Frame-Mode MPLS Configuration and Verification

Step 1.

Enable CEF – CEF is an essential component for label switching and is responsible for imposition and disposition of labels in an MPLS network. Configure CEF globally on routers R1, R2, R3, and R4 by issuing the ip cef [distributed] command. Ensure that CEF is not disabled on the interface. If disabled, enable CEF on the interface by issuing ip route-cache cef in interface mode. Use the distributed keyword in the global configuration mode for Cisco platform capable of distributed CEF switching. Example 2-1 highlights the configuration to enable CEF on R2. Similarly enable CEF on R1, R3, and R4.

Example 2-1. Enable CEF

R2(config)#ip cef distributed R2(config)#do show running-config interface s0/0 | include cef no ip route-cache cef R2(config)#interface s0/0 R2(config-if)#ip route-cache cef  

Step 2.

Configure IGP routing protocol – Configure the IGP routing protocol; in this case, OSPF. Enable the interfaces on R1, R2, R3, and R4 that are part of the provider network in OSPF using network ip-address wild-card-mask area area-id command under the OSPF routing process. Example 2-2 highlights the OSPF configuration on R2. Similarly configure OSPF on R1, R3, and R4.

Example 2-2. Configure IGP Routing Protocol on R2

R2(config)#router ospf 100 R2(config)#network 10.10.10.0 0.0.0.255 area 0

Enabling the label distribution protocol is an optional step. TDP is deprecated, and by default, LDP is the label distribution protocol. The command mpls label protocol {ldp | tdp} is configured only if LDP is not the default label distribution protocol or if you are reverting from LDP to TDP protocol or vice versa. The command can be configured in the global as well as in the interface configuration mode. The interface configuration command will, however, override the global configuration.

Step 3.

Assign LDP router ID – LDP uses the highest IP address on a loopback interface as the LDP router ID. If there is no loopback address defined, the highest IP address on the router becomes the LDP router ID. To force an interface to be an LDP router ID, mpls ldp router-id interface-type number command can be used. The loopback interface address is recommended because it always remains up. Configure the loopback 0 interface on the R2 router to be the LDP router ID as shown in Example 2-3. Repeat the configuration on R1, R3, and R4, assigning the local loopback interface as LDP router-id.

Example 2-3. Assign LDP Router ID

R2(config)#mpls ldp router-id loopback 0  

Step 4.

Enable IPv4 MPLS or label forwarding on the interface – Example 2-4 demonstrates the step to enable MPLS forwarding on the interface.

Example 2-4. Enable MPLS Forwarding

R2(config)#interface serial 0/0 R2(config-if)#mpls ip R2(config)#interface serial 0/1 R2(config-if)#mpls ip  

Step 1.

Example 2-5 verifies whether CEF is globally enabled or disabled on the router by issuing the show ip cef command. As shown in Example 2-5, CEF is disabled on R2. Example 2-5 shows if CEF is enabled on the router interfaces.

Example 2-5. CEF Verification

R2#show ip cef %CEF not running Prefix Next Hop Interface _____________________________________________________________________ R2#show cef interface serial 0/0 Serial0/0 is up (if_number 5) (Output truncated) IP CEF switching enabled IP CEF Fast switching turbo vector (Output Truncated) _____________________________________________________________________ R2#show cef interface serial 0/1 Serial0/1 is up (if_number 6) (Output Truncated) IP CEF switching enabled IP CEF Fast switching turbo vector  

Step 2.

Verify MPLS forwarding is enabled on the interfaces by issuing the show mpls interfaces command. Example 2-6 shows that MPLS is enabled on the serial interfaces. The IP column depicts Yes if IP label switching is enabled on the interface. The Tunnel column is Yes if LSP tunnel labeling (discussed later in Chapter 9, "MPLS Traffic Engineering") is enabled on the interface, and the Operational column is Yes if packets are labeled on the interface.

Example 2-6. MPLS Forwarding Verification

R2#show mpls interfaces Interface IP Tunnel Operational Serial0/0 Yes (ldp) No Yes Serial0/1 Yes (ldp) No Yes  

Step 3.

Verify the status of the Label Distribution Protocol (LDP) discovery process by issuing show mpls ldp discovery. This command displays neighbor discovery information for LDP and shows the interfaces over which the LDP discovery process is running. Example 2-7 shows that R2 has discovered two LDP neighbors, 10.10.10.101 (R1) and 10.10.10.103 (R3). The xmit/recv field indicates that the interface is transmitting and receiving LDP discovery Hello packets.

Example 2-7. LDP Discovery Verification

R2#show mpls ldp discovery Local LDP Identifier: 10.10.10.102:0 Discovery Sources: Interfaces: Serial0/0 (ldp): xmit/recv LDP Id: 10.10.10.101:0 Serial0/1 (ldp): xmit/recv LDP Id: 10.10.10.103:0  

Step 4.

Issue show mpls ldp neighbor to verify the status of the LDP neighbor sessions. Example 2-8 shows that the LDP session between R2 and R1 (10.10.10.101), as well as between R2 and R3 (10.10.10.103), is operational. Downstream indicates that the downstream method of label distribution is being used for this LDP session in which the LSR advertises all of its locally assigned (incoming) labels to its LDP peer (subject to any configured access list restrictions).

