What Is BGP?

Roles in BGP Message Exchange

Speaker: A routing device that sends BGP messages is called a BGP speaker. The speaker receives or generates new routing information, and then advertises the routing information to other BGP speakers. After receiving a route from another AS, the BGP speaker compares the route with its local routes. If the route is better than its local routes or is new, the speaker advertises it to all other BGP speakers except the one that advertised the route.

Peer: BGP speakers that exchange messages with each other are called peers.

BGP Messages

BGP runs by sending five types of messages: Open, Update, Notification, Keepalive, and Route-refresh.

• Open: It is the first message sent after a TCP connection is set up. This type of message is used to set up a BGP peer relationship. After a peer receives an Open message and the negotiation between the local device and peer succeeds, the peer sends a Keepalive message to confirm and maintain the peer relationship. Then, the peers can exchange Update, Notification, Keepalive, and Route-refresh messages.

• Update: This type of message is used to exchange routing information between peers. An Update message can advertise multiple reachable routes with the same attributes and can also be used to delete multiple unreachable routes.

• Notification: If BGP detects an error, it sends a Notification message to its peers. The BGP connections are then torn down immediately.

• Keepalive: BGP periodically sends Keepalive messages to peers to ensure the validity of BGP connections.

• Route-refresh: This type of message is used to request that peers re-send all reachable routes to the local device.

BGP Processing

Because BGP uses TCP as the transport layer protocol, peers must first establish a TCP connection before they can establish a BGP peer relationship. Then, to establish a BGP peer relationship, the peers negotiate parameters by exchanging Open messages. After the peer relationship is established, BGP peers exchange BGP routing tables. BGP sends Keepalive messages to maintain BGP connections between peers. It does not periodically update routing tables, but updates them incrementally through Update messages when BGP routes change. If BGP detects an error (for example, it receives an error message), it sends a Notification message to report the error, and the BGP connection is torn down accordingly. Figure 1 shows the process of establishing a peer relationship.

Peer relationship establishment process

BGP Finite State Machine

The BGP Finite State Machine (FSM) has six states: Idle, Connect, Active, Open-Sent, Open-Confirm, and Established.

Among them, Idle, Active, and Established are common states during the establishment of BGP peer relationships.

• In the Idle state, BGP denies all connection requests. Idle is the initial state of BGP.

• In the Connect state, BGP decides subsequent operations after a TCP connection is established.

• In the Active state, BGP attempts to establish a TCP connection. Active is an intermediate state of BGP.

• In the Open-Sent state, BGP waits for an Open message from a peer.

• In the Open-Confirm state, BGP waits for a Notification or Keepalive message.

• In the Established state, BGP peers can exchange Update, Route-refresh, Keepalive, and Notification messages.

A BGP peer relationship can be established only when both BGP peers are in the Established state. Both peers send Update messages to exchange routing information.

BGP Attributes

BGP attributes (also called BGP route attributes) are a set of parameters that describe specific BGP routes, and BGP can filter and select routes based on these attributes. BGP route attributes are classified into four types:

• Well-known mandatory: This type of attribute can be identified by all BGP devices and must be carried in Update messages. Without this attribute, errors occur in routing information.

• Well-known discretionary: This type of attribute can be identified by all BGP routers. It is optional and, therefore, is not necessarily carried in Update messages.

• Optional transitive: This indicates the transitive attribute between ASs. Even if a BGP device does not support this type of attribute, the device will still receive messages carrying it and advertise such messages to other peers.

• Optional non-transitive: If a BGP device does not support this type of attribute, the device ignores it and does not advertise messages carrying it to other peers.

Some common BGP route attributes are as follows:

• Origin is a well-known mandatory attribute. It defines the origin of path information and identifies how a route becomes a BGP route. The Origin attribute value can be IGP, EGP, or Incomplete.

• AS_Path is a well-known mandatory attribute. It records the numbers of all ASs through which a route passes from the local end to the destination in the vector order.

• Next_Hop is a well-known mandatory attribute.

• Multi-Exit-Discriminator (MED) is an optional non-transitive attribute. It is transmitted only between two neighboring ASs. The AS that receives the MED attribute will not advertise it to any third AS.

• Local_Pref is a well-known discretionary attribute that indicates the preference of a BGP route on a routing device. It is valid only between IBGP peers and is not advertised to other ASs.

Route Import

BGP itself cannot discover routes. Therefore, it needs to import routes from other routing protocols, such as IGP routes or static routes, and then store them in the BGP routing table. Imported routes can be transmitted within an AS or between ASs.

BGP can import routes in either Import or Network mode. The Import mode enables BGP to import routes by protocol type, such as RIP, OSPF, IS-IS, static, and direct routes. The Network mode imports a route with the specified prefix and mask into the BGP routing table, and is more precise than the Import mode.

Route Selection

When multiple routes to the same destination are available, BGP selects routes using routing policies. For example, BGP prefers routes that do not recurse to an SRv6 TE Policy in the Graceful Down state (the SRv6 TE Policy is in the delayed deletion state), routes in descending order of Valid, Not Found, and Invalid after BGP origin AS validation results are applied to route selection in scenarios where the device is connected to a Resource Public Key Infrastructure (RPKI) server, and routes without bit errors.

Route Summarization

The size of the BGP routing table typically increases as the scale of a network grows. To reduce the size, route summarization can be used.

Route summarization summarizes multiple routes into a summary route. After route summarization, BGP advertises to BGP peers only the summary route rather than all the specific routes.

IPv4 supports both automatic and manual summarization, whereas IPv6 supports only manual summarization.

BGP Route Advertisement