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The SoO attribute specifies the site where the route update is originated. It prevents the receiving
router from advertising the route update back to the originating site. If the AS-path attribute is lost,
the router can use the SoO attribute to avoid routing loops.
The SoO attribute has the following formats:
•
16-bit AS number
:
32-bit user-defined number
. For example, 100:1.
•
32-bit IPv4 address
:
16-bit user-defined number
. For example, 172.1.1.1:1.
•
32-bit AS number
:
16-bit user-defined number
, where the minimum value of the AS number is
65536. For example, 65536:1.
NOTE:
A route update can contain one SoO attribute at most.
MP-BGP
MP-BGP advertises VPN composition information and routes between PEs. It is backward
compatible and supports both traditional IPv4 address family and other address families, such as
VPN-IPv4 address family.
Using MP-BGP can guarantee that private routes of a VPN are advertised only in the VPN and
implement communications between MPLS VPN members.
Routing policy
In addition to the import and export extended communities for controlling VPN route advertisement,
you can also configure import and export routing policies to control the redistribution and
advertisement of VPN routes more precisely.
An import routing policy can further filter the routes that can be advertised to a VPN instance by using
the route target attribute of import target attribute. It can reject the routes selected by the
communities in the import target attribute. An export routing policy can reject the routes selected by
the communities in the export target attribute.
After a VPN instance is created, you can configure an import routing policy, an export routing policy,
or both as needed.
Tunneling policy
A tunneling policy is used to select the tunnel for the packets of a specific VPN instance to use.
After a VPN instance is created, you can optionally configure a tunneling policy for the VPN instance.
By default, only one tunnel is selected (no load balancing) in this order: LSP tunnel, GRE tunnel,
CR-LSP tunnel. A tunneling policy takes effect only within the local AS.
MPLS L3VPN packet forwarding
For basic MPLS L3VPN applications in a single AS, VPN packets are forwarded with the following
layers of labels:
•
Layer 1 labels
—Outer labels, used for label switching inside the backbone. They indicate
LSPs from the local PEs to the remote PEs. Based on Layer 1 labels, VPN packets can be label
switched along the LSPs to the remote PEs.
•
Layer 2 labels
—Inner labels, used for forwarding packets from the remote PEs to the CEs. An
inner label indicates to which site, or more precisely, to which CE the packet should be sent. A
PE finds the interface for forwarding a packet according to the inner label.
If two CEs belong to the same VPN and are connected to the same PE, each CE only needs to know
how to reach the other CE.