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.
If you redistribute OSPF routes into BGP in each VRF, you do not want the OSPF
routes that point to sham links to be redistributed into BGP. If they were redistributed,
multiple BGP routes for a single OSPF route would exist: one BGP route at each
endpoint of a sham link.
Use the dont-install-routes command to prevent OSPF routes pointing to the sham
link from being installed in the IP routing table of the VRF, and thus to prevent them
from being redistributed into BGP. Forwarding still works using the MP-IBGP routes
received from the remote PE router.
Use the ttl command to configure a TTL for the remote neighbor because the neighbor
might be more than a single hop away. Use the update-source command to specify
the loopback address used as the source address for the OSPF connection to the
remote neighbor.
If you do not configure a sham link between each pair of PE routers for which a
backdoor link exists, then you need to redistribute BGP routes back into OSPF.
For more information about OSPF remote neighbors, see Remote Neighbors in the
JUNOSe IP, IPv6, and IGP Configuration Guide.
dont-install-routes
â– Use to prevent any OSPF routes that point directly to the OSPF remote neighbor
from being installed in the IP routing table of the VR or VRF in which OSPF is
running.
â– Using this command avoids having many BGP routes to the same prefix by
preventing OSPF routes learned over the sham link from being redistributed
back into BGP even when you have configured redistribution of OSPF routes into
BGP.
484 â– OSPF and BGP/MPLS VPNs
JUNOSe 11.1.x BGP and MPLS Configuration Guide
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