routed to the nearest (lowest cost) location. Used for load balancing and high availability.

ExaBGP use case: Health check + route announcement for anycast services (DNS, CDN, load balancers).

See also: Anycast Management

AS (Autonomous System)

A collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. Identified by an AS Number (ASN).

AS_PATH

BGP attribute (Type 2) - Well-known mandatory attribute listing the sequence of ASNs the route has traversed. Used for loop detection and path selection.

See also: Attribute Reference - AS_PATH

AS Path Prepending

Traffic engineering technique where the local AS is prepended multiple times to the AS_PATH to make the path less attractive to neighboring ASes.

Example:

announce route 10.0.0.0/24 next-hop 192.0.2.1 as-path [65001 65001 65001]

See also: Attribute Reference - AS_PATH

ASN (Autonomous System Number)

Unique identifier for an Autonomous System. Originally 16-bit (0-65535), now extended to 32-bit (0-4294967295) by RFC 4893.

Ranges:

  • 1-64511 - Public ASNs
  • 64512-65534 - Private ASNs (16-bit)
  • 65535 - Reserved
  • 64496-64511 - Documentation ASNs
  • 4200000000-4294967294 - Private ASNs (32-bit)

ATOMIC_AGGREGATE

BGP attribute (Type 6) - Indicates route was aggregated and AS path information may be incomplete.

See also: Attribute Reference - ATOMIC_AGGREGATE

B

BGP (Border Gateway Protocol)

The routing protocol used to exchange routing information between autonomous systems on the Internet. Current version is BGP-4 (RFC 4271).

ExaBGP relationship: ExaBGP implements BGP-4 protocol but does NOT manipulate the RIB/FIB. It only speaks the BGP protocol.

See also: RFC 4271

BGP FSM (Finite State Machine)

The state machine that governs BGP session establishment and maintenance.

States:

  1. Idle - Initial state
  2. Connect - Waiting for TCP connection
  3. Active - Trying to establish TCP connection
  4. OpenSent - OPEN message sent
  5. OpenConfirm - OPEN message received and validated
  6. Established - BGP session active, exchanging routes

See also: BGP State Machine

BGP-LS (BGP Link State)

RFC 7752 - BGP extension for distributing network topology information (IGP link-state data) to SDN controllers and centralized path computation engines.

ExaBGP use case: Topology collection for SDN, traffic engineering, network visualization.

See also: BGP-LS Overview

Blackhole

Routing technique where traffic is dropped by setting the next-hop to a discard interface. Used for DDoS mitigation.

Remote Triggered Blackhole (RTBH): Upstream provider drops traffic based on BGP announcement with special community (e.g., 65535:666).

See also: DDoS Mitigation

BoRR (Beginning of Route Refresh)

RFC 7313 - Marker indicating the start of route refresh. Part of Enhanced Route Refresh capability.

C

Capability

Optional features negotiated during BGP session establishment via the OPEN message. Examples: ADD-PATH, Graceful Restart, 4-byte ASN, Route Refresh.

See also: Capabilities

CIDR (Classless Inter-Domain Routing)

IP address allocation and routing methodology using variable-length subnet masks. Notation: 10.0.0.0/24.

CLUSTER_LIST

BGP attribute (Type 10) - Used by route reflectors to prevent routing loops between clusters. Lists cluster IDs the route has traversed.

See also: Attribute Reference - CLUSTER_LIST, Route Reflection

Community

BGP attribute (Type 8) - Optional transitive attribute used to tag routes for policy application. Format: ASN:Value (both 16-bit).

Well-known communities:

  • NO_EXPORT (65535:65281) - Don't advertise to EBGP
  • NO_ADVERTISE (65535:65282) - Don't advertise to any peer
  • NOPEER (65535:65284) - Don't advertise to peers

See also: Attribute Reference - COMMUNITY, Communities

Confederation

RFC 5065 - Technique for dividing a large AS into sub-ASes while appearing as a single AS to external peers. Used for IBGP scaling.

D

DDoS (Distributed Denial of Service)

Attack where multiple compromised systems flood a target with traffic. BGP FlowSpec is used to mitigate DDoS attacks.

ExaBGP use case: Detect attacks → Announce FlowSpec rules → Block/rate-limit malicious traffic.

See also: DDoS Mitigation, FlowSpec

DSCP (Differentiated Services Code Point)

6-bit field in IP header used for QoS classification. FlowSpec can mark traffic with DSCP values.

