MPLS Label Switching 11

MPLS Label Switching

Until you see MPLS label switching in operation, it may still be all smoke and mirrors. This section runs through a quick and dirty example that shows how MPLS label switching works.

MPLS Network Components

Figure 1.8 illustrates a simple service provider network that we’ll use for the example in this section.

F I G U R E 1 . 8 A simple service provider network

The routers in the network are labeled CE1, PE1, P1, P2, PE2, and CE2. These names are acronyms for:

CE A customer edge (CE) device. This is a router that connects to the customer network and to a service provider.

PE A provider edge (PE) device. This is a service provider piece of equipment that connects to a customer and into the provider (P) network.

P A provider (P) device. This is a service provider piece of equipment that exists entirely in the provider (P) network and only connects to other service provider devices (not to customers).

In addition, the PE and P routers are label switch routers. There are two types of label switch routers:

LSR A label switch router (LSR) is a Cisco IOS router/switch that is capable of forwarding packets based on labels. The CE, or customer, devices are not LSRs and can handle regular unlabeled IP packets.

12 Chapter 1 An Introduction to MPLS

Edge-LSR An edge label switch router (edge-LSR) is a more specific term for the PE routers. An edge-LSR is an edge device that is also an LSR. For an MPLS network, this is the device that takes unlabeled IP traffic and imposes, or in MPLS terms, pushes an MPLS label and switches the traffic to the next LSR. The edge-LSR also takes labeled traffic and deposes, or in MPLS terms, pops the label and forwards it to the next hop. A PE device is an edge-LSR in MPLS-based networks.

The network addresses for the provider devices are listed in Table 1.1. The IP addresses of the clients and client connections will be shown in a later example.

T A B L E 1 . 1 Service Provider IP Addressing

RIPv2 and MPLS have been enabled on each of the P and PE devices in Figure 1.8.

Device Output

As you may recall from the MPLS architecture section, I asked you to repeat to yourself, “Labels are bound to routes in the routing table.” The only exceptions to this rule are BGP routes. So, you can expand the mantra to, “Labels get bound to subnets in the routing table, with the exception of BGP routes.”

In the following output, each network device in the simple network illustrated in Figure 1.8 has both a routing table and labels bound to network routes. As you look at the following device outputs, take a careful

MPLS Label Switching 13

look at the IP prefixes in the routing table and then look to find the corresponding label.

The following output shows the RIPv2 routes on PE1:

PE1#show ip route rip

... Output Omitted

R 192.168.1.16/30 [120/2] via 192.168.1.21, 00:00:20, Serial0/0 R 192.168.1.4/32 [120/3] via 192.168.1.21, 00:00:20, Serial0/0 R 192.168.1.2/32 [120/1] via 192.168.1.21, 00:00:20, Serial0/0 R 192.168.1.3/32 [120/2] via 192.168.1.21, 00:00:20, Serial0/0 R 192.168.1.12/30 [120/1] via 192.168.1.21, 00:00:20, Serial0/0

The following output shows the label mappings on PE1:

The following output shows the RIPv2 routes on P1:

P1#show ip route rip

... Output Omitted

R 192.168.1.16/30 [120/1] via 192.168.1.17, 00:00:27, Serial0/1 R 192.18.1.1/32 [120/1] via 192.168.1.22, 00:00:25, Serial0/0 R 192.168.1.4/32 [120/2] via 192.168.1.17, 00:00:27, Serial0/1 R 192.168.1.3/32 [120/1] via 192.168.1.17, 00:00:27, Serial0/1

The following output shows the label mappings on P1:

14 Chapter 1 An Introduction to MPLS

The following output shows the RIPv2 routes on P2:

P2#show ip route rip

... Output Omitted

R 192.168.1.1/32 [120/2] via 192.168.1.18, 00:00:27, Serial0/0 R 192.168.1.4/32 [120/1] via 192.168.1.13, 00:00:00, Serial0/1 R 192.168.1.2/32 [120/1] via 192.168.1.18, 00:00:27, Serial0/0 R 192.168.1.8/30 [120/1] via 192.168.1.18, 00:00:27, Serial0/0

The following output shows the label mappings on P2:

The following output shows the RIPv2 routes on PE2:

PE2#show ip route rip

... Output Omitted

R 192.168.1.1/32 [120/3] via 192.168.1.14, 00:00:22, Serial0/0 R 192.168.1.2/32 [120/2] via 192.168.1.14, 00:00:22, Serial0/0 R 192.168.1.3/32 [120/1] via 192.168.1.14, 00:00:22, Serial0/0 R 192.168.1.12/30 [120/1] via 192.168.1.14, 00:00:22, Serial0/0 R 192.168.1.8/30 [120/2] via 192.168.1.14, 00:00:22, Serial0/0

The following output shows the label mappings on PE2: