Information Technology Reference
In-Depth Information
Usually, BGP synchronization is disabled, which allows BGP to converge more quickly
because BGP does not have to wait for the IGP to converge. It can be disabled if the AS is not
a transit AS, or if all transit routers run BGP. You can disable BGP synchronization with the
following command:
no bgp synchronization
BGP Attributes, Weight, and the BGP Decision Process
BGP is a simple protocol that uses route attributes to make a selection for the best path to a
destination. This section describes BGP attributes, the use of weight to influence path selection,
and the BGP decision process.
BGP Path Attributes
BGP uses several attributes for the path selection process. BGP uses path attributes to
communicate routing policies. BGP path attributes include the following: next hop, local
preference, number of AS path hops, origin, Multi-Exit Discriminator (MED), atomic
aggregate, and aggregator.
BGP attributes can be categorized as well-known or optional . Well-known attributes are
recognized by all BGP implementations. Optional attributes do not need to be supported by the
BGP process.
Well-known attributes can be further subcategorized as mandatory or discretionary. Mandatory
attributes are always included in BGP update messages. Discretionary attributes might or might
not be included in the BGP update message.
Optional attributes can be further subcategorized as transitive or nontransitive . Routers must
advertise the route with transitive attributes to its peers even if is does not support the attribute
locally. If the path attribute is nontransitive, the router does not need to advertise the route to its
peers.
Each attribute category is covered in the sections that follow. Know the category of each
attribute for the exam.
Next-Hop Attribute
The next-hop attribute is simply the IP address of the next eBGP hop that you use to reach the
destination. The next-hop attribute is a well-known, mandatory attribute. With eBGP, the next
hop is set by the eBGP peer when it announces the route. The next-hop attribute is used on
multiaccess networks where there is more than one BGP speaker. With iBGP, routers advertise
the next hop as the external (eBGP) peer instead of itself. In fully meshed multiaccess networks,
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