Juniper BGP, Configuration Manual

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Use the no version to halt the substitution of the AS numbers. Use the default
version to remove the explicit configuration from the peer or peer group and
reestablish inheritance of the feature configuration.
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See neighbor as-override.
Preventing Routing Loops
Routing loops can occur when routes learned from a peer are later advertised back
to that peer. Normally such routing loops are prevented by the AS path attribute.
However, the AS path cannot prevent routing loops in a network configuration with
the following characteristics:
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BGP is running between CE and PE routers.
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You use a single AS number for all customer sites, and have issued the neighbor
as-override command for the PE routers.
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A CE router is dual-homed to two or more PE routers.
The site-of-origin extended community attribute enables BGP to filter out such routes
to prevent routing loops in this network. You can use the
set extcommunity
command to specify a site of origin and then use the match extcommunity command
and an outbound route map to filter routes; for more information, see Extended
Community Lists in the JUNOSe IP Services Configuration Guide .
Alternatively, you can use the neighbor site-of-origin command alone to achieve
the same effect in such a network configuration. Consider the network shown in
Figure 96 on page 449, which enables PE 3 to advertise back to CE 1 routes that it
learned from PE 1 that originated with CE 1. In a typical network configuration, CE
1 rejects these routes because it determines from the AS path that a routing loop
exists. In this particular network, the
neighbor as-override command prevents this
method of detection.
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Configuring BGP VPN Services
JUNOSe 11.1.x BGP and MPLS Configuration Guide