Mpls Vpn Model

It is important to become familiar with the terminology concerning MPLS VPN. Look at Figure 7-1 for a schematic overview of the MPLS VPN model. A service provider is providing the common public infrastructure that customers use.

Figure 7-1 MPLS VPN Schematic Overview

Customer 1

ri ri

Site A

Service Provider

Pi

P

PE1

li

PE

V

P

Site B

A PE router is a provider edge (PE) router. It has a direct connection with the customer edge (CE) router at Layer 3. A provider (P) router is a router without the direct connection to the routers of the customer. In the MPLS VPN implementation, both P and PE routers run MPLS. This means that they must be able to distribute labels between them and forward labeled packets.

A CE router has a direct Layer 3 connection with the PE router. A customer (C) router is a router without a direct connection with the PE router. A CE router does not need to run MPLS.

Because the CE and PE routers interact at Layer 3, they must run a routing protocol (or static routing) between them. The CE router has only one peer outside of its own site: the PE router. If the CE router is multihomed, it can peer with multiple PE routers. The CE router does not peer with any of the CE routers from the other sites across the service provider network, as with the overlay model. The name peer-to-peer model is derived from the fact that the CE and PE form a peer at Layer 3.

The P in VPN stands for private. As such, the customers of the service provider are allowed to have their own IP addressing scheme. This means that they can use registered IP addresses but also private IP addresses (see RFC 1918) or even IP addresses that are also used by other customers who are connecting to the same service provider (referred to as overlapping IP addressing). If the packets were to be forwarded as IP packets within the service provider network, this would cause problems, because the P routers would be confused. If the private and overlapping IP addressing scheme is not allowed, then every customer must be using a unique address range. In that case, the packets can be forwarded by looking up the destination IP address on every router in the service provider network. This means that all P and PE routers must have the complete routing table of every customer. This would be a large routing table. The only routing protocol that is capable of carrying a large number of routes is Border Gateway Protocol (BGP). This would mean that all P and PE routers would have to run internal BGP (iBGP) among them. However, this is not a VPN scheme, because it is not private to the customers.

Another solution is that every P and PE router has a private routing table for each customer. Several processes of one routing protocol (one process per VPN) could be running on all the routers to distribute the VPN routes. Running one routing process per VPN on every P router is not very scalable. Each time one VPN is added to the network, a new routing process must be added to every P router. Furthermore, if an IP packet enters a P router, how does the P router determine which VPN the packet belongs to to figure out which private routing table to use to forward the packet? If the packet is an IP packet, this is not possible. You could add one extra field to the IP packet indicating which VPN the IP packet belongs to. The P routers could then forward the IP packets by looking at this extra field and at the destination IP address. Again, all P routers would have to be aware of this extra field.

A scalable solution would be to have the P routers completely unaware of the VPNs. Then the P routers would not be burdened with having routing information for VPN routes. Can you achieve this by using MPLS? The answer is yes. The customer IP packets are labeled in the service provider network to achieve a private VPN for each customer. Furthermore, the P routers no longer need to have the routing table of the customers by using two MPLS labels. Therefore, BGP is not needed on the P routers. See the section "BGP-Free Core" in Chapter 1 for an explanation on this. The VPN routes are only known on the PE routers. As such, the VPN knowledge is present only on the edge routers of the MPLS VPN network, which makes the MPLS VPN solution scalable.

Figure 7-2 shows the MPLS VPN model: label switching packets in the service provider network and PE routers that are VPN aware.

Figure 7-2 MPLS VPN Model

VPN A Site 1

Customer Equipment

Provider Equipment

Provider Equipment

Isolated Routing Between VPNs

Provider Equipment

Service Provider Running MPLS

Data

Label(s)

Provider Equipment

Customer Equipment

VPN A Site 2

Provider Equipment

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