MPLS TE explicit paths are usually used in combination with an offline network analysis and simulation tool to optimize the backbone utilization based on observed flows across the backbone.
In this part of the exercise, you’ll use the MPLS TE explicit paths to force the traffic between PE routers to take the longer path through the P router. You will use dynamic path computation as a fallback option.
| Task 1 | Determine the Explicit Path |
Explicit MPLS TE path must specify hop-by-hop path across an MPLS backbone, listing ingress IP addresses of all LSRs between the tunnel headend and tunnel tailend (including the tailend IP address). The simplest way to get these addresses is to perform a trace command from the tunnel headend toward the tunnel tailend using loose source routing.
The following steps have to be performed on all PE routers:
| Step 1 | Do a trace toward the tunnel tailend with loose source routing through the P-router. You will get a printout similar to the one below: |
| Step 2 | Create an explicit MPLS TE route, listing the IP addresses displayed by the trace command as the next-hop addresses in the explicit route. Add the tailend router’s loopback address as the last hop in the explicit route. |
| Step 3 | Configure the explicit route defined above as the preferred path option for the tunnel interface configured in the previous section, with the dynamically computed route being a fallback option. |
PE1#trace
Protocol [ip]:
Target IP address: 192.168.1.3
Source address:
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]: 10
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]: l
Source route: 192.168.1.5
Loose, Strict, Record, Timestamp, Verbose[LV]: v
Loose, Strict, Record, Timestamp, Verbose[L]:
Type escape sequence to abort.
Tracing the route to PE3 (192.168.1.3)
1 192.168.1.21 20 msec 16 msec 20 msec
2 192.168.1.17 32 msec 32 msec 36 msec
3 192.168.1.13 48 msec * 48 msec
ip explicit-path name PE3 enable
next-address 192.168.1.21
next-address 192.168.1.17
next-address 192.168.1.13
next-address 192.168.1.3
interface Tunnel0
tunnel mpls traffic-eng path-option 1 explicit name PE3
tunnel mpls traffic-eng path-option 99 dynamic
PE3#trace
Protocol [ip]:
Target IP address: 192.168.1.1
Source address:
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]:
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]: l
Source route: 192.168.1.5
Loose, Strict, Record, Timestamp, Verbose[LV]: v
Loose, Strict, Record, Timestamp, Verbose[L]:
Type escape sequence to abort.
Tracing the route to PE1 (192.168.1.1)
1 192.168.1.14 32 msec 32 msec 32 msec
2 192.168.1.18 64 msec 68 msec 64 msec
3 192.168.1.22 96 msec * 96 msec
ip explicit-path name PE1 enable
next-address 192.168.1.14
next-address 192.168.1.18
next-address 192.168.1.22
next-address 192.168.1.1
interface Tunnel0
tunnel mpls traffic-eng path-option 1 explicit name PE1
tunnel mpls traffic-eng path-option 99 dynamic
PE2#trace
Protocol [ip]:
Target IP address: 192.168.1.4
Source address:
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]:
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]: l
Source route: 192.168.1.5
Loose, Strict, Record, Timestamp, Verbose[LV]: v
Loose, Strict, Record, Timestamp, Verbose[L]:
Type escape sequence to abort.
Tracing the route to PE4 (192.168.1.4)
1 192.168.1.17 40 msec 32 msec 32 msec
2 192.168.1.13 120 msec 468 msec 576 msec
3 192.168.1.9 76 msec * 80 msec
interface Tunnel0
tunnel mpls traffic-eng path-option 1 explicit name PE4
tunnel mpls traffic-eng path-option 99 dynamic
ip explicit-path name PE4 enable
next-address 192.168.1.17
next-address 192.168.1.13
next-address 192.168.1.9
next-address 192.168.1.4
PE4#trace
Protocol [ip]:
Target IP address: 192.168.1.2
Source address:
Numeric display [n]:
Timeout in seconds [3]:
Probe count [3]:
Minimum Time to Live [1]:
Maximum Time to Live [30]:
Port Number [33434]:
Loose, Strict, Record, Timestamp, Verbose[none]: l
Source route: 192.168.1.5
Loose, Strict, Record, Timestamp, Verbose[LV]: v
Loose, Strict, Record, Timestamp, Verbose[L]:
Type escape sequence to abort.
Tracing the route to PE2 (192.168.1.2)
1 192.168.1.10 32 msec 32 msec 32 msec
2 192.168.1.14 64 msec 64 msec 72 msec
3 192.168.1.18 112 msec * 96 msec
interface Tunnel0
tunnel mpls traffic-eng path-option 1 explicit name PE4
tunnel mpls traffic-eng path-option 99 dynamic
ip explicit-path name PE2 enable
next-address 192.168.1.10
next-address 192.168.1.14
next-address 192.168.1.18
next-address 192.168.1.2
Explicit Path Verification
| Step 4 | Perform the steps outlined in the section Simple MPLS TE verification to verify proper operation of the MPLS TE tunnels. |
| Step 5 | Perform trace from A1 ® A2 and from B1 ® B2. The traffic should flow over the P router. |
A1#trace a2
Type escape sequence to abort.
Tracing the route to A2 (201.1.0.2)
1 150.1.11.1 28 msec 28 msec 28 msec
2 192.168.1.14 [MPLS: Labels 24/25 Exp 0] 408 msec 400 msec 400 msec
3 192.168.1.18 [MPLS: Labels 27/25 Exp 0] 360 msec 660 msec 440 msec
4 150.1.11.9 [MPLS: Label 25 Exp 0] 232 msec 268 msec 356 msec
5 150.1.11.10 144 msec * 144 msec
A1#
B1#trace b2
Type escape sequence to abort.
Tracing the route to B2 (201.2.0.2)
1 150.1.12.1 28 msec 28 msec 28 msec
2 192.168.1.17 [MPLS: Labels 25/26 Exp 0] 396 msec 400 msec 396 msec
3 192.168.1.13 [MPLS: Labels 26/26 Exp 0] 364 msec 364 msec 1056 msec
4 150.1.12.9 [MPLS: Label 26 Exp 0] 228 msec 232 msec 232 msec
5 150.1.12.10 164 msec * 548 msec
B1#
