Juniper JUNOSE 11.2.X BGP AND MPLS, Руководство по настройке

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However, when the OSPF backdoor link connects two sites that are in the same OSPF
area, the undesired result is that the path over the OSPF backdoor link is always preferred
over the path over the backbone.
In Figure 112 on page 482, the OSPF backdoor link connects customer site 4 to customer
site 5 directly, without going through the backbone. OSPF uses the backdoor path for
traffic flow between these two sites for the following reasons:
• At CE 4 and CE 5, the path over the OSPF backdoor link is an intra-area path, whereas
the path over the backbone is an interarea path. OSPF always uses intra-area paths
before interarea paths.
• At PE 2 and PE 3, the OSPF routes received from the respective directly connected CE
routers have a better administrative distance than the IBGP routes received from the
remote PE router. OSPF uses routes with better administrative distances.
OSPF Sham Links
Figure 113 on page 483 shows how you can use OSPF sham links to avoid the problem
created by the intra-area backdoor link. The sham link is a logical intra-area link between
VRF B on PE 2 and PE 3. OSPF creates an adjacency and exchanges LSAs across the
sham link. As a result, OSPF sees both the path over the backdoor link and the path over
the backbone as intra-area paths. OSPF then selects the best path based on the metrics
of the links and selects the sham link path, ensuring that the backdoor link is not used.
NOTE: If the VPN sites are not connected by an OSPF backdoor link or if the VPN sites
are in different OSPF areas, the problem does not exist and you do not need to configure
an OSPF sham link.
Figure 113: OSPF Sham Link
Use the remote-neighbor command to configure the OSPF sham link on both VRFs
joined by the link. If a BGP route and an OSPF route to the same destination are both
installed in the IP routing table, OSPF uses the OSPF route because it has a better
administrative distance by definition.
483 Copyright © 2010, Juniper Networks, Inc.
Chapter 6: Configuring BGP-MPLS Applications