(Optional) Enabling Next-Hop-based Label Allocation on the ASBR

In an inter-AS VPN Option B scenario, ASBRs can be enabled to allocate labels to VPN routes based on next hops. This saves labels on the ASBRs.

Next-hop-based label allocation means to allocate the same label for the routes with the same forwarding behavior. In other words, VPN routes with the same forwarding path and outbound label are assigned the same label. Different from the prefix-based label allocation mode that is used by default, next-hop-based label allocation enrich the label allocation modes and allows for flexible label allocation. In addition, when an ASBR functions as a PE device, next-hop-based label allocation can be used together with one label per instance mode to save labels on the ASBR.

As shown in Figure 1, the inter-AS VPN Option B networking is established; two VPN instances, VPN1 and VPN2, are configured on PE1; the label allocation mode is one label per VPN instance. CE1 in VPN1 and CE2 in VPN2 are respectively imported with 1 thousand VPN routes. When the next-hop-based label allocation feature is not enabled for VPN routes on ASBRs, the 2 thousand routes of PE1 advertised by ASBR1 to ASBR2 use 2 thousand labels; after the next-hop-based label allocation feature is enabled for VPN routes on ASBR1, ASBR1 only assigns one label for VPN routes of the same next hop and outgoing label. As a result, ASBR1 needs to allocate only two labels for 2 thousands routes.

Figure 1 Next-hop-based label allocation for VPN routes on ASBR

After next-hop-based label allocation is enabled or disabled, the label allocated by the ASBR for a route changes, which leads to packet loss.

Perform the following steps on the ASBR.