See also: FlowSpec Actions - Mark

E

EBGP (External BGP)

BGP sessions between routers in different autonomous systems.

Characteristics:

  • Different AS numbers
  • Next-hop is typically changed to advertising router
  • AS_PATH is modified (local AS prepended)
  • TTL typically set to 1 (for directly connected peers)

Contrast: IBGP

ECMP (Equal-Cost Multi-Path)

Routing technique where traffic is load-balanced across multiple equal-cost paths.

ExaBGP integration: ExaBGP announces routes → Router with ECMP distributes traffic across multiple next-hops.

See also: Service High Availability

Encoder

ExaBGP term - Format for API messages. Options: text or json.

Configuration:

process announce-routes {
    run /path/to/script.py;
    encoder text;  # or json
}

See also: Text API Reference, JSON API Reference

EoR (End-of-RIB)

Marker indicating initial BGP route convergence is complete. Implemented as an UPDATE message with no NLRI.

See also: API Commands - eor

EoRR (End of Route Refresh)

RFC 7313 - Marker indicating the end of route refresh. Part of Enhanced Route Refresh capability.

EVPN (Ethernet VPN)

RFC 7432 - BGP MPLS-based Ethernet VPN for data center fabrics, VXLAN control plane, and multi-tenant networks.

ExaBGP support: Full EVPN route types (1-5), MAC/IP advertisement, inclusive multicast routes.

See also: EVPN Overview

Extended Community

BGP attribute (Type 16) - 8-byte communities (vs 4-byte standard communities). Used for VPN route targets, FlowSpec actions, link bandwidth.

Types:

  • Route Target (RT)
  • Route Origin (RO)
  • Redirect (FlowSpec)
  • Traffic Rate (FlowSpec)
  • Traffic Marking (FlowSpec)

See also: Attribute Reference - EXTENDED_COMMUNITY

F

FIB (Forwarding Information Base)

The actual forwarding table used by routers to forward packets. Derived from RIB.

ExaBGP does NOT manipulate FIB. External processes must install routes into the OS routing table (e.g., via ip route add).

See also: RIB/FIB

FlowSpec (Flow Specification)

RFC 5575/8955 - BGP extension for distributing traffic filtering rules. Used for DDoS mitigation, traffic filtering, and QoS.

ExaBGP achievement: First open-source implementation of FlowSpec (now also in GoBGP, FRR, BIRD).

Components:

  • Match conditions: source, destination, port, protocol, packet length, TCP flags, etc.
  • Actions: discard, rate-limit, redirect, mark DSCP, community

See also: FlowSpec Overview, DDoS Mitigation

Forwarding

The act of sending packets from one interface to another based on routing table entries.

ExaBGP does NOT perform forwarding. It only speaks BGP protocol.

G

Graceful Restart

RFC 4724 - BGP capability allowing session restart without losing forwarding state. Minimizes traffic disruption during control plane restarts.

See also: Graceful Restart Capability

GTSM (Generalized TTL Security Mechanism)

RFC 5082 - Security mechanism using TTL field to prevent BGP session hijacking. Packets with incorrect TTL are dropped.

ExaBGP configuration:

incoming-ttl 1;  # Require TTL=255 (direct connection)

See also: Security Best Practices

H

Health Check

Mechanism to monitor service availability and announce/withdraw routes based on health.

ExaBGP built-in healthcheck:

python3 -m exabgp healthcheck --cmd "curl http://localhost"

See also: Service High Availability, Example

Hold Time

BGP timer (in seconds) specifying maximum time without receiving KEEPALIVE or UPDATE before declaring session dead.

Default: 180 seconds Keepalive: Hold time / 3

Configuration:

hold-time 180;

I

IBGP (Internal BGP)

BGP sessions between routers within the same autonomous system.

Characteristics:

  • Same AS number for local-as and peer-as
  • Next-hop is typically NOT changed
  • AS_PATH is NOT modified
  • LOCAL_PREF is propagated
  • Requires full mesh or route reflection

Contrast: EBGP

IGP (Interior Gateway Protocol)

Routing protocols used within an autonomous system. Examples: OSPF, IS-IS, RIP, EIGRP.

Relationship to BGP: BGP relies on IGP for next-hop reachability.

ExaBGP does NOT run IGP. It only speaks BGP.

K

KEEPALIVE

BGP message sent periodically to maintain session. Sent every (hold-time / 3) seconds.

See also: Hold Time

L

L2VPN (Layer 2 VPN)

VPN operating at Layer 2 (Ethernet). Includes VPLS (Virtual Private LAN Service).

