aggregating routes can include the aggregator attribute to indicate the router and AS
where the aggregation was performed.
•
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.
Figure 6: Transit Service
Each ISP provides nontransit service to other ISPs. For example, Figure 7 on page 13 shows
that ISP 1 does not permit traffic between ISP 2 and ISP 3 to cross its backbone. If ISP 1
permits such traffic, it squanders its own resources with no benefit to its customers or
itself.
Copyright © 2010, Juniper Networks, Inc.12
JunosE 11.2.x BGP and MPLS Configuration Guide
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