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In this solution, PEs exchange VPN-IPv4 routes over a multihop MP-EBGP session. Each PE must
have a route to the peer PE and a label for the route so that the inter-AS public tunnel between the
PEs can be set up. Inter-AS option C sets up a public tunnel by using the following methods:
•
A label distribution protocol within the AS, for example, LDP.
•
Labeled IPv4 unicast route advertisement by ASBRs through BGP.
Labeled IPv4 unicast route advertisement refers to the process of assigning MPLS labels to
IPv4 unicast routes and advertising IPv4 unicast routes and their labels.
Figure 54 Network diagram for inter-AS option C
As shown in
Figure
54, in VPN 1, routes are advertised from CE 1 to CE 3 by using the following
process:
1.
PE 1 advertises the VPN routes learned from CE 1 as VPN-IPv4 routes to PE 3 through
multihop MP-EBGP.
Assume that the inner label assigned by PE 1 for the routes is Lx.
2.
PE 3 advertises the received routes to CE 3.
Setting up an inter-AS public tunnel is difficult in this solution. A public tunnel, for example, the one
from PE 3 to PE 1, is set up by using the following process:
1.
Within AS 100, the public tunnel from ASBR 1 to PE 1 is set up by using a label distribution
protocol, for example, LDP.
Assume that the outgoing label for the public tunnel on ASBR 1 is L1.
2.
ASBR 1 advertises labeled IPv4 unicast routes to ASBR 2 through EBGP.
The route destined for PE 1 and the label (L2) assigned by ASBR 1 for the route are advertised
from ASBR 1 to ASBR 2. The next hop of the route is ASBR 1. The public tunnel from ASBR 2
to ASBR 1 is set up. The incoming label for the public tunnel on ASBR 1 is L2.
3.
ASBR 2 advertises labeled IPv4 unicast routes to PE 3 through IBGP.
The route destined for PE 1 and the label (L3) assigned by ASBR 2 for the route are advertised
from ASBR 2 to PE 3. The next hop for the route is ASBR 2. The public tunnel from PE 3 to
ASBR 2 is set up. 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. Within AS 200, the 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.
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