ExaBGP support: VPLS announcements.

See also: VPLS Configuration

L3VPN (Layer 3 VPN)

RFC 4364 - MPLS-based VPN operating at Layer 3 (IP). Uses VPNv4/VPNv6 address families.

Components:

  • Route Distinguisher (RD): Makes routes unique
  • Route Target (RT): Import/export filter

ExaBGP support: Full VPNv4/VPNv6 support.

See also: L3VPN Overview

Label

MPLS label - 20-bit value used for MPLS forwarding. VPN routes carry labels.

See also: L3VPN

Large Community

RFC 8092 - BGP attribute (Type 32) with 12 bytes (vs 4 bytes for standard communities). Format: ASN:Value1:Value2 (all 32-bit).

Advantage: Native support for 4-byte ASNs and larger value space.

See also: Attribute Reference - LARGE_COMMUNITY

LOCAL_PREF (Local Preference)

BGP attribute (Type 5) - Well-known discretionary attribute indicating degree of preference for a route (IBGP only). Higher value = more preferred.

Range: 0-4294967295 Default: 100

See also: Attribute Reference - LOCAL_PREF

M

MD5 Authentication

RFC 2385 - TCP MD5 signature option for BGP session authentication. All packets are signed with shared secret.

ExaBGP configuration:

md5-password "MySecretPassword";

See also: Security Best Practices

MED (Multi-Exit Discriminator)

BGP attribute (Type 4) - Optional non-transitive attribute influencing route preference from neighboring AS. Lower value = more preferred.

Range: 0-4294967295 Default: 0

Use case: Multi-homed networks signaling preferred entry point.

See also: Attribute Reference - MED

MP-BGP (Multiprotocol BGP)

RFC 4760 - Extension to BGP-4 supporting multiple address families beyond IPv4 unicast. Uses MP_REACH_NLRI and MP_UNREACH_NLRI attributes.

Address families: IPv6, VPNv4, VPNv6, FlowSpec, EVPN, BGP-LS, VPLS, MVPN, etc.

See also: Multiprotocol BGP

MPLS (Multiprotocol Label Switching)

Forwarding mechanism using labels instead of IP addresses. Foundation for L3VPN, VPLS, traffic engineering.

ExaBGP relationship: ExaBGP can announce VPN routes with labels but does NOT perform MPLS forwarding.

MRT (Multi-Threaded Routing Toolkit)

File format for storing BGP messages and routing table snapshots. ExaBGP can output MRT format.

See also: MRT Documentation

MVPN (Multicast VPN)

RFC 6514 - BGP-based multicast VPN for distributing multicast traffic across MPLS networks.

ExaBGP support: MVPN route announcements.

See also: Configuration Example

N

Neighbor

BGP peer. In ExaBGP configuration, defines a BGP session.

Configuration:

neighbor 192.168.1.1 {
    router-id 192.168.1.2;
    local-as 65001;
    peer-as 65000;
}

See also: Configuration Syntax

Next-Hop

BGP attribute (Type 3) - IP address to forward packets to. Well-known mandatory for IPv4 unicast.

EBGP: Changed to advertising router IBGP: Preserved (not changed)

⚠️ Warning: next-hop self is an EXPERIMENTAL feature. For production use, specify an explicit next-hop IP address instead.

See also: Attribute Reference - NEXT_HOP

NLRI (Network Layer Reachability Information)

The actual routing information (prefixes) carried in BGP UPDATE messages.

Examples:

  • 10.0.0.0/24 (IPv4 unicast)
  • 2001:db8::/64 (IPv6 unicast)
  • FlowSpec rules
  • VPN routes with RD

NO_ADVERTISE

Well-known community (65535:65282) - Routes with this community must not be advertised to any peer.

See also: Attribute Reference - COMMUNITY

NO_EXPORT

Well-known community (65535:65281) - Routes with this community must not be advertised to EBGP peers.

See also: Attribute Reference - COMMUNITY

NOTIFICATION

BGP message type sent when errors occur or session is being closed. Contains error code and subcode.

Common error codes:

  • Message Header Error
  • OPEN Message Error
  • UPDATE Message Error
  • Hold Timer Expired
  • Finite State Machine Error
  • Cease

See also: Debugging Guide

O

OPEN

BGP message type sent during session establishment. Contains BGP version, AS number, hold time, router ID, and capabilities.

See also: BGP FSM

ORIGIN

BGP attribute (Type 1) - Well-known mandatory attribute indicating origin of route.

