Types of VPNs
A virtual private network (VPN) consists of two topological areas: the provider’s network and the customer’s network. The customer’s network is commonly located at multiple physical sites and is also private (non-Internet). A customer site would typically consist of a group of routers or other networking equipment located at a single physical location. The provider’s network, which runs across the public Internet infrastructure, consists of routers that provide VPN services to a customer’s network as well as routers that provide other services. The provider’s network connects the various customer sites in what appears to the customer and the provider to be a private network.
To ensure that VPNs remain private and isolated from other VPNs and from the public Internet, the provider’s network maintains policies that keep routing information from different VPNs separate. A provider can service multiple VPNs as long as its policies keep routes from different VPNs separate. Similarly, a customer site can belong to multiple VPNs as long as it keeps routes from the different VPNs separate.
The Junos® Operating System (Junos OS) provides several types of VPNs; you can choose the best solution for your network environment. Each of the following VPNs has different capabilities and requires different types of configuration:
Layer 2 VPNs
Implementing a Layer 2 VPN on a router is similar to implementing a VPN using a Layer 2 technology such as ATM or Frame Relay. However, for a Layer 2 VPN on a router, traffic is forwarded to the router in Layer 2 format. It is carried by MPLS over the service provider’s network and then converted back to Layer 2 format at the receiving site. You can configure different Layer 2 formats at the sending and receiving sites. The security and privacy of an MPLS Layer 2 VPN are equal to those of an ATM or Frame Relay VPN.
On a Layer 2 VPN, routing occurs on the customer’s routers, typically on the CE router. The CE router connected to a service provider on a Layer 2 VPN must select the appropriate circuit on which to send traffic. The PE router receiving the traffic sends it across the service provider’s network to the PE router connected to the receiving site. The PE routers do not need to store or process the customer’s routes; they only need to be configured to send data to the appropriate tunnel.
For a Layer 2 VPN, customers need to configure their own routers to carry all Layer 3 traffic. The service provider needs to know only how much traffic the Layer 2 VPN needs to carry. The service provider’s routers carry traffic between the customer’s sites using Layer 2 VPN interfaces. The VPN topology is determined by policies configured on the PE routers.
Layer 3 VPNs
In a Layer 3 VPN, the routing occurs on the service provider’s routers. Therefore, Layer 3 VPNs require more configuration on the part of the service provider, because the service provider’s PE routers must store and process the customer’s routes.
In the Junos OS, Layer 3 VPNs are based on RFC 4364, BGP/MPLS IP Virtual Private Networks (VPNs). This RFC defines a mechanism by which service providers can use their IP backbones to provide Layer 3 VPN services to their customers. The sites that make up a Layer 3 VPN are connected over a provider’s existing public Internet backbone.
VPNs based on RFC 4364 are also known as BGP/MPLS VPNs because BGP is used to distribute VPN routing information across the provider’s backbone, and MPLS is used to forward VPN traffic across the backbone to remote VPN sites.
Customer networks, because they are private, can use either public addresses or private addresses, as defined in RFC 1918, Address Allocation for Private Internets. When customer networks that use private addresses connect to the public Internet infrastructure, the private addresses might overlap with the private addresses used by other network users. BGP/MPLS VPNs solve this problem by prefixing a VPN identifier to each address from a particular VPN site, thereby creating an address that is unique both within the VPN and within the public Internet. In addition, each VPN has its own VPN-specific routing table that contains the routing information for that VPN only.
VPLS
Virtual private LAN service (VPLS) allows you to connect geographically dispersed customer sites as if they were connected to the same LAN. In many ways, it works like a Layer 2 VPN. VPLS and Layer 2 VPNs use the same network topology and function similarly. A packet originating within a customer’s network is sent first to a CE device. It is then sent to a PE router within the service provider’s network. The packet traverses the service provider’s network over an MPLS LSP. It arrives at the egress PE router, which then forwards the traffic to the CE device at the destination customer site.
The key difference in VPLS is that packets can traverse the service provider’s network in a point-to-multipoint fashion, meaning that a packet originating from a CE device can be broadcast to PE routers in the VPLS. In contrast, a Layer 2 VPN forwards packets in a point-to-point fashion only. The destination of a packet received from a CE device by a PE router must be known for the Layer 2 VPN to function properly.
In a Layer 3 network only, you can configure virtual private LAN service (VPLS), to connect geographically dispersed Ethernet local area networks (LAN) sites to each other across an MPLS backbone. For ISP customers who implement VPLS, all sites appear to be in the same Ethernet LAN even though traffic travels across the service provider's network. VPLS is designed to carry Ethernet traffic across an MPLS-enabled service provider network. In certain ways, VPLS mimics the behavior of an Ethernet network. When a PE router configured with a VPLS routing instance receives a packet from a CE device, it first checks the appropriate routing table for the destination of the VPLS packet. If the router has the destination, it forwards it to the appropriate PE router. If it does not have the destination, it broadcasts the packet to all the other PE routers that are members of the same VPLS routing instance. The PE routers forward the packet to their CE devices. The CE device that is the intended recipient of the packet forwards it to its final destination. The other CE devices discard it.
Virtual-Router Routing Instances
A virtual-router
routing instance, like a VPN routing and forwarding (VRF) routing
instance, maintains separate routing and forwarding tables for each
instance. However, many configuration steps required for VRF routing
instances are not required for virtual-router routing instances. Specifically,
you do not need to configure a route distinguisher, a routing table
policy (the vrf-export, vrf-import, and route-distinguisher statements), or MPLS between the P routers.
However, you need to configure separate logical interfaces between each of the service provider routers participating in a virtual-router routing instance. You also need to configure separate logical interfaces between the service provider routers and the customer routers participating in each routing instance. Each virtual-router instance requires its own unique set of logical interfaces to all participating routers.
Figure 1 shows how this works. The service provider routers G and H are configured for virtual-router routing instances Red and Green. Each service provider router is directly connected to two local customer routers, one in each routing instance. The service provider routers are also connected to each other over the service provider network. These routers need four logical interfaces: a logical interface to each of the locally connected customer routers and a logical interface to carry traffic between the two service provider routers for each virtual-router instance.
Layer 3 VPNs do not have this configuration requirement. If you configure several Layer 3 VPN routing instances on a PE router, all the instances can use the same logical interface to reach another PE router. This is possible because Layer 3 VPNs use MPLS (VPN) labels that differentiate traffic going to and from various routing instances. Without MPLS and VPN labels, as in a virtual-router routing instance, you need separate logical interfaces to separate traffic from different instances.
One method of providing this logical interface between the service provider routers is by configuring tunnels between them. You can configure IP Security (IPsec), generic routing encapsulation (GRE), or IP-IP tunnels between the service provider routers, terminating the tunnels at the virtual-router instance.