Configuring Basic MPLS CE Routers

Problem

You want to configure the "customer" CE routers for MPLS.

Solution

CE routers do not require any special software or configuration to work with an MPLS carrier. You just need to ensure that there are appropriate routing table entries to allow sites to communicate across the MPLS network. We will do this with static routes for now:

Router-CE-A1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router-CE-A1(config)#interface FastEthernet0/0.1
Router-CE-A1(config-if)#encapsulation dot1Q 101
Router-CE-A1(config-if)#ip address 192.168.1.5 255.255.255.0
Router-CE-A1(config-if)#exit
Router-CE-A1(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.1
Router-CE-A1(config)# exit
Router-CE-A1#

Discussion

In this example, we used static routes to communicate across the MPLS core. You could also use various routing protocols such as RIP, OSPF, EIGRP, or BGP to communicate between the CE and PE routers. We will discuss these options in more depth in Recipes 26.5, 26.6, 26.7, and 26.8. Another simple solution is to use GRE tunnels between your CE routers. You can then continue to use static routes to carry traffic between the CE and PE routers, but still have the all of the advantages of a routing protocol.

It is important to remember that the customer data passes through the MPLS network in a VLAN tunnel. If you look at any common IP information, such as the Time To Live (TTL) value, in the IP header or the output of a TRaceroute command, the entire MPLS cloud looks like a single hop:

Router-CE-A1#traceroute ip 192.168.2.9

Type escape sequence to abort.
Tracing the route to 192.168.2.9

 1 192.168.1.1 0 msec 4 msec 4 msec
 2 192.168.2.1 4 msec 4 msec 4 msec
 3 192.168.2.9 4 msec * 4 msec
Router-CE-A1#

In this case, 192.168.1.1 is the address of the PE router at this site, 192.168.2.1 is the PE router at the other site, and 192.168.2.9 is the address of the CE router at the other site.

We also note that we have used an Ethernet subinterface in this example to associate the PE-CE link with a VLAN. We did this for two reasons. First, the practical reason was that this router was limited in its physical Ethernet interfaces, so if we wanted to connect to other downstream C routers via Ethernet, we could still do so by putting them on a different VLAN. The second reason was to stress that the connection between PE and CE happens at Layer 3, so we can use any Layer 2 technology that is the most convenient. This is, in fact, one of the most attractive features of MPLS. The provider can deliver the service over any available medium. Ethernet is a popular delivery method because it allows the CE router to be essentially any inexpensive access router.