MPLS VPNs 715
The Problem: Duplicate Chump Abode Ranges
When an SP connects to a advanced array of barter application a Layer 2 WAN account such as Frame
Relay or ATM, the SP does not affliction about the IP acclamation and subnets acclimated by those customers.
However, in adjustment to drift those aforementioned barter to a Layer 3 WAN service, the SP charge learn
address ranges from the assorted barter and again acquaint those routes into the SP’s network.
However, alike if the SP capital to apperceive about all subnets from all its customers, abounding enterprises
use the aforementioned abode ranges—namely, the clandestine IP arrangement numbers, including the ever-popular
network 10.0.0.0.
If you approved to abutment assorted barter application MPLS unicast IP acquisition alone, the routers would
be abashed by the overlapping prefixes, as apparent in Figure 19-11. In this case, the arrangement shows
five of the SP’s routers central a cloud. Three barter (A, B, and C) are shown, with two
customer routers affiliated to the SP’s network. All three barter use arrangement 10.0.0.0, with
the three chump sites on the appropriate all application subnet 10.3.3.0/24.
Figure 19-11 The Main Challenge with Supporting Layer 3 VPNs
The aboriginal and best basal ambition for a Layer 3 VPN account is to acquiesce chump A sites to
communicate with chump A sites—and alone chump A sites. However, the arrangement in Figure
19-11 fails to accommodated this ambition for several reasons. Because of the overlapping abode spaces,
several routers would be faced with the bind of allotment one customer’s avenue to 10.3.3.0/24
as the best route, and blank the avenue to 10.3.3.0/24 abstruse from addition customer. For
example, PE2 would apprentice about two altered 10.3.3.0/24 prefixes. If PE2 chooses one of the two
possible routes—for example, if PE2 best the avenue to CE-A2 as best—then PE2 could not
CE-A1
CE-A4
CE-B1
CE-C1
Customer A
Customer B
Customer C
CE-C2
Customer C
Subnet
10.3.3.0/24
PE2
PE4
PE1 P1
SP Network
P2
IGP
IGP
IGP
10.3.3.0/24
IGP
10.3.3.0/24
IGP
10.3.3.0/24
IGP
IGP
IGP
IGP
CE-A2
Customer A
Subnet
10.3.3.0/24
CE-B2
Customer B
Subnet
10.3.3.0/24
Which avenue to
10.3.3.0/24 is better?
forward packets to chump B’s 10.3.3.0/24 off router CE-B2. Also, a possibly worse aftereffect is that
hosts in one chump armpit may be able to accelerate and accept packets with hosts in addition customer’s
network. Following this aforementioned example, hosts in chump B and C sites could advanced packets to
subnet 10.3.3.0/24, and the routers ability advanced these packets to chump A’s CE-A2 router.