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Chapter 8

Configuring Basic BGP

Now let's discuss some of these criteria separately. We will also learn how

to configure them.

Configuring the Atomic Aggregate Attribute

When using classless interdomain routing (CIDR), you can create aggregate
routes to minimize the size of routing tables. You can configure aggregate routes
in BGP by redistributing an aggregate route into BGP. The Atomic Aggregate
attribute can be configured using the aggregate-address command. This
command allows you to configure an aggregate or summary entry in the BGP
table. The command has several syntaxes. Let's look at the command and the
possible syntaxes:

aggregate-address

ip-address mask [summary-only] [as-set]

The ip-address and mask indicate the aggregate address to be created.

By default, BGP advertises both aggregate routes and more specific routes.
By using the summary-only syntax, the BGP router will advertise only the
aggregate route. If you use the as-set syntax, the BGP router will advertise
the route as coming from your AS and will set the Atomic Aggregate
attribute to show that information regarding the route may be missing.

Configuring the Weight Attribute

The Weight attribute is a Cisco proprietary attribute used for path selection.
This attribute, which is also considered a metric, allows a system adminis-
trator to manually assign a value to all paths learned from other BGP peers.
The larger the weight value, the more desirable the path.

This metric is particularly helpful when a router is connected to multiple

autonomous systems. The weight assigned stays local to the router on which
it is configured. When paths are learned from multiple sources, the Weight
metric can be used to force BGP to select a specified interface over the others.

This metric is configured using the following command from within the

BGP routing session:

neighbor [

ip-address | peer-group-name] weight weight

The ip-address is the IP address of the neighbor. The peer-group-name

may be used when assigning weight to all routes learned via the BGP peer
group. The weight value has a range from 0 to 65,535. The default value is
32,768.

Configuring the Local Preference Attribute

The Local Preference attribute is used to assign metric values that are used
among IBGP peers. We learned that the Weight metric remains local to a
router. The Local Preference attribute is useful when multiple iBGP peers
have their own eBGP peers.

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