Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual page 431

Copyright © 2010, Juniper Networks, Inc.
On PE 1, you see that MP-BGP receives a labeled VPN-IPv4 prefix of 10.24.0.0/16 with
an out label of 16. MP-BGP on PE 2 advertised this label with the prefix. In the data plane,
MPLS traffic is sent by PE 1 to PE 2 with this label.
host1:pe1#show ip bgp vpn all field out-label
Prefix Out-label
10.12.0.0/16 none
10.24.0.0/16 16
On PE 2, you see that MP-BGP receives a labeled VPN-IPv4 prefix of 10.12.0.0/16 with
an out label of 24. MP-BGP on PE 1 advertised this label with the prefix. In the data plane,
MPLS traffic is sent by PE 2 to PE 1 with this label.
host2:pe2#show ip bgp vpn all field out-label
Prefix Out-label
10.12.0.0/16 24
10.24.0.0/16 none
The data packets are transported within a VPN across the service provider core by MPLS.
This transport process requires two layers of MPLS labels, stacked one upon the other.
The inner labels are assigned by each PE router for each VRF. When an MPLS packet
arrives at the egress PE router, that egress PE router uses the inner label to determine
which VRF the packet is destined for. In the default, per-VRF label allocation mode
(described in "Creating Labels per FEC" on page 439), the egress PE router does an IP
lookup in the IP forwarding table of that VRF using the IP destination address in the IP
packet that is encapsulated in the MPLS packet. The egress PE router then forwards the
IP packet (without the MPLS header) to the appropriate customer site. The inner labels
themselves are communicated between PE routers in the MP-BGP extended update
messages as described in the previous section.
MPLS uses the outer labels to forward data packets from the ingress PE router through
a succession of P routers across the core. This succession of P routers constitutes a
label-switched path (LSP), also referred to as an MPLS tunnel. The labels are assigned
to links in the path.
At each P router, MPLS pops the outer label from a data packet. The label is an index
into the P router's forwarding table, from which it determines both the next hop along
the LSP and another label. The router pushes the label on to the label stack and forwards
the packet to the next P router. The combination of popping one label and pushing another
is known as a label swap. At the egress PE router, MPLS pops the outer label, then the
inner label. The inner label determines the CE router to which the packet is sent. The P
routers never examine the inner MPLS label or the destination IP address encapsulated
in the MPLS packet.
In many cases, the PE routers are fully meshed by means of LSPs. You can use tunnel
profiles to simplify the LSP configuration process. See "Configuring MPLS" on page 275,
for procedures to configure an LSP.
Each LSP is unidirectional for data traffic, so you must establish LSPs in both directions
for two-way data transport. Figure 74 on page 396 shows that two LSPs have been created
between PE 1 and PE 2. PE 1 and PE 2 have an MP-BGP session as shown previously in
Figure 73 on page 394.
Chapter 6: Configuring BGP-MPLS Applications
395