Border Gateway Protocol - CCIE Routing and Switching Exam Certification Guide - page 444

routes advertised by different peers in the same autonomous system. The second, bgp always-
compare-med , enables the comparison of the MED values from neighbors in different auton-
omous systems. The bgp always-compare-med command compares the MED attribute from
different autonomous systems to select a preferred path.
Community Attribute
Although not an attribute in the routing decision process, the community attribute group routes
and applies policies or decisions (accept, prefer) to those routes.
Atomic Aggregate and Aggregator Attributes
The Atomic Aggregate attribute informs BGP peers that the local router is using a less specific
(aggregated) route to a destination.
If a BGP speaker selects a less specific route, when a more specific route is available, it must
attach the Atomic Aggregate attribute when propagating the route. The Atomic Aggregate
attribute lets the BGP peers know that the BGP speaker used an aggregated route.
When you use the Atomic Aggregate attribute, the BGP speaker has the option to send the
Aggregator attribute. The Aggregator attribute includes the AS number and the IP address of
the router that originated the aggregated route. In Cisco routers, the IP address is the RID of the
router that performs the route aggregation. Atomic Aggregate is a well-known attribute and
Aggregator is an optional, transitive attribute.
Weight
Weight is assigned locally on a router to specify a preferred path if multiple paths exist out of
a router for a destination. Weights can be applied to individual routes or to all routes that are
received from a peer. Weight is specific to Cisco routers and is not propagated to other routers.
The weight value ranges from 0 to 65,535. Routes with a higher weight are preferred when
multiple routes exist to a destination. Routes that are originated by the local router have a
default weight of 32,768.
Weight can be used instead of local preference to influence the selected path to external BGP
peers. The difference is that weight is configured locally and not exchanged in BGP updates.
However, the local preference attribute is exchanged between iBGP peers and configured at the
gateway router.
Figure 9-8 shows an example of the use of weight to influence the preferred route.