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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
2-18
The advantage of this MP-EBGP scheme is that you are not required to configure a sub-interface
for each site of VPN users on an ASBR. You are also not required to set up the inter-AS LSP.
The VPN routes are directly transmitted between single-hop MP-EBGP neighbors. The VPN
routing information, however, is maintained and spread by the ASBRs between ASs. If a large
number of VPN routes exist, the ASBRs are faced with heavy pressures. Since the ASBRs also
generally assume forwarding tasks of IP packets on the public network, high requirements are
imposed on these devices. In addition, the ASBRs cancel the RT filtering function for received
VPN routes. The VPN routes on PEs may be spread to the ASBRs in another AS. This may lead
to the leakage of VPN routes. As a result, the SPs, who exchange VPN routes, must reach trust
agreements on route exchanging. The ASBRs should trust each other and perform
corresponding route filtering policies. The OptionB scheme is applicable to networks with lots of
inter-AS VPN services.
OptionB has two schemes:
The ASBR does not change the next hop of a VPN route.
The ASBR changes the next hop of a VPN route.
The following describes the configuration procedures of the two schemes.
Scheme 1: Next Hop Unchanged
When an ASBR receives VPN routes sent from the ASBR in another AS and sends the routes to
the MP-IBGP neighbors in the local AS, the next hop of the routes is not changed. This mode is
called the "OptionB Next Hop Unchanged Scheme". In this mode, the PEs and ASBRs in an AS
still set up MP-IBGP sessions to exchange VPN routes and the two ASBRs set up MP-EBGP
sessions to directly exchange VPN routes. When sending routes to an MP-IBGP neighbor, the
ASBR does not change the next hop of the VPN routes received from the MP-EBGP neighbor.
This requires that the PE in the AS should have a route to the next hop address (that is, the
ASBR in another AS). For this purpose, you can configure the local ASBR to redistribute routes
destined for the other ASBR to the IGP protocol in the local AS. In this manner, the address of
the ASBR in another AS becomes reachable and you can set up an LSP through the LDP.
The configuration procedure is as follows:
Configuring Route Exchanging Between PEs and CEs
Configuring an IGP and MPLS Signaling Protocol in an AS
Configuring an ASBR to Cancel the Default RT Filtering Function
Configuring PEs and ASBRs in the Same AS to Exchange VPN Routing Information