The Border Gateway Protocol (BGP) is an exterior routing protocol used for exchanging routing information between autonomous systems. BGP is used for exchange of routing information between multiple transit autonomous systems as well as between transit and stub autonomous systems. BGP is related to EGP but operates with more capability, greater flexibility, and less required bandwidth. BGP uses path attributes to provide more information about each route, and in particular maintain an AS path, which includes the AS number of each autonomous system the route has transited, providing information sufficient to prevent routing loops in an arbitrary topology. Path attributes may also be used to distinguish between groups of routes to determine administrative preferences, allowing greater flexibility in determining route preference to achieve a variety of administrative ends.
BGP supports two basic types of sessions between neighbours, internal (sometimes refered to as IBGP) and external. Internal sessions are run between routers in the same autonomous system, while external sessions run between routers in different autonomous systems. When sending routes to an external peer the local AS number is prepended to the AS path, hence routes received from an external peer are guaranteed to have the AS number of that peer at the start of the path. Routes received from an internal neighbour will not in general have the local AS number prepended to the AS path, and hence will in general have the same AS path that the route had when the originating internal neighbour received the route from an external peer. Routes with no AS numbers in the path may be legitimately received from internal neighbours; these indicate that the received route should be considered internal to your own AS.
The BGP implementation supports three versions of the BGP protocol, versions 2, 3 and 4. BGP versions 2 and 3 are quite similar in capability and function. They will only propagate classed network routes, and the AS path is a simple array of AS numbers. BGP 4 will propagate fully general address-and-mask routes, and the AS path has some structure to represent the results of aggregating dissimilar routes.
External BGP sessions may or may not include a single metric, which BGP calls the Multi-Exit Discriminator, in the path attributes. For BGP versions 2 and 3 this metric is a 16-bit unsigned integer, for BGP version 4 it is a 32-bit unsigned integer. In either case smaller values of the metric are to be preferred. Currently this metric is only used to break ties between routes with equal preference from the same neighbour AS. Internal BGP sessions carry at least one metric in the path attributes, which BGP calls the LocalPref. The size of the metric is identical to the MED. For BGP versions 2 and 3 this metric is considered better when its value is smaller, for version 4 it is better when it is larger. BGP version 4 sessions may optionally carry a second metric on internal sessions, this being an internal version of the Multi-Exit Discriminator. The use of these metrics is dependent on the type of internal protocol processing which is specified.
BGP collapses routes with similar path attributes into a single update for advertisement. Routes that are received in a single update will be readvertised in a single update. The churn caused by the loss of a neighbor will be minimized and the initial advertisement sent during peer establishment will be maximally compressed. BGP does not read information from the kernel message-by-message, but fills the input buffer. It processes all complete messages in the buffer before reading again. BGP also does multiple reads to clear all incoming data queued on the socket. This feature may cause other protocols to be blocked for prolonged intervals by a busy peer connection.
All unreachable messages are collected into a single message and sent prior to reachable routes during a flash update. For these unreachable announcements, the next hop is set to the local address on the connection, no metric is sent and the path origin is set to incomplete. On external connections the AS path in unreachable announcements is set to the local AS, on internal connections the AS path is set to zero length.
The BGP implementation expects external peers to be directly attached to a shared subnet, and expects those peers to advertise next hops which are host addresses on that subnet (though this constraint can be relaxed by configuration for testing). For groups of internal peers, however, there are several alternatives which may be selected from by specifying the group type. Type internal groups expect all peers to be directly attached to a shared subnet so that, like external peers, the next hops received in BGP advertisements may be used directly for forwarding. Type routing groups instead will determine the immediate next hops for routes by using the next hop received with a route from a peer as a forwarding address, and use this to look up an immediate next hop in an IGP's routes. Such groups support distant peers, but need to be informed of the IGP whose routes they are using to determine immediate next hops. Finally, type igp groups expect routes from the group peers to not be used for forwarding at all. Instead they expect that copies of the BGP routes received will also be received via an IGP, and that the BGP routes will only be used to determine the path attributes associated with the IGP routes. Such groups also support distant peers, and also need to be informed of the IGP they are running with.
For internal BGP group types (and for test groups), where possible a single outgoing message is built for all group peers based on the common policy. A copy of the message is sent to every peer in the group, with possible adjustments to the next hop field as appropriate to each peer. This minimizes the computational load of running large numbers of peers in these types of groups. BGP allows unconfigured peers to connect if an appropriate group has been configured with an allow clause.
bgp yes | no | on | off
preference preference ;
defaultmetric metric ;
traceoptions trace_options ;
group type ( external peeras autonomous_system )
| ( internal peeras autonomous_system )
| ( igp peeras autonomous_system proto proto )
| ( routing peeras autonomous_system proto proto
interface interface_list )
| ( test peeras autonomous_system )
network mask mask
network masklen number
[ metricout metric ]
[ localas autonomous_system ]
[ nogendefault ]
[ gateway gateway ]
[ preference preference ]
[ preference2 preference ]
[ lcladdr local_address ]
[ holdtime time ]
[ version number ]
[ passive ]
[ sendbuffer number ]
[ recvbuffer number ]
[ indelay time ]
[ outdelay time ]
[ keep [ all | none ] ]
[ analretentive ]
[ noauthcheck ]
[ noaggregatorid ]
[ keepalivesalways ]
[ v3asloopokay ]
[ nov4asloop ]
[ logupdown ]
[ ttl ttl ]
[ traceoptions trace_options ]
} ] ;
external | internal | igp | test
The bgp statement enables or disables BGP. By default BGP is disabled. The default metric for announcing routes via BGP is not to send a metric.
BGP peers are grouped by type and the autonomous system of the peers. Any number of groups may be specified, but each must have a unique combination of type and peer autonomous system. There are four possible group types:
The proto names the interior protocol to be used to resolve BGP route next hops, and may be the name of any IGP in the configuration. By default the next hop in BGP routes advertised to type routing peers will be set to the local address on the BGP connection to those peers, as it is assumed a route to this address will be propagated via the IGP. The interface_list can optionally provide a list interfaces whose routes are carried via the IGP for which third party next hops may be used instead.
The BGP statement has group clauses and peer subclauses. Any number of peer subclauses may be specified within a group. A group clause usually defines default parameters for a group of peers, these parameters apply to all subsidiary peer subclauses. Any parameters from the peer subclause may be specified on the group clause to provide defaults for the whole group (which may be overridden for individual peers).
Within a group, BGP peers may be configured in one of two ways. They may be explicitly configured with a peer statement, or implicitly configured with the allow statement. Both are described here:
Within each group clause, individual peers can be specified or a group of potential peers can be specified using allow. Allow is used to specify a set of address masks. If GateD receives a BGP connection request from any address in the set specified, it will accept it and set up a peer relationship.
The BGP peer subclause allows the following parameters, which can also be specified on the group clause. All are optional.
logupdown Causes a message to be logged via the syslog mechanism whenever a BGP peer enters or leaves the ESTABLISHED state.
Note that the state option works with BGP, but does not provide true state transition information.
Packet tracing options (which may be modified with detail, send and recv):