H3C Technologies H3C S6800 Series Switches - Manual (Page 184)

171
ASBR 2 assigns a label (L3) to the route destined for PE 1, and advertises the route and its label (L3)
to PE 3. The next hop for the route is ASBR 2. The incoming label for the public tunnel on ASBR 2
is L3, and the outgoing label is L2.
4. MPLS packets cannot be forwarded directly from PE 3 to ASBR 2. Therefore, another public tunnel
from PE 3 to ASBR 2 is required to be set up hop by hop through a label distribution protocol, for
example, LDP.
Assume that the outgoing label for the public tunnel on PE 3 is Lv.
After route advertisement and public tunnel setup, a packet is forwarded from CE 3 to CE 1 by using the
following process:
1. PE 3 performs the following routing table lookups for the packet:
a. Finds a matching route with next hop PE 1 and inner label Lx, and encapsulates the packet with
label Lx.
b. Finds the route to PE 1 with next hop ASBR 2 and label L3, and encapsulates the packet with
label L3 as the outer label.
c. Finds the route to ASBR 2 with outgoing label Lv, and encapsulates the packet with label Lv as
the outmost label.
2. AS 200 transmits the packet to ASBR 2 by the outmost label.
3. ASBR 2 removes the outmost label, replaces L3 with L2, and forwards the packet to ASBR 1.
4. ASBR 1 replaces L2 with L1, and forwards the packet.
5. AS 100 transmits the packet to PE 1 by the outer label.
6. PE 1 removes the outer label, and forwards the packet to CE 1 according to the inner label Lx.
As shown in Figure 49 , to improve scalability, you can specify an RR in each AS to exchange VPN-IPv4
routes with PEs in the same AS. The RR in each AS maintains all VPN-IPv4 routes. The RRs in two ASs
establish a multihop MP-EBGP session to advertise VPN-IPv4 routes.
Figure 49 Network diagram for inter-AS option C using RRs
文件中找不到关系 ID 为 rId71 的图像部件。