DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
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As shown below, if the link between PE and CE is in non-0 area, then PE will redistribute the
VPNv4 route advertised by MP-BGP to OSPF and restore to type-3 LSA to be advertised to CE1,
which will not calculate the non-backbone area LSAs. Therefore, these LSAs will not be
advertised to routers in Area0, and intra-VPN sites won't learn the route to other sites.
Accordingly, special attention shall be paid when deploying non-0 area between PE and CE.
Generally, in L3VPN applications, if OSP protocol is run between PE and CE to exchange VPN
routes, it is suggested not to deploy backbone area at intra-VPN site. In practice, if intra-VPN
routers other than the PE sites also fall into the backbone area, then there must be at least one
router at this intra-VPN site to connect with PE, and the link between CE and PE must belong to
Area 0, so that intra-area routes and external routes can be propagated between PE and VPN
site.
Sham link
Sham-link is not a real link. It refers to the "virtual link" established between the VRFs of two PEs.
The Sham-link is same as the normal OSPF link. With its own OSPF interface, it can send OSPF
protocol packets, establish neighbors and send LSAs. When LSAs are flooded over the sham
link, all OSPF route types won't be changed, as shown below.
