Juniper JUNOSE, Configuration Manual

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The following are some of the most important path attributes:
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AS-path specifies the sequence of autonomous systems that must be crossed to
reach a certain destination. This path attribute is used to avoid routing loops and
to prefer shorter routes over longer routes.
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Next-hop specifies the IP address of the ingress router in the next autonomous
system on the path to the destination.
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Local-pref and multiexit discriminator (MED) are metrics that administrators can
tune to ensure that certain routes are more attractive over other routes. The
local-pref attribute specifies a degree of preference that enables a router to select
among multiple routes to the same prefix. The MED is used for ASs that have
more than one connection to each other. The administrator of one AS sets the
MED to express a degree of preference for one link versus another; the BGP peer
in the other AS uses this MED to optimize traffic.
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Originator-ID specifies the IP address of the router that originates the route. The
router ignores updates that have this attribute set to its own IP address.
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Atomic-aggregate and aggregator inform peers about actions taken by a BGP
speaker regarding aggregation of routes. If a BGP speaker aggregates routes that
have differing path attributes, it includes the atomic-aggregate attribute with the
aggregated prefix to inform update recipients that they must not deaggregate
the prefix. A BGP speaker aggregating routes can include the aggregator attribute
to indicate the router and AS where the aggregation was performed.
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Community and extended community identify prefixes as sharing some common
attribute, providing a means of grouping prefixes and enacting routing policies
on the group of prefixes. A prefix can belong to more than one community. You
can specify a community name as a 32-bit string, a standards-defined well-known
community, or an AS number combined with a 32-bit number to create a unique
identifier. An extended community name consists of either an IP address or an
AS number, combined with a 32-bit or 16-bit number to create a unique identifier.
Transit and Nontransit Service
While an ISP provides connectivity to its customers, it also provides connectivity to
customers of other ISPs. In doing this, an ISP must be able to ensure the appropriate
use of its resources.
For example, Figure 6 on page 12 shows three ISPs and three customers. ISP 1, ISP
2, and ISP 3 are directly connected to one another through a physical link and a
corresponding EBGP session (represented here by a single line). Customer 1 is
connected to ISP 1 through a physical link and corresponding EBGP session. Customer
2 is similarly connected to ISP 2, and Customer 3 is similarly connected to ISP 3.
Each ISP provides
transit service to its own customers. Figure 6 on page 12 illustrates
how the ISP permits traffic to transit across its backbone from its own customers or
to its own customers.
Overview
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Chapter 1: Configuring BGP Routing