3Com 8800 Configuration Manual page 450

3Com Switch 8800 Configuration Guide
LSPs must be set up between PEs for VPN data traffic forwarding with MPLS LSP. The
PE router which receives packets from CE and create label protocol stack is called
ingress LSR, while the BGP next hop (egress PE router) is egress LSR. Using LDP to
create fully connected LSPs among PEs.
3) Between PE and CE
A CE can learn remote VPN routes from the PE connected through static routes, RIP,
OSPF or EBGP.
With above-mentioned steps, reachable routes can be established between CEs, for
transmission of VPN private routing information over public network.
II. Forwarding VPN packets
On the ingress PE, two-layer label stack is formed for each VPN packet:
Interior-layer label, also called MPLS label, is at the bottom of the label stack and
distributed by M-BGP when the egress PE advertises routing information (in VPN
forwarding table) to ingress GE. When VPN packets from public network reach the CE,
they can be forwarded from the designated interface to the designated CE or site by
searching for the target MPLS forwarding table according to the labels contained.
Exterior-layer label, known as LSP initialization label, distributed by MPLS LDP, is at
the top of the label stack and indicates an LSP from the ingress PE to egress PE. By the
switching of exterior-layer label, VPN packets can be forwarded along the LSP to the
peer PE.
Figure 37-4 illustrates the details:
CE1 CE1
site1 site1
1.1.1.1/24 1.1.1.1/24
Figure 37-4 Forwarding VPN packets
1) Site 1 sends an IPv4 packet with the destination address 1.1.1.2 of to CE1. CE1
looks up the IP routing table for a matched entry and sends the packet to PE1
according to the matched entry.
2) Depending on the interface the packet reaches and the destination of it, PE1 looks
up the VPN-instance entry to obtain interior-layer label, exterior-layer label, BGP
next hop (PE2), and output interfaces. After the establishment of labels, PE1
forwards MPLS packets to the first P of LSP through output interface.
Layer1 Layer1
Layer2 Layer2
1.1.1.2 1.1.1.2
1.1.1.2 1.1.1.2
PE1 PE1
P P
Chapter 37 BGP/MPLS VPN Configuration
Layer2 Layer2
1.1.1.2 1.1.1.2
1.1.1.2 1.1.1.2
PE2 PE2
P P
37-6
CE2 CE2
site2 site2
1.1.1.2/24 1.1.1.2/24