H3C Technologies H3C S7500E Series Switches - Manual (Page 19)

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Normally, when an OSPF route is imported to the BGP routing table as a BGP route on a PE, some
attributes of the OSPF route get lost. When the BGP route is imported to the OSPF routing table on
the remote CE, not all the attributes of the original OSPF routes can be restored. As a result, the
route cannot be distinguished from the routes imported from other domains. In order to distinguish
OSPF routes imported from different OSPF domains, the OSPF routes to be imported to the BGP
routing tables on PEs must carry an attribute (the OSPF domain ID) used to identify the OSPF
domains. The domain ID of an OSPF process is contained in the routes generated by the process.
When an OSPF route is imported to BGP, the domain ID is added to BGP VPN routes as the
extended BGP community.
In cases where a VPN have multiple MCE devices attached to it, when a MCE device advertises the
routes learned from BGP within the VPN, the routes may be learned by other MCE devices, thus
generating route loops. To prevent route loops, you can configure route tags for different VPN
instances on each MCE. It is recommended that a VPN be assigned the same route tag on multiple
MCEs.
IS-IS
Similar to those in OSPF, IS-IS processes can be bound to VPN instances for private network routes
to be exchanged between MCEs and sites. An IS-IS process can be bound to only one VPN
instance.
EBGP
To use EBGP to exchange private routes between an MCE and a site, you need to configure BGP
peers for VPN instances on MCEs and redistribute IGP routing information from corresponding
VPNs. Normally, sites reside in different ASs, so EBGP is used for route exchange. In this case, the
following configurations are needed.
1) Configuring to use EBGP to import IGP routes from each site
To advertise private network routes to PEs properly, IGP routes in the sites directly connected to an
MCE device need to be first imported to the BGP routing table of the MCE device.
2) Configuring a peer group for each VPN instance
For proper route exchange between an MCE and a site, you need to configure a peer group for each
VPN instance and assign AS numbers for these peer groups in BGP IPv4 address family view.
3) Applying filtering policies for route filtering
To make sure that routing information is exchanged between sites and PE devices properly, filtering
policies are applied to filter routes received or to be advertised.
Route Exchange between MCE and PE
Routing information entries are bound to specific VPN instances on an MCE device, and packets of
each VPN instance are forwarded between MCE and PE according to interface. As a result, VPN
routing information can be transmitted by performing relatively simple configurations between MCE
and PE, such as importing the VPN routing entries on MCE devices to the routing table of the
routing protocol running between MCE and PEs.