Enterasys Networks X-PeditionTM Defining BGP Path Attributes, Update, Keepalive, Notification

Overview

•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•Unfeasible routes length: Length of the field that lists the routes being withdrawn from service because they are judged no longer reachable.

•Withdrawn routes: IP address prefixes for the routes being withdrawn from service.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 timers bgp keep-alivecommand. If you 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.

XSR User’s Guide 6-3