189
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
768HFigure 53, to improve scalability, you can specify a route reflector (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 53 Network diagram for inter-AS option C using RRs
227BCarrier's carrier
If a customer of an MPLS L3VPN service provider is also a service provider:
• The MPLS L3VPN service provider is called the provider carrier or the Level 1 carrier.
• The customer is called the customer carrier or the Level 2 carrier.
This networking model is referred to as carrier's carrier.
The PEs of the Level 2 carrier directly exchange customer networks over a BGP session. The Level
1 carrier only learns the backbone networks of the Level 2 carrier, without learning customer
networks.
VPN 1
PE 1
PE 3
VPN 1
CE 3
CE 2
VPN 2 VPN 2
PE 4
CE 4
CE 1
AS 100
PE 2
MP-IBGP
MP-IBGP
ASBR 1
(PE)
RR 1
RR 2
ASBR 2
(PE)
IBGP
IBGP
EBGP
Multi-hop MP-EBGP
VPN LSP
LSP
AS 200
MP-IBGP
M
P
-IBGP
MPLS backbone
MPLS backbone
Private LSP
Public LSP
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