On an IPv6 provider edge (6PE) networking, by default, each 6PE route is assigned. Therefore, each route advertised to other 6PE peers needs to apply for a label. The number of required labels is directly proportional to the number of 6PE routes. When there are many 6PE routes, a large number of labels are required.

After 6PE routes sharing the explicit null label is enabled, all 6PE routes share the explicit null label, without applying for labels. In such a case, the number of required labels is irrelevant to the number of 6PE routes, saving label resources on 6PE routers.

The explicit null label is a special label which needs to be popped out on the egress PE. The packets then must be forwarded on the basis of IPv6.

In the 6PE networking shown in Figure 9-23, 6PE routes sharing the explicit null label is enabled on 6PE1. 6PE1 then can advertise routes sharing the explicit null label to 6PE2 without applying for a label for each route. When 6PE2 sends data to 6PE1, the data packet carries two labels, the top label being the label assigned by LDP and the bottom label is the explicit null label assigned by MP-BGP. After the data packet reaches 6PE1, 6PE1 pops the explicit null label and forwards the IPv6 data packet to CE1.

Figure 9-23 6PE networking diagram

Note that when you enable or disable 6PE routes sharing the explicit null label after a 6PE peer relationship is set up, temporary packet loss occurs. Therefore, enable this function prior to setting up a 6PE peer relationship.