Инструкции По Установке для 3com S7906E

 

1-20 

z 

Finally, PE 2 redistributes the BGP VPN routes into OSPF and advertises them to CE 21 and CE 

22. 

Figure 1-16 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.  

To avoid routing loops, when creating Type 3 LSAs, the PE always sets the flag bit DN for BGP VPN 

routes learnt from MPLS/BGP, regardless of whether the PE and the CEs are connected through the 

OSPF backbone. When performing route calculation, the OSPF process of the PE ignores the Type 3 

LSAs whose DN bit is set. 

If the PE needs to advertise to a CE the routes from other OSPF domains, it must indicate that it is the 

ASBR, and advertise the routes using Type 5 LSAs. 

Generally, BGP peers carry routing information on the MPLS VPN backbone through the BGP extended 

community attributes. The OSPF that runs on the remote PE can use the information to create Type 3 

summary LSAs to be transmitted to the CEs. As shown in 

, both site 1 and site 2 belong to 

VPN 1 and OSPF area1. They are connected to different PEs, PE 1 and PE 2. There is an intra-area 

OSPF link called backdoor link between them. In this case, the route connecting the two sites through 

PEs is an inter-area route. It is not preferred by OSPF because its preference is lower than that of the 

intra-area route across the backdoor link.