Operation Manual – MPLS L3VPN
H3C S9500 Series Routing Switches
Chapter 1 MPLS L3VPN Configuration
1-20
can exchange OSPF routing information as they are using dedicated lines. This
improves the network management and makes OSPF applications more effective.
As shown in
Figure 1-15
, PE 1 and PE 2 are connected through the MPLS backbone,
while CE 11, CE 21, and CE 22 belong to VPN 1. Assumes that all the devices in the
figure belong to the same AS, that is, CE 11, CE 21, and CE 22 belong to the same
OSPF domain. The advertisement procedure of VPN 1 routes is as follows:
z
At first, PE 1 redistributes OSPF routes from CE 11 into BGP.
z
Then, PE 1 advertises the VPN routes to PE 2 through BGP.
z
Finally, PE 2 redistributes the BGP VPN routes into OSPF and advertises them to
CE 21 and CE 22.
Figure 1-15
Application of OSPF in VPN
With the standard BGP/OSPF interaction, PE 2 advertises the BGP VPN routes to CE
21 and CE 22 through Type 5 LSAs (ASE LSAs). However, CE 11, CE 21, and CE 22
belong to the same OSPF domain, and the route advertisement between them should
use Type 3 LSAs (inter-provider routes).
To solve the above problems, PE uses an extended BGP/OSPF interaction process
called BGP/OSPF interoperability to advertise routes from one site to another,
differentiating the routes from real AS-External routes. The process requires that
extended BGP community attributes carry the information for identifying the OSPF
attributes.
It is required that each OSPF domain has a configurable domain ID. It is recommended
to configure for all OSPF instances in the network related to each VPN instance the
same domain ID, or adopt the default ID. Thus, the system can know that all VPN
routes with the same domain ID are from the same VPN instance.
3) Routing
loop
detection
If OSPF runs between CEs and PEs and a VPN site is connected to multiple PEs, when
a PE advertises the BGP VPN routes learnt from MPLS/BGP to the VPN site through
LSAs, the LSAs may be received by another PE, resulting in a routing loop.
