[实验手册]MPLS/***分解：防止PE-CE的路由环路

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Router CE1 and Router CE2 are located in the same OSPF
area. These CE routers are linked together by a Layer 3 *** over Router
PE1 and Router PE2. In addition, Router CE1 and Router CE2 are
connected by an intraarea link used as a backup.

OSPF treats the link through
the Layer 3 *** as an interarea link. By default, OSPF prefers
intraarea links to interarea links, so OSPF selects the backup
intraarea link as the active path. This is not acceptable in
configurations where the intraarea link is not the expected primary
path for traffic between the CE routers. An OSPF sham link is also an intraarea link, except that it is configured between the PE routers. You can configure the metric for the sham link to
ensure that the path over the Layer 3 *** is preferred to a backup path
over an intraarea link connecting the CE routers.

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You should configure an OSPF sham link under the following circumstances:
Two CE routers are linked together by a Layer 3 ***.
These CE routers are in the same OSPF area.
An intraarea link is configured between the two CE routers.

If there is no intraarea link between the CE routers, you do not need to configure an OSPF sham link.

For more information on OSPF sham links, see the Internet draft draft-ietf-l3***-ospf-2547-01.txt, OSPF as the PE/CE Protocol in BGP/MPLS ***s.

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