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Figure 53 Network diagram for inter-AS option B
VPN 1
CE 1
MPLS backbone
AS 100
CE 2
VPN 2
As shown in
process:
1.
PE 1 advertises the VPN routes learned from CE 1 to ASBR 1 through MP-IBGP.
Assume that the inner label assigned by PE 1 for the routes is L1.
2.
ASBR 1 advertises the VPN-IPv4 routes to ASBR 2 through MP-EBGP.
Before advertising the routes, ASBR 1 modifies the next hop as its own address, assigns a new
inner label (L2) to the routes, and associates L1 with L2.
3.
ASBR 2 advertises the VPN-IPv4 routes to PE 3 through MP-IBGP.
Before advertising the routes, ASBR 2 modifies the next hop as its own address, assigns a new
inner label (L3) to the routes, and associates L2 with L3.
4.
PE 3 advertises the received routes to CE 3.
A packet is forwarded from CE 3 to CE 1 by using the following process:
1.
PE 3 encapsulates the received packet with two labels, and forwards the encapsulated packet
to ASBR 2.
One of the labels is L3, and the other is the outer tag for the public tunnel from PE 3 to ASBR 2.
2.
ASBR 2 removes the outer tag, replaces L3 with L2, and forwards the packet to ASBR 1.
Packets between ASBR 1 and ASBR 2 carry only one inner label.
3.
ASBR 1 replaces L2 with L1, adds the outer tag of the public tunnel from ASBR 1 to PE 1, and
forwards the packet to PE 1.
4.
PE 1 removes the inner label and outer tag and forwards the packet to CE 1.
In this solution, ASBRs must receive all inter-AS VPN routes. Therefore, ASBRs cannot filter
incoming VPN-IPv4 routes by route targets.
Inter-AS option B has better scalability than option A. However, it requires that ASBRs maintain and
advertise VPN routes.
Inter-AS option C
The Inter-AS option A and option B solutions require that the ASBRs maintain and advertise
VPN-IPv4 routes. When every AS needs to exchange a great amount of VPN routes, the ASBRs
might become bottlenecks, which hinders network extension. Inter-AS option C has better scalability
because it makes PEs directly exchange VPN-IPv4 routes.
ASBR 1
PE 1
(PE)
MP-EBGP
PE 2
VPN LSP 1
VPN LSP2
LSP 1
Figure
53, in VPN 1, routes are advertised from CE 1 to CE 3 by using the following
VPN 1
ASBR 2
PE 3
(PE)
MPLS backbone
AS 200
PE 4
VPN LSP 3
LSP 2
Private LSP
VPN 2
Public LSP
196
CE 3
CE 4
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