Information Technology Reference
In-Depth Information
The P routers maintain LSPs among PE devices using LDP/TDP or other label distribution
protocols and thus are unaware of any VPN information encapsulated inside LSPs. All
provider devices, including P routers and PE devices, use an IGP so that all links and other
local addresses can be reached within the provider network.
The CE routers are conventional IP routers. Neither VPN nor MPLS is needed on CE
routers. These routers communicate only with routers in the same VPN, as dictated by
the PE devices.
Multiprotocol BGP comes into play when PE devices communicate with each other. Spe-
cifically, PE devices use multiprotocol iBGP to advertise VPNv4 prefixes to each other.
Similar to IPv4, route reflection and confederation can be used to increase iBGP scalability.
The VPN routing information can be exchanged between a PE and a CE in several ways,
including RIPv2, OSPF, BGP, EIGRP, and static routes.
P routers do not need to run multiprotocol iBGP and do not maintain VPNv4 information.
NOTE
The forwarding path is best illustrated by a layered approach, as shown in Figure 10-6. The
packets between a CE and a PE are encapsulated in IP. In the IP layer, two CE devices and
two PE devices are Layer 3 peers (this is why VPNs built in this way are called
Layer 3
VPNs
). On a PE, several layers are used. There are two layers of LSPs between the ingress
PE and the egress PE:
•
LSPv to identify the VPN
•
LSPi to identify the remote PE
Thus, the original IP packet from the CE now carries a label stack of two entries. A P router
has only one LSP, so the P router is unaware of the VPN LSP. All the P routers swap or pop
the top label. The VPN label is popped by the egress PE, and the original packet is
forwarded to the connected CE.
Figure 10-6
Layered Visualization of the MPLS VPN Forwarding Path
CE
CE
PE
P
PE
IP
IP
IP
IP
LSPv
LSPv
LSPi
LSPi
LSPi