Example 2-8. LDP Neighbor Verification

R2#show mpls ldp neighbor Peer LDP Ident: 10.10.10.101:0; Local LDP Ident 10.10.10.102:0 TCP connection: 10.10.10.101.646 - 10.10.10.102.11012 State: Oper; PIEs sent/rcvd: 26611/26601; Downstream Up time: 2w2d LDP discovery sources: Serial0/0, Src IP addr: 10.10.10.1 Addresses bound to peer LDP Ident: 10.10.10.101 10.10.10.1 Peer LDP Ident: 10.10.10.103:0; Local LDP Ident 10.10.10.102:0 TCP connection: 10.10.10.103.11002 - 10.10.10.102.646 State: Oper; Msgs sent/rcvd: 2374/2374; Downstream Up time: 1d10h LDP discovery sources: Serial0/1, Src IP addr: 10.10.10.6 Addresses bound to peer LDP Ident: 10.10.10.6 10.10.10.103 10.10.10.9  

Step 1.

Example 2-9 shows that R1 sends an implicit null or the POP label to R2. A value of 3 represents the implicit-null label. R1 propagates the implicit-null label to its penultimate Router R2, which performs the POP function in the data forwarding from R4 to 10.10.10.101/32. If R1 propagates an explicit-null label, the upstream LSR R2 does not POP the label but assigns a label value of 0 and sends a labeled packet to R2.

Example 2-9. MPLS Label Bindings on R1

R1#show mpls ldp bindings tib entry: 10.10.10.101/32, rev 4 local binding: tag: imp-null remote binding: tsr: 10.10.10.102:0, tag: 16  

Step 2.

Example 2-10 shows R2 assigning an LSP label 16 to 10.10.10.101/32. This label value is propagated to R3. This label value is imposed by R3 in the data forwarding path (for example, a packet originating from R4 to prefix 10.10.10.101/32 on R1).

Example 2-10. Label Allocation and Distribution Verification on R2

R2#show mpls forwarding-table Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 16 Pop tag 10.10.10.101/32 0 Se0/0 point2point 17 Pop tag 10.10.10.8/30 0 Se1/0 point2point 18 Pop tag 10.10.10.103/32 0 Se1/0 point2point 19 19 10.10.10.104/32 0 Se1/0 point2point  

Step 3.

Example 2-11 shows that on R3, prefix 10.10.10.101/32 has been assigned a local label of 17 and an outgoing label of 16. The outgoing label is received from the Router R2. The local label of 17 has been propagated during label distribution to Router R4. Label 17 is used by R4 in the data forwarding path for data destined to prefix 10.10.10.101/32 located on R1 from R4.

Example 2-11. Label Allocation and Distribution Verification on R3

R3#show mpls forwarding-table Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 16 Pop tag 10.10.10.0/30 0 Se0/0 point2point 17 16 10.10.10.101/32 0 Se0/0 point2point 18 Pop tag 10.10.10.102/32 0 Se0/0 point2point 19 Pop tag 10.10.10.104/32 0 Se1/0 point2point  

1.

As shown in Figure 2-3, R4 imposes label 17 on the data packet originating from R4 destined to 10.10.10.101/32.

2.

R3 does an LFIB lookup and swaps label 17 for 16 and forwards that data packet to R2.

3.

R2 receives the data packet from R3, does a penultimate hop pop function, removes label 16, and forwards the data packet to R1.

Step 1.

Follow the steps shown in the "Basic Frame-Mode MPLS Configuration Steps" section. These steps are the same for frame-mode MPLS over RFC 2684 routed PVC. Follow those steps to configure frame-mode MPLS on R1 and R2:

Step 1. – Enable CEF

Step 2. – Enable IGP routing protocol

Step 3. – Assign LDP router ID

Step 2.

Enable IPv4 MPLS or label forwarding on the interface – Configure the ATM PVCs 2/200 on each of the appropriate subinterfaces on R1 and R2. The encapsulation used on the PVC is ATM aal5snap. Example 2-16 highlights the steps to configure ATM PVC.

Example 2-16. Configure PVCs on R1 and R2

R1(config)#interface ATM2/0.2 point-to-point R1(config-subif)# pvc 2/200 R1(config-if-atm-vc)# encapsulation aal5snap R1(config-if-atm-vc)# mpls ip _____________________________________________________________________ R2(config)#interface ATM2/0.2 point-to-point R2(config-subif)#pvc 2/200 R2(config-if-atm-vc)#encapsulation aal5snap R2(config-if-atm-vc)# mpls ip  

Step 1.

Verify the operation of MPLS over RFC 2684 by performing a view of the MPLS forwarding information base (LFIB), as shown in Example 2-18.

Example 2-18. Verification of LFIB

R1#show mpls forwarding-table Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 36 Pop tag 10.10.10.104/32 0 AT2/0.2 point2point 37 Pop tag 10.10.20.128/30 0 AT2/0.2 point2point R1# _____________________________________________________________________ R2#show mpls forwarding-table Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 16 Pop tag 10.10.10.101/32 0 AT2/0.2 point2point 18 Pop tag 10.10.20.192/30 0 AT2/0.2 point2point  

Step 2.

As shown in Example 2-19, verify connectivity by issuing pings.

Example 2-19. Verify Connectivity

R1#ping 10.10.10.104 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.10.10.101, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms R4# R2#ping 10.10.10.101 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.10.10.101, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms R4#