Values:

  • igp (0) - Learned from IGP (most preferred)
  • egp (1) - Learned from EGP
  • incomplete (2) - Learned by other means (e.g., static, redistribution)

See also: Attribute Reference - ORIGIN

ORIGINATOR_ID

BGP attribute (Type 9) - Optional non-transitive attribute created by route reflectors. Contains router ID of route originator. Used for loop prevention.

See also: Attribute Reference - ORIGINATOR_ID, Route Reflection

P

Path Attribute

Information attached to BGP routes (NLRI) in UPDATE messages. Examples: AS_PATH, NEXT_HOP, LOCAL_PREF, MED, COMMUNITY.

See also: Attribute Reference

Peer

Another BGP router with which a BGP session is established. Synonym: neighbor.

Prefix

IP address range in CIDR notation. Example: 10.0.0.0/24, 2001:db8::/64.

PREFIX_SID

BGP attribute (Type 40) - Carries Segment Routing prefix segment identifier. Used in SRv6 and Segment Routing deployments.

See also: SRv6 and MUP

Process

ExaBGP term - External program that communicates with ExaBGP via API (STDIN/STDOUT).

Configuration:

process announce-routes {
    run /path/to/script.py;
    encoder text;
}

See also: Process Configuration, API Overview

Q

QoS (Quality of Service)

Mechanisms for prioritizing network traffic. FlowSpec can mark DSCP for QoS classification.

See also: FlowSpec Actions

R

RD (Route Distinguisher)

64-bit value prepended to IP address to create unique VPNv4/VPNv6 routes. Allows overlapping IP addresses in different VPNs.

Formats:

  • Type 0: ASN:Value (16-bit ASN, 32-bit value)
  • Type 1: IP:Value (IPv4 address, 16-bit value)
  • Type 2: 4ByteASN:Value (32-bit ASN, 16-bit value)

See also: L3VPN Overview

Redirect

FlowSpec action - Redirects matching traffic to a VRF or IP address using extended communities.

Types:

  • Redirect to VRF: redirect 65001:100
  • Redirect to IP: redirect 1.2.3.4

See also: FlowSpec Actions - Redirect

RIB (Routing Information Base)

The routing table containing all routes learned by BGP. Separate from FIB (forwarding table).

ExaBGP does NOT manipulate RIB/FIB. It only speaks the BGP protocol. External processes must install routes into the OS routing table.

See also: RFC Support

Route Distinguisher

See RD (Route Distinguisher)

Route Reflection

RFC 4456 - IBGP scaling technique where route reflectors (RR) redistribute routes to clients, avoiding full mesh.

Attributes used:

  • ORIGINATOR_ID (Type 9)
  • CLUSTER_LIST (Type 10)

ExaBGP support: Can act as route reflector.

See also: Route Reflection

Route Reflector

Router that reflects IBGP routes to route reflector clients. Avoids IBGP full mesh requirement.

See also: Route Reflection

Route Refresh

RFC 2918 - BGP capability allowing request for re-advertisement of routes without tearing down session.

Enhanced Route Refresh (RFC 7313): Adds BoRR and EoRR markers.

See also: Route Refresh Capability

Route Target

See RT (Route Target)

Router ID

32-bit BGP identifier (IPv4 address format) uniquely identifying BGP speaker within an AS.

Configuration:

router-id 192.168.1.2;

Note: Must be an IPv4 address even for IPv6-only sessions.

RT (Route Target)

Extended community used for VPN route import/export filtering. Routes with matching RT are imported into VRF.

Format: target:ASN:Value

See also: L3VPN Overview

RTBH (Remote Triggered Blackhole)

DDoS mitigation technique where victim IP is announced with special community (e.g., 65535:666) and next-hop set to discard interface. Upstream provider drops traffic to that IP.

See also: DDoS Mitigation

S

SAFI (Subsequent Address Family Identifier)

Sub-identifier within AFI defining specific routing information type.

Common SAFI values:

  • 1 - Unicast
  • 2 - Multicast
  • 4 - MPLS Labels
  • 128 - VPN
  • 133 - FlowSpec
  • 134 - FlowSpec VPN

See also: Address Family

SDN (Software-Defined Networking)

Network architecture where control plane is centralized and programmable. BGP-LS distributes topology to SDN controllers.

ExaBGP use case: BGP-LS for topology collection, API for programmable route control.

