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Nokia Network Voyager for IPSO 4.0 Reference Guide 405
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.
The following table displays the path attributes and their definitions
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
Path Attribute Definition
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.