MPLS Traffic Engineering Fast Reroute — Link Protection

Fast Reroute Operation

The example in Figure 1 illustrates how Fast Reroute link protection is used to protect traffic carried in a TE tunnel between devices R1 and R9, as it traverses the link between devices R2 and R3. The TE tunnel from R1 to R9 is considered to be the primary tunnel and is defined by labels 37, 14, and Pop. To protect this link, you create a backup tunnel that runs from R2 to R3 by way of R6 and R7. This backup tunnel is defined by labels 17, 22, and Pop.

When R2 is notified that the link between it and R3 is no longer available, it simply forwards traffic destined for R3 through the backup tunnel. That is accomplished by pushing label 17 onto packets destined to R3 after the normal swap operation (which replaces label 37 with label 14) has been performed. Pushing label 17 onto packets forwards them along the backup tunnel, thereby routing traffic around the failed link. The decision to reroute packets from the primary tunnel to the backup tunnel is made solely by R2 upon detection of link failure.

Figure 1 Backup Tunnel—Fast Reroute

For more details about general MPLS traffic engineering, see the Cisco IOS 12.1(3)T MPLS Traffic Engineering and Enhancements document athttp://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t3/traffeng.htm.

Benefits

The Fast Reroute feature has two noticeable benefits: the increased reliability it gives to IP traffic service and the high scalability inherent in its design:

•Increased Reliability for IP Services—MPLS traffic engineering with Fast Reroute uses fail over times that match the capabilities of SONET link restoration. This leverages a very high degree of resiliency for IP traffic that flows over a service provider's backbone, leading to more robust IP services and higher end-customer satisfaction.

•High Scalability Solution—The Fast Reroute feature uses the highest degree of scalability by supporting the mapping of all primary tunnels that traverse a link onto a single backup tunnel. This capability bounds the growth of backup tunnels to the number of links in the backbone rather than the number of TE tunnels that run across the backbone.

Configuration Tasks

You configure Fast Reroute link protection in three overall tasks:

•Configure Fast Reroute protection for a particular link

•Configure a backup tunnel around the protected link

•Configure the protected link to use the backup tunnel

Before or after entering these commands, you must enable the MPLS traffic-engineering tunnel capability globally on the router. The command to do that is mpls traffic-eng tunnels.

Configure Fast Reroute protection for a particular link

To configure a primary tunnel and assign it for Fast Reroute protection, perform the following steps:

	Command	Purpose
Step 1	Router(config)# interface Tunnel1	Creates a tunnel interface and enters interface configuration mode.
Step 2	Router(config-if)# ip unnumbered loopback0	Gives the tunnel interface an IP address. An MPLS traffic engineering tunnel interface should be unnumbered since it represents a unidirectional link.
Step 3	Router(config-if)# tunnel destination A.B.C.D	Specifies the IP address of the device at the far end of the tunnel.
Step 4	Router(config-if)# tunnel mode mpls traffic-eng	Sets the tunnel's encapsulation mode to MPLS traffic engineering.
Step 5	Router(config-if)# tunnel mpls traffic-eng autoroute announce	Causes the IGP to use the tunnel in its enhanced SPF calculation.
Step 6	Router(config-if)# tunnel mpls traffic-eng priority 0 0	Configures setup and reservation priorities of the tunnel.
Step 7	Router(config-if)# tunnel mpls traffic-eng bandwidth 100	Configures the tunnel's bandwidth (in kilobits).
Step 8	Router(config-if)# tunnel mpls traffic-eng path-option 1 explicit identifier 1	Directs the tunnel to use an explicit path.
Step 9	Router(config-if)# tunnel mpls traffic-eng fast-reroute	Enables the tunnel to use a backup tunnel in the event of a link failure. (Establishing the backup tunnel is accomplished by the following two tables' commands.)

