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•
Hold time: Number of seconds that the sender proposes for the value of the Hold Timer. The
hold time defines the interval that can elapse without the receipt of an Update or KeepAlive
message before the peer is assumed to be disabled.
•
BGP identifier: IP address of the BGP node (Router ID).
•
Parameter field length and the parameter itself: Optional fields.
Update
BGP nodes send update messages to swap network reachability data between BGP peers. The
nodes use this information to build a graph describing the relationships among all known ASs.
Update messages comprise the BGP header plus the following optional fields:
•
Unfeasible routes length: Length of the field that lists the routes being withdrawn from service
because they are judged no longer reachable.
•
Withdrawn routes: IP address prefixes for the routes being withdrawn from service.
•
Total path attribute length: Length of the field that lists the path attributes for a feasible route to
a destination.
•
Path attributes: Properties of the routes, including the path origin, the multiple exit
discriminator (MED), the originating node's preference for the route, and data about
aggregation, communities, confederations, and route reflection.
•
Network layer reachability information (NLRI): IP address prefixes of feasible routes being
advertised in the update message.
Keepalive
BGP nodes exchange keepalive messages to learn whether a link or host is down or unavailable.
Keepalive messages are exchanged often enough so that the hold timer does not expire. They
consist only of the BGP header.
When a link is initiated, BGP negotiates the hold time with the neighbor and selects the lesser
interval. The keepalive timer is then set based on the negotiated hold and configured keepalive
intervals with the XSR maintaining a hold interval that is three times the keepalive.
You can reset the default keepalive interval with the
set the variable to 0, keepalives will not be sent. Hold intervals cannot be configured.
Notification
BGP nodes send notification messages when an error condition is learned. After the message is
sent, the BGP session and TCP connection between the BGP nodes are closed. Notification
messages consist of the BGP header, the error code and subcode, and data that describes the error.
Defining BGP Path Attributes
BGP path attributes are defined as a set of parameters describing the traits of a path to a
destination IP prefix. They are used extensively in the route selection process to select the best of
multiple routes, and to build routing policies by matching and setting attributes.
timers bgp keep-alive
Overview
command. If you
XSR User's Guide 6-3
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Troubleshooting