See also: BGP-LS Overview

Segment Routing

Source routing paradigm where path is encoded in packet header as list of segments. SRv6 uses IPv6 addresses as segments.

ExaBGP support: PREFIX_SID attribute, SRv6, Mobile User Plane (MUP).

See also: SRv6 and MUP

SRv6 (Segment Routing over IPv6)

Segment Routing implementation using IPv6 addresses. Used in 5G networks, data centers, and WAN.

ExaBGP support: SRv6 prefix SID, MUP (Mobile User Plane).

See also: SRv6 and MUP

Static Route

Route configured manually (not learned dynamically).

ExaBGP configuration:

static {
    route 10.0.0.0/24 next-hop 192.168.1.1;
}

See also: Configuration Syntax

T

TCP MD5

See MD5 Authentication

Template

ExaBGP feature - Configuration inheritance for reducing duplication.

See also: Templates and Inheritance, Example

Traffic Engineering

Technique for controlling network traffic flow to optimize performance, avoid congestion, or meet SLAs.

BGP tools:

  • AS path prepending
  • LOCAL_PREF manipulation
  • MED hints
  • Communities for policy triggers

See also: Attribute Reference

TTL (Time To Live)

IP header field decremented at each hop. Used for loop prevention and security (GTSM).

See also: GTSM

U

Unicast

One-to-one routing (single destination). Contrast: multicast (one-to-many).

Address families:

  • ipv4 unicast
  • ipv6 unicast

See also: IPv4 Unicast, IPv6 Unicast

UPDATE

BGP message type carrying routing information (NLRI) and attributes. Can announce new routes or withdraw existing routes.

V

VPNv4

IPv4 routes for L3VPN (MPLS VPN). Uses Route Distinguisher (RD) and Route Target (RT).

Address family: l3vpn ipv4 or vpnv4

See also: L3VPN Overview

VPNv6

IPv6 routes for L3VPN.

Address family: l3vpn ipv6 or vpnv6

See also: L3VPN Overview

VPLS (Virtual Private LAN Service)

RFC 4761/4762 - L2VPN technology for multi-point Ethernet VPN over MPLS.

ExaBGP support: VPLS route announcements.

See also: VPLS Configuration

VRF (Virtual Routing and Forwarding)

Separate routing table instance in L3VPN. Each customer has isolated VRF.

See also: L3VPN

VXLAN (Virtual Extensible LAN)

Network virtualization technology for Layer 2 overlay networks. EVPN provides control plane for VXLAN.

See also: EVPN Overview

W

Well-Known Attribute

BGP attribute that must be recognized by all BGP implementations.

Types:

  • Well-Known Mandatory: ORIGIN, AS_PATH, NEXT_HOP
  • Well-Known Discretionary: LOCAL_PREF, ATOMIC_AGGREGATE

See also: Attribute Reference

Withdraw

Remove previously announced route.

API command:

withdraw route 10.0.0.0/24

See also: Command Reference - withdraw

Acronym Quick Reference

Acronym Full Name
ACK Acknowledgment
AFI Address Family Identifier
AIGP Accumulated IGP Metric
AS Autonomous System
ASN Autonomous System Number
BGP Border Gateway Protocol
BGP-LS BGP Link State
BoRR Beginning of Route Refresh
CIDR Classless Inter-Domain Routing
DDoS Distributed Denial of Service
DSCP Differentiated Services Code Point
EBGP External BGP
ECMP Equal-Cost Multi-Path
EGP Exterior Gateway Protocol
EoR End-of-RIB
EoRR End of Route Refresh
EVPN Ethernet VPN
FIB Forwarding Information Base
FSM Finite State Machine
GTSM Generalized TTL Security Mechanism
IBGP Internal BGP
IGP Interior Gateway Protocol
L2VPN Layer 2 VPN
L3VPN Layer 3 VPN
MED Multi-Exit Discriminator
MP-BGP Multiprotocol BGP
MPLS Multiprotocol Label Switching
MRT Multi-Threaded Routing Toolkit
MVPN Multicast VPN
NLRI Network Layer Reachability Information
QoS Quality of Service
RD Route Distinguisher
RIB Routing Information Base
RT Route Target
RTBH Remote Triggered Blackhole
SAFI Subsequent Address Family Identifier
SDN Software-Defined Networking
SRv6 Segment Routing over IPv6
TTL Time To Live
VPNv4 VPN IPv4
VPNv6 VPN IPv6
VPLS Virtual Private LAN Service
VRF Virtual Routing and Forwarding
VXLAN Virtual Extensible LAN

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