How Does BGP Work?

BGP uses TCP port 179 for communication. BGP is considered a path-vector protocol, containing end-to-end path descriptions for destinations. BGP neighbors can either be internal (iBGP) or external (eBGP):

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iBGP – Neighbor is in the same AS.

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eBGP – Neighbor is in a different AS.

Paths are advertised in UPDATE messages that are tagged with various path attributes. AS_PATH and NEXT_HOP are the two most important attributes that describe the path of a route in a BGP update message.

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AS_PATH: Indicates the ASs that the route is traveling from and two. In the example below, the AS_PATH is from AS 7675 to AS 12345. For internal BGP, the AS_PATH specifies the same AS for both the source and destination.

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NEXT_HOP: Indicates the IP address of the next router the path travels to. Paths advertised across AS boundaries inherit the NEXT_HOP address of the boundary router. BGP relies on interior routing protocols to reach NEXT_HOP addresses.

BGP Finite State Machine

RFC 1771, which defines BGP, describes the operation of BGP in terms of the following state machine. The table following BGP Finite State Machine provides additional information on the various states.

BGP Finite State Machine

BGP Finite State Machine: States
State	Description
Idle	Waiting for Start event, after establishing new BGP session or resetting an existing session. In the event of errors, falls back to the Idle state. After a Start event, BGP initializes, resets connect retry timer, initiates TCP transport connection, and listens for connections
Connect	Once the TCP layer is up, transition to OpenSent, and send OPEN. If no TCP, transition to Active. If the connect retry timer expires, remain in Connect, reset the timer, and initiate a transport connection. Otherwise, transition back to Idle.
Active	Try to establish TCP connection with peer. If successful, transition to OpenSent and send OPEN. If connect retry expires, restart the timer and fall back to the Connect state. Also actively listen for connection by another peer. Go back to Idle in case of other events. Connect to Active flapping indicates a TCP transport problem, for example, TCP retransmissions or unreachability of a peer.
OpenSent	Waiting for OPEN message from peer. Validate on receipt. On validation failure, send NOTIFICATION and go to Idle. On success, send KEEPALIVE and reset the keepalive timer. Negotiate hold time, smaller value wins. If zero, hold timer and keepalive timer are not restarted.
OpenConfirm	Wait for KEEPALIVE or NOTIFICATION. If KEEPALIVE is received, transition to Established. If UPDATE or KEEPALIVE is received, restart the hold timer (unless the negotiated hold time is zero). If NOTIFICATION is received, transition to Idle. Periodic KEEPALIVE messages are sent. If TCP layer breaks, transition to Idle. If an error occurs, send a NOTIFICATION with error code, transition to Idle.
Established	Session up, exchange updates with peers. If a NOTIFICATION is received, transition to Idle. Updates are checked for errors. On error, send NOTIFICATION, and transition to Idle. In case of hold time expiration, disconnect TCP.

BGP Messages

BGP communication includes the following types of messages

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Open – The first message between BGP peers after TCP session establishment. Contains the necessary information to establish a peering session, for example, ASN, hold time, and capabilities such as multi-product extensions and route-refresh.

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Update – These messages contain path information, such as route announcements or withdrawals.

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Keepalive – Periodic messages to keep TCP layer up, and to advertise liveliness.

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Notification – A request to terminate the BGP session. Non-fatal notifications contain the error code “cease”. Subcodes provide further detail:

Notification Subcodes
Subcode	Description
1 – Maximum number of prefixes reached	The configured “neighbor maximum-prefix” value was exceeded
2 – Administratively shutdown	Session was administratively shutdown
3 – Peer unconfigured	Peer configuration has been removed
4 – Administratively reset	Session was administratively reset
5 – Connection rejected	Rejection (sometimes temporary) of BGP session
6 – Other configuration change	Session was administratively reset for some reason

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Route-refresh – A request for the peer to resend its routes.

BGP Attributes

BGP update messages can include the following attributes:

BCP Attributes
Value	Code
1	ORIGIN
2	AS_PATH
3	NEXT_HOP
4	MULTI_EXIT_DISC
5	LOCAL_PREF
6	ATOMIC_AGGREGATE
7	AGGREGATOR
8	COMMUNITY
9	ORIGINATOR_ID
10	CLUSTER_LIST
11	DPA
12	ADVERTISER (Historic)
13	RCID_PATH / CLUSTER_ID (Historic)
14	MP_REACH_NLRI
15	MP_UNREACH_NLRI
16	EXTENDED COMMUNITIES
17	AS4_PATH
18	AS4_AGGREGATOR
19	SAFI Specific Attribute (SSA) (deprecated)
20	Connector Attribute (deprecated)
21	AS_PATHLIMIT (deprecated)
22	PMSI_TUNNEL
23	Tunnel Encapsulation Attribute
24	Traffic Engineering
25	IPv6 Address Specific Extended Community
26	AIGP (TEMPORARY - expires 2011-02-23)
27-254	Unassigned
255	Reserved for development

For more information on BGP attributes, see: http://www.iana.org/assignments/bgp-parameters/bgp-parameters.xml.