3com S7906E Instruccion De Instalación

 

1-5 

An import routing policy can further filter the routes that can be advertised to a VPN instance by using 

the VPN 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 import and/or export routing policies as needed. 

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. By default, LSPs are 

used as tunnels and no load balancing occurs (in other words, the number of tunnels for load balancing 

is 1). In addition, a tunneling policy takes effect only within the local AS. 

For basic MPLS L3VPN applications in a single AS, VPN packets are forwarded with two layers of 

labels:  

z 

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. 

z 

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 sites (CEs) belong to the same VPN and are connected to the same PE, each of them only needs 

to know how to reach the remote CE. 

The following takes 

 as an example to illustrate the VPN packet forwarding procedure. 

Figure 1-3 VPN packet forwarding 

 

 

1)  Site 1 sends an IP packet with the destination address of 1.1.1.2. CE 1 transmits the packet to PE 

1. 

2)  PE 1 searches VPN instance entries based on the inbound interface and destination address of the 

packet. Once finding a matching entry, PE 1 labels the packet with both inner and outer labels and 

forwards the packet out. 

3)  The MPLS backbone transmits the packet to PE 2 by outer label. Note that the outer label is 

removed from the packet at the penultimate hop. 

4)  PE 2 searches VPN instance entries according to the inner label and destination address of the 

packet to determine the outbound interface and then forwards the packet out the interface to CE 2.