The MPLS VPN Abstracts Plane
The explanations of the VRF, RD, and RT appearance explain best of the capacity of the MPLS VPN
control plane. VRFs acquiesce PEs to abundance routes abstruse from assorted CEs, alike if the prefixes
overlap. The RD allows PEs to acquaint routes as different prefixes, alike if the IPv4 prefixes happen
to overlap. Finally, the RT tells the PEs which routes should be added to anniversary VRF, which provides
greater ascendancy and the adeptness to acquiesce sites to be attainable from assorted VPNs.
MPLS VPNs 727
At the end of the process, however, to abutment the forwarding of packets, admission PEs charge appropriate
FIB entries, with Ps and PEs defective adapted LFIB entries. This area focuses on
explaining how LSRs ample the FIB and LFIB back application MPLS VPNs.
As accepted for this chapter, this area focuses on how to advanced packets to subnet 10.3.3.0/24 in
the chump A VPN. To activate this assay of the MPLS VPN abstracts plane, accede Amount 19-
18. This amount repeats the aforementioned forwarding archetype in Amount 19-13 but now shows a few details
about the FIB in the admission PE and the LFIB entries in the P and departure PE routers.
Figure 19-18 The Admission PE FIB and Other Routers’ LFIBs
The numbered accomplish in the amount are as follows:
1. An unlabeled packet arrives on an interface assigned to VRF-A, which will account admission PE1
to use VRF-A’s FIB to accomplish a forwarding decision.
2. Admission PE1’s VRF-A FIB access for 10.3.3.0/24 lists an approachable interface of S0/0/1, and a
label assemblage with two labels—an close characterization of 3333 and an alien characterization of 1111. So PE1
forwards the packet with these two labels pushed in advanced of the IP header.
3. P1 uses the LFIB access for admission (local) characterization 1111, swapping this alien characterization amount to
2222.
4. PE2 does two LFIB lookups. PE2 finds characterization 2222 in the table and ancestor that label, abrogation the
inner label. Then PE2 looks up the close characterization 3333 in the LFIB, acquainted the pop activity as well,
along with the approachable interface. So PE2 assiduously the unlabeled packet out interface S0/1/0.
PE1 P1 PE2
CE-A1 CE-A2
Inner: IP
3333
IP
IP
Inner: IP
3333
Outer:
1111
Outer:
1111
Int. In
VRF-A
S0/0/1 S0/1/0
Int. In S0/0/1
VRF-A
1
2 3
2 3
4
4
In Label
2222
3333
Action
pop
pop
Out Int.
N/A
S0/1/0
PE2 LFIB
In Label
1111
Out Label
2222
Out Int.
S0/1/0
P1 LFIB
Prefix
10.3.3.0/24
Out Int.
S0/0/1
CEF FIB — VRF-A
Out Label(s)
1111, 3333
The archetype shows the mechanics of what happens in the abstracts alike already the actual FIB and
LFIB entries accept been added. The blow of this affair about the MPLS VPN abstracts alike examines
how MPLS VPN LSRs body these actual entries. While account this section, it is accessible to keep
in apperception a brace of capacity about the purpose of the close and alien characterization acclimated for MPLS VPNs:
■ The alien characterization identifies the segments of the LSP amid the admission PE and the departure PE,
but it does not analyze how the departure PE should advanced the packet.
■ The close characterization identifies the departure PE’s forwarding details, in accurate the outgoing
interface for the unlabeled packet.
NOTE As was the case with the archetype apparent in Amount 19-13, the capacity at Accomplish 3 and 4
will alter hardly in practice, as a aftereffect of the PHP feature, which is explained about Figure
19-23 at the end of this chapter.