Border Gateway Protocol BGP
TAGS: BGPBorder Gateway Protocol

Border Gateway Protocol BGP

The Border Gateway Protocol BGP is an inter-Autonomous System routing protocol.  It is built on experience gained with EGP as defined in RFC 904 and EGP usage in the NSFNET Backbone as described in RFC 1092 and RFC 1093.

The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems.  This network reachability information includes information on the full path of Autonomous Systems (ASs) that traffic must transit to reach these networks.  This information is sufficient to construct a graph of AS connectivity from which routing loops may be pruned and some policy decisions at the AS level may be enforced.

To characterize the set of policy decisions that can be enforced using BGP, one must focus on the rule that an AS advertise to its neighbor ASs only those routes that it itself uses.  This rule reflects the “hop-by-hop” routing paradigm generally used throughout the current Internet.  Note that some policies cannot be supported by the “hop-by-hop” routing paradigm and thus require techniques such as source routing to enforce.  For example, BGP does not enable one AS to send traffic to a neighbor AS intending that that traffic take a different route from that taken by traffic originating in the neighbor AS.  On the other hand, BGP can support any policy conforming to the “hop-by-hop” routing paradigm.  Since the current Internet uses only the “hop-by-hop” routing paradigm and since BGP can support any policy that conforms to that paradigm, BGP is highly applicable as an inter-AS routing protocol for the current Internet.

BGP runs over a reliable transport protocol.  This eliminates the need to implement explicit update fragmentation, retransmission, acknowledgment, and sequencing.  Any authentication scheme used by the transport protocol may be used in addition to BGP’s own authentication mechanisms.  The error notification mechanism used in BGP assumes that the transport protocol supports a “graceful” close, i.e., that all outstanding data will be delivered before the connection is closed.

BGP uses TCP as its transport protocol.  TCP meets BGP’s transport requirements and is present in virtually all commercial routers and hosts.  In the following descriptions the phrase “transport protocol connection” can be understood to refer to a TCP connection.  BGP uses TCP port 179 for establishing its connections.

Autonomous System

The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other ASs.  Since this classic definition was developed, it has become common for a single AS to use several interior gateway protocols and sometimes several sets of metrics within an AS.  The use of the term Autonomous System here stresses the fact that, even when multiple Interior Gateway Protocols and metrics are used, the administration of an AS appears to other ASs to have a single coherent interior routing plan and presents a consistent picture of what networks are reachable through it.  From the standpoint of exterior routing, an AS can be viewed as monolithic: reachability to networks directly connected to the AS must be equivalent from all border gateways of the AS.

Summary of Operation

Two systems form a transport protocol connection between one another. They exchange messages to open and confirm the connection parameters. The initial data flow is the entire BGP routing table.  Incremental updates are sent as the routing tables change.  BGP does not require periodic refresh of the entire BGP routing table.  Therefore, a BGP speaker must retain the current version of the entire BGP routing tables of all of its peers for the duration of the connection. KeepAlive messages are sent periodically to ensure the liveness of the connection.  Notification messages are sent in response to errors or special conditions.  If a connection encounters an error condition, a notification message is sent and the connection is closed.

The hosts executing the Border Gateway Protocol need not be routers. A non-routing host could exchange routing information with routers via EGP or even an interior routing protocol.  That non-routing host could then use BGP to exchange routing information with a border router in another Autonomous System.  The implications and applications of this architecture are for further study.

If a particular AS has multiple BGP speakers and is providing transit service for other ASs, then care must be taken to ensure a consistent view of routing within the AS.  A consistent view of the interior routes of the AS is provided by the interior routing protocol.  A consistent view of the routes exterior to the AS can be provided by having all BGP speakers within the AS maintain direct BGP connections with each other.  Using a common set of policies, the BGP speakers arrive at an agreement as to which border routers will serve as exit/entry points for particular networks outside the AS.  This information is communicated to the AS’s internal routers, possibly via the interior routing protocol.  Care must be taken to ensure that the interior routers have all been updated with transit information before the BGP speakers announce to other ASs that transit service is being provided.

Connections between BGP speakers of different ASs are referred to as “external” links.

BGP connections between BGP speakers within the same AS are referred to as “internal” links.

Routes: Advertisement and Storage

For purposes of this protocol a route is defined as a unit of information that pairs a destination with the attributes of a path to that destination:
  1. Routes are advertised between a pair of BGP speakers in UPDATE messages: The destination is the systems whose IP addresses are reported in the Network Layer Reachability Information (NLRI) field, and the path is the information reported in the path attributes fields of the same UPDATE message.
  2. Routes are stored in the Routing Information Bases (RIBs): namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes that will be advertised to other BGP speakers must be present in the Adj-RIB-Out; routes that will be used by the local BGP speaker must be present in the Loc-RIB, and the next hop for each of these routes must be present in the local BGP speaker’s forwarding information base; and routes that are received from other BGP speakers are present in the Adj-RIBs-In.
If a BGP speaker chooses to advertise the route, it may add to or modify the path attributes of the route before advertising it to a peer.

BGP provides mechanisms by which a BGP speaker can inform its peer that a previously advertised route is no longer available for use. There are three methods by which a given BGP speaker can indicate that a route has been withdrawn from service:

  1. The IP prefix that expresses destinations for a previously advertised route can be advertised in the WITHDRAWN ROUTES field in the UPDATE message, thus marking the associated route as being no longer available for use.
  2. A replacement route with the same Network Layer Reachability Information can be advertised.
  3. The BGP speaker – BGP speaker connection can be closed, which implicitly removes from service all routes which the pair of speakers had advertised to each other.

Routing Information Bases

The Routing Information Base (RIB) within a BGP speaker consists of three distinct parts:
  1. Adj-RIBs-In: The Adj-RIBs-In store routing information that has been learned from inbound UPDATE messages. Their contents represent routes that are available as an input to the Decision Process.
  2. Loc-RIB: The Loc-RIB contains the local routing information that the BGP speaker has selected by applying its local policies to the routing information contained in its Adj-RIBs-In.
  3. Adj-RIBs-Out: The Adj-RIBs-Out store the information that the local BGP speaker has selected for advertisement to its peers. The routing information stored in the Adj-RIBs-Out will be carried in the local BGP speaker’s UPDATE messages and advertised to its peers.

In summary, the Adj-RIBs-In contain unprocessed routing information that has been advertised to the local BGP speaker by its peers; the Loc-RIB contains the routes that have been selected by the local BGP speaker’s Decision Process; and the Adj-RIBs-Out organize the routes for advertisement to specific peers by means of the local speaker’s UPDATE messages.

Although the conceptual model distinguishes between Adj-RIBs-In, Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an implementation must maintain three separate copies of the routing information. The choice of implementation (for example, 3 copies of the information vs 1 copy with pointers) is not constrained by the protocol.

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