BGP Path Attributes

A path attribute is a list of AS numbers that a route has traversed to reach a destination. BGP uses path attributes to provide more information about each route and to help prevent routing loops in an arbitrary topology. You can also use path attributes to determine administrative preferences.

BGP collapses routes with similar path attributes into a single update for advertisement. Routes that are received in a single update are readvertised in a single update. The churn caused by the loss of a neighbor is minimized, and the initial advertisement sent during peer establishment is maximally compressed.

BGP does not read information that the kernel forms message by message. Instead, it fills the input buffer. BGP processes all complete messages in the buffer before reading again. BGP also performs multiple reads to clear all incoming data queued on the socket.

Note - This feature might cause a busy peer connection to block other protocols for prolonged intervals.

Path attributes

Attribute	Description
AS_PATH	Identifies the autonomous systems through which routing information carried in an UPDATE message passed. Components of this list can be AS_SETs or AS_SEQUENCES.
NEXT_HOP	Defines the IP address of the border router that should be used as the next hop to the destinations listed in the UPDATE message.
MULTI_EXIT_DISC	Discriminates among multiple exit or entry points to the same neighboring autonomous system. Used only on external links.
LOCAL_PREF	Determines which external route should be taken and is included in all iBGP UPDATE messages. The assigned BGP speaker sends this message to BGP speakers within its own autonomous system but not to neighboring autonomous systems. Higher values of a LOCAL_PREF are preferred.
ATOMIC_AGGREGATE	Specifies to a BGP speaker that a less specific route was chosen over a more specific route. The BGP speaker attaches the ATOMIC_AGGREGATE attribute to the route when it reproduces it to other BGP speakers. The BGP speaker that receives this route cannot remove the ATOMIC_AGGREGATE attribute or make any Network Layer Reachability Information (NLRI) of the route more specific. This attribute is used only for debugging purposes.

All unreachable messages are collected into a single message and are sent before 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 length is set to zero.

Routing information shared between peers in BGP has two formats: announcements and withdrawals. A route announcement indicates that a router either learned of a new network attachment or made a policy decision to prefer another route to a network destination. Route withdrawals are sent when a router makes a new local decision that a network is no longer reachable.