Creating LFIB Entries to Forward Packets to the Egress PE
The outer label defines the LSP from the ingress PE to the egress PE. More specifically, it defines
an LSP used to forward packets to the BGP next-hop address as advertised in BGP Updates. In
concept, the ingress PE adds the outer label to make a request of the core of the MPLS network to
“deliver this packet to the egress PE—which advertised this particular BGP next-hop address.”
MPLS VPNs use an IGP and LDP to learn routes and labels, specifically to learn the label values
to use in the outer label. To link the concepts together, it can be helpful to think of the full control
plane process related to the LSP used for the outer label, particularly Step 4 onward:
1. A PE, which will be an egress PE for this particular route, learns routes from some CE.
2. The egress PE uses IBGP to advertise the routes to an ingress PE.
3. The learned IBGP routes list some next-hop IP address.
4. For MPLS VPNs to work, the PE and P routers must have advertised a route to reach the BGP
next-hop addresses.
5. Likewise, for MPLS VPNs to work, the PE and P routers must have advertised labels with
LDP for the routes to reach the BGP next-hop addresses.
730 Chapter 19: Multiprotocol Label Switching
6. Each P and PE router adds its part of the full end-to-end LSP into its LFIB, supporting the
ingress PE’s ability to send a packet to the egress PE.
For example, Figure 19-19 shows PE2 advertising two routes to PE1, both with BGP next-hop IP
address 3.3.3.3. For MPLS to work, the collective PE and P routers need to advertise an IGP route
to reach 3.3.3.3, with LDP advertising the labels, so that packets can be label switched toward the
egress PE. Figure 19-20 shows the basic process; however, note that this part of the process works
exactly like the simple IGP and LDP process shown for unicast IP forwarding in the first half of
this chapter.
Figure 19-20 Creating the LFIB Entries to Reach the Egress PE’s BGP Next Hop
The steps in the figure focus on the LFIB entries for prefix 3.3.3.3/32, which matches PE2’s BGP
next-hop IP address, as follows. Note that the figure does not show all LDP advertisements but
only those that are particularly interesting to the example.
1. PE2, upon learning a route for prefix 3.3.3.3/32, allocates a local label of 2222.
2. PE2 updates its LFIB for the local label, listing a pop action.
3. As normal, PE2 advertises to its LDP neighbors the label binding of prefix 3.3.3.3/32 with
label 2222.
P1
P2
PE1 PE2
S0/1/0
S0/1/1
S0/0/1
4
3 1
In Label
1111
Out Label
2222
Out Int.
S0/1/0
P1 LFIB
Action
swap
4 In Label
5555
Out Label
2222
Out Int.
S0/1/1
P2 LFIB
Action
swap
2
In Label
2222
PE2 LFIB
Action
pop
Out Int.
Loop0
LDP:
3.3.3.3/32,
Label 2222
3 LDP:
3.3.3.3/32,
Label 2222
5 LDP:
3.3.3.3/32,
Label 5555
5 LDP:
3.3.3.3/32,
Label 1111
Loopback0:
3.3.3.3/32
Allocate a local
label (2222) for
route 3.3.3.3/32
4. P1 and P2 both independently learn about prefix 3.3.3.3/32 with the IGP, allocate a local label
(1111 on P1 and 5555 on P2), and update their LFIBs.
5. P1 and P2 advertise the binding of 3.3.3.3/32, along with their respective local labels, to their
peers.
Figure 19-18 showed the FIB and LFIB entries required for forwarding a packet from CE-A1 to
CE-A2, specifically into subnet 10.3.3.0/24. Figures 19-19 and 19-20, and their associated text,
explained how all the LFIB entries were created. Next, the focus turns to the FIB entry required
on PE1.