Virtual Acquisition and Forwarding Tables
To abutment assorted customers, MPLS VPN standards accommodate the abstraction of a basic router. This
feature, alleged a VRF table, can be acclimated to abundance routes alone for altered chump VPNs.
The use of abstracted tables solves allotment of the problems of preventing one customer’s packets from
leaking into addition customer’s arrangement due to overlapping prefixes, while acceptance all sites in the
same chump VPN to communicate.
A VRF exists central a distinct MPLS-aware router. Typically, routers charge at atomic one VRF for each
customer absorbed to that accurate router. For example, in Amount 19-12, router PE2 connects to
CE routers in barter A and B but not in chump C, so PE2 would not charge a VRF for customer
C. However, PE1 connects to CE routers for three customers, so PE1 will charge three different
VRFs.
For added circuitous designs, a PE ability charge assorted VRFs to abutment a distinct customer. Using
Figure 19-12 afresh as an example, PE1 connects to two CEs of chump A (CE-A1 and CE-A4).
If hosts abreast CE-A1 were accustomed to admission a centralized aggregate account (not apparent in the figure)
and hosts abreast CE-A4 were not accustomed access, again PE1 would charge two VRFs for chump A—
one with routes for the aggregate service’s subnets and one after those routes.
720 Chapter 19: Multiprotocol Label Switching
Each VRF has three capital components, as follows:
■ An IP acquisition table (RIB)
■ A CEF FIB, busy based on that VRF’s RIB
■ A abstracted instance or action of the acquisition agreement acclimated to barter routes with the CEs
that charge to be accurate by the VRF
For example, Amount 19-14 shows added detail about router PE2 from Amount 19-12, now with
MPLS VPNs implemented. In this case, PE2 will use RIP-2 as the IGP to both chump A (router
CE-A2) and chump B (router CE-B2). (The best of acquisition agreement acclimated from PE-CE is
unimportant to the abyss of explanations apparent here.)
Figure 19-14 Adding Routes Learned from a CE to VRFs on Router PE2
The amount shows three alongside accomplish that action with anniversary of the two customers. Note that footfall 1
for anniversary chump does not action at the aforementioned burning in time, nor does footfall 2, nor footfall 3; the figure
lists these accomplish with the aforementioned numbers because the aforementioned action occurs at anniversary step. The explanation
of the accomplish is as follows:
1. The CE router, which has no ability of MPLS at all, advertises a avenue for 10.3.3.0/24 as
normal—in this case with RIP-2.
2. In the top instance of footfall 2, the RIP-2 amend arrives on PE3’s S0/1/0, which has been
assigned to chump A’s VRF, VRF-A. PE2 uses a abstracted RIP action for anniversary VRF, so
PE2’s VRF-A RIP action interprets the update. Similarly, the VRF-B RIP action analyzes
the amend accustomed on S0/1/1 from CE-B2.
CE-A2
CE-B2
PE2
Prefix
10.3.3.0/24
Out Int.
S0/1/0
RIB – VRF-A
Next-Hop
192.168.37.7
3
1
2
2
VRF-A RIP
Process:
Prefix
10.3.3.0/24
Out Int.
S0/1/1
RIB – VRF-B
Next-Hop
192.168.38.8
3 VRF-B RIP
Process:
S0/1/0 —VRF-A 192.168.37.7
S0/1/1 —VRF-B 192.168.38.8
Subnet
10.3.3.0/24
Customer A
Customer B
Subnet
10.3.3.0/24
RIP-2
10.3.3.0/24
1 RIP-2
10.3.3.0/24
3. In the top instance of footfall 3, the VRF-A RIP action adds an access for 10.3.3.0/24 to the RIB
for VRF-A. Similarly, the basal instance of footfall 3 shows the RIP action for VRF-B adding
a avenue to prefix 10.3.3.0/24 to the VRF-B RIB.
NOTE Anniversary VRF additionally has a FIB, which was not included in the figure. IOS would add an
appropriate FIB access for anniversary RIB entry.