216
Based on these solutions, there are three ways to implement inter-AS MD VPN:
•
MD VPN inter-AS option A
•
MD VPN inter-AS option B
•
MD VPN inter-AS option C
MD VPN inter-AS option A
•
Two VPN instances are in AS 1 and AS 2.
•
PE 3 and PE 4 are ASBRs for AS 1 and AS 2, respectively.
•
PE 3 and PE 4 are interconnected through their respective VPN instance and treat each other
as a CE device.
Figure 78 MD VPN inter-AS option A
To implement MD VPN inter-AS option A, a separate MD must be created in each AS. Multicast data
is transmitted between the VPNs in different ASs through the MDs.
Multicast packets of VPN instance 1 are delivered as follows:
1.
CE 1 forwards the multicast packet of VPN instance 1 to PE 1.
2.
PE 1 encapsulates the multicast packet into a public network packet and forwards it to PE 3
through the MTI interface in MD 1.
3.
PE 3 considers PE 4 as a CE device of VPN instance 1, so PE 3 forwards the multicast packet
to PE 4.
4.
PE 4 considers PE 3 as a CE device of VPN instance 2, so it forwards the multicast packet to
PE 2 through the MTI interface in MD 2 on the public network.
5.
PE 2 forwards the multicast packet to CE 2.
Because only VPN multicast data is forwarded between ASBRs, different PIM modes can run within
different ASs. However, the same PIM mode must run on all interfaces that belong to the same VPN
(including interfaces with VPN bindings on ASBRs).
MD VPN inter-AS option B
In MD VPN inter-AS option B, RPF vector and BGP connector are introduced:
•
RPF vector—Attribute encapsulated in a PIM join message. It is the next hop of BGP MDT route
from the local PE device to the remote PE device. Typically, it is the ASBR in the local AS.
When a device receives the join message with the RPF vector, it first checks whether the RPF
vector is its own IP address. If so, the device removes the RPF vector, and sends the message
to its upstream neighbor according to the route to the remote PE device. Otherwise, it keeps the
RPF vector, looks up the route to the RPF vector, and sends the message to the next hop of the
MD 2
AS 2
PE 1'
PE 3'
CE 1
PE 1"
PE 3"
CE 2
PE 2"
PE 4'
PE 2'
AS 1
MD 1
PE 4"
VPN instance 1
Public instance
MT 1
MT 2
VPN instance 2
P 1
P 2
MTI 1
MTI 1
MTI 2
MTI 2
VPN instance-VPN instance
ASBR
ASBR
