3com S7906E 설치 설명서

 

1-7 

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.   

Similar to those in OSPF, IS-IS processes can be bound to VPN instances for private network routes to 

be exchanged between CEs and sites. An IS-IS process can be bound to only one VPN instance.  

To use EBGP to exchange private routes between a CE and a site, you need to configure BGP peers for 

VPN instances on CEs and import 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 a CE 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.  

Routing information entries are bound to specific VPN instances on a MCE device, and packets of each 

VPN instance are forwarded between CE and PE according to interface. As a result, VPN routing 

information can be transmitted by performing relatively simple configurations between CE and PE, such 

as importing the VPN routing entries on MCE devices to the routing table of the routing protocol running 

between CEs and PEs.  

The following routing protocols can be used between CE and PE for routing formation exchange:  

z 

Static route