3com 3031 Instruccion De Instalación

VPN packets are forwarded by adopting ta wo-layer label mode:

Interior-layer label, also called MPLS label, is at the bottom in a label stack and 
distributed 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, is at the top in label stack and 
indicates an LSP from the ingress PE to egress PE. By switching exterior-layer label, 
VPN packets can be forwarded along the LSP to the peer PE. 

Figure 168 illustrates the details: 

Figure 168   Forwarding VPN packets

1 Site 1 sends an IPv4 packet with destination address 1.1.1.2 to CE1. CE1 looks up 

routing information in the IP routing table and sends the packet to PE1. 

2 PE1 looks up in the VPN-instance table according to the destination interface and 

destination address to get MPLS label (or interior-layer label), LSP initialization label 
(or exterior-layer label), BGP next hop (PE2), egress interface etc. When the label 
stack is established, PE1 forwards via the egress interface the MPLS packet to the 
first P on the LSP. 

3 Every P router on the LSP forwards the MPLS packet according to the exterior-layer 

label till the packet reaches the penultimate router, i.e. the P router right before 
the PE2, which extracts the exterior-layer label and forwards the packet to PE2.

4 PE2 looks up in the MPLS forwarding table according to the interior-layer label and 

destination address to determine the egress interface for labeling operation and 
the packet. It then extracts the interior-layer label and forwards through the egress 
interface the IPv4 packet to CE2. 

5 CE2 looks up in the routing table and sends the packet in normal IPv4 packet 

forwarding mode to the site2. 

As PE is required to aggregate multiple VPN routes on a BGP/MPLS VPN, it is prone 
to forming a bottleneck in a large-scale deployment or in the case where that PE 
capacity is small. To solve the problem, 3Com introduced the HoVPN (Hierarchy of 
VPN, Hierarchical BGP/MPLS VPN) solution.

Hierarchical BGP/MPLS VPN divides an MPLS VPN into several MPLS VPNs in a 
hierarchical network structure. Each VPN takes on a role depending on its level. 

PE2

site1

CE1

PE1

P

P

1.1.1.1/24

CE2

1.1.1.2

Layer2

Layer1

1.1.1.2

Layer2

1.1.1.2

1.1.1.2

site2

1.1.1.2/24