Configure a backup tunnel around the protected link

	Command	Purpose
Step 1	Router(config)# interface Tunnel1000	Creates a tunnel interface and enters interface configuration mode.
Step 2	Router(config-if)# ip unnumbered loopback0	Gives the tunnel interface an IP address. An MPLS traffic engineering tunnel interface should be unnumbered since it represents a unidirectional link.
Step 3	Router(config-if)# tunnel destination A.B.C.D	Specifies the IP address of the device at the far end of the tunnel.
Step 4	Router(config-if)# tunnel mode mpls traffic-eng	Sets encapsulation mode of the tunnel to MPLS traffic engineering.
Step 5	Router(config-if)# tunnel mpls traffic-eng priority 0 0	Configures setup and reservation priorities of the tunnel.
Step 6	Router(config-if)# tunnel mpls traffic-eng path-option 1 explicit identifier 2	Directs the tunnel to use an explicit path.

To configure a backup tunnel around the protected link, perform the following steps:

Configure the protected link to use the backup tunnel

To configure the protected link to use the backup tunnel, perform the following steps:

	Command	Purpose
Step 1	Router(config)# interface interface-id	Moves configuration to the physical interface level, directing subsequent configuration commands to the specific physical interface identified by theinterface-id. (In this release it is a POS interface.)
Step 2	Router(config-if)# ip address A.B.C.D E.F.G.H	Sets a primary IP address for this interface and a mask for the associated IP subnet.
Step 3	Router(config-if)# mpls traffic-eng tunnels	Enables the MPLS traffic engineering tunnel feature on this physical interface.
Step 4	Router(config-if)# mpls traffic-eng backup-path Tunnel1000	Configures the interface to use the backup tunnel in the event of a detected failure on the interface.
Step 5	Router(config-if)# pos ais-shut	Enables the alarm signal-line (AIS-L) to be sent when the POS interface is placed in administrative shutdown state.
Step 6	Router(config-if)# pos report lrdi	Permits selected SONET alarms to be logged to the console for a POS interface.
Step 7	Router(config-if)# ip rsvp bandwidth 2480000 2480000	Enables RSVP on this interface, and limits the amount of bandwidth RSVP can reserve on this interface. For a description of IP RSVP command syntax, see the Cisco IOS Quality of Service Command Reference. [http://www.cisco.com/univercd/ cc/td/doc/product/software/ios121/121cgcr/ qos_r/qrdcmd2.htm]

Verifying Fast Reroute for MPLS Traffic Engineering

Verify whether MPLS traffic engineering Fast Reroute link protection has been enabled.

Enter the show mpls traffic-eng fast-reroute database command.

router# show mpls traffic-eng fast-reroute database 12.0.0.0

The sample display output is as follows:

Tunnel head fast reroute information:

Prefix  Tunnel In-label Out intf/label FRR intf/label Status

12.0.0.0/16 Tu111 Tun hd PO0/0:Untagged Tu4000:16 ready  

12.0.0.0/16 Tu449 Tun hd PO0/0:Untagged Tu4000:736 ready  

12.0.0.0/16 Tu314 Tun hd PO0/0:Untagged Tu4000:757 ready  

12.0.0.0/16 Tu313 Tun hd PO0/0:Untagged Tu4000:756 ready 

Configuration Examples

This section provides the following configuration examples:

•Configuring a Primary Tunnel and Assigning It for Fast Reroute Link Protection

•Configuring a Backup Tunnel Around the Protected Link

•Configuring the Protected Link to Use the Backup Tunnel

Configuring a Primary Tunnel and Assigning It for Fast Reroute

Enter the following commands to configure a primary tunnel (at the headend) and to assign it for Fast Reroute link protection:

interface Tunnel1

ip unnumbered loopback0

tunnel destination 25.1.1.1

tunnel mode mpls traffic-eng

tunnel mpls traffic-eng autoroute announce

tunnel mode mpls traffic-eng priority 0 0

tunnel mpls traffic-eng bandwidth 100

tunnel mpls traffic-eng path-option 1 explicit identifier 1

tunnel mpls traffic-eng fast-reroute

Configuring a Backup Tunnel Around the Protected Link