Dec 2 2008
Traffic Engineering – Fast Reroute
Fast reroute (FRR) is one of the main drivers for implementing Traffic Engineering. Fast Reroute uses the concept of a backup tunnel. When the path used by the primary tunnel is detected as down, traffic is dynamically switched over to the backup tunnel.
Fast reroute provides two types of protection.
1-Link Protection
2-Node Protection
Lets examine the topology below.
Link Protection
Lets suppose the primary path for a tunnel whose headend is at PE1 and tailend is at PE2, that the primary path is via PE3.
If the link between PE1 and PE3 fails then fast reroute link protection reroutes the traffic using the following path.
PE1–>P1–>PE3–>PE2
As you can see the reroute mechanism has rereouted around the failed link. These type of FRR backup tunnel are known as Next-Hop back tunnels(NHOP).
Node Protection
Lets suppose the primary path for a tunnel whose headend is at PE1 and tailend is at PE2, that the primary path is via PE3 and not P1.
If the node PE3 fails then fast reroute link protection reroutes the traffic using the following path.
PE1–>P1–>PE2
As you can see the reroute mechanism has rereouted around the failed node. These type of FRR backup tunnels are known as Next-Next Hop back tunnels(NNHOP).
Configuring FRR
The configuration of Traffic engineering has evolved over the years, In the beginning both the primary and backup tunnel had to be configured manually. Then the backup tunnel evolved to be dynamic configuration and now even the primary tunnel is almost fully automated.
Below we will show an how to configure an autotunnel Primary and Backup
To configure the autotunnel on PE1 simply enter the following command.
PE1(config)#mpls traffic-eng auto-tunnel primary onehop
You will then see a primary tunnel comeup for each directly connected traffic engineering enabled neighbor.
00:06:57: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel65336, changed state to up
00:06:58: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel65337, changed state to up
PE1#show mpls traffic-eng tunnels brief
TUNNEL NAME DESTINATION UP IF DOWN IF STATE/PROT
PE1_t65336 4.4.4.4 - Fa2/0 up/up
PE1_t65337 2.2.2.2 - Se1/1.1 up/up
Displayed 2 (of 2) heads, 0 (of 0) midpoints, 0 (of 0) tails
To configure the backup tunnels use the following command:
PE1(config)#mpls traffic-eng auto-tunnel backup
Shortly after entering the above command you will see 2 backup tunnels come up.
PE1(config)#
00:03:20: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel65436, changed state to up
00:03:21: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel65437, changed state to up
The command below shows that tunnels 65436 and 65437 are acting as backup tunnels for 65336 and 65337.
PE1#show ip rsvp fast-reroute
Primary Protect BW Backup
Tunnel I/F BPS:Type Tunnel:Label State Level Type
------ ------- -------- ------------- ------ ----- ------
PE1_t65337 Se1/1.1 0:G Tu65436:0 Ready any-unl Nhop
PE1_t65336 Fa2/0 0:G Tu65437:0 Ready any-unl Nhop
I’ll talk more about these tunnels in another post.
