The creation of the segment routing Flex-Algorithm forwarding information results in the label forwarding tables on the router. On a Nokia router, it is possible to look both at the tunnel table and the routing table to understand the Flex-Algorithm path toward a destination prefix.
For example, algorithm 128 has been configured to use the delay metric and consequently forwards traffic using the lowest delay through the network. In the following figure, Node B is configured with IP address 10.20.1.2/32, the A-B path has the best default IGP metric, and the A-C-B path has the best delay.
Figure: Selecting the lowest delay path
Example: tunnel-table command output
A:Dut-A# show router tunnel-table
===============================================================================
IPv4 Tunnel Table (Router: Base)
===============================================================================
Destination Owner Encap TunnelId Pref Nexthop Metric
Color
-------------------------------------------------------------------------------
10.10.10.2/32 isis (0) MPLS 524298 11 10.10.10.2 0
10.10.10.6/32 isis (0) MPLS 524292 11 10.10.10.6 0
10.20.1.2/32 isis (0) MPLS 524296 11 10.10.10.2 10
10.20.1.2/32 isis (0) MPLS 524306 11 10.10.10.6 200
10.20.1.3/32 isis (0) MPLS 524294 11 10.10.10.6 10
10.20.1.3/32 isis (0) MPLS 524307 11 10.10.10.6 100
-------------------------------------------------------------------------------
Flags: B = BGP or MPLS backup hop available
L = Loop-Free Alternate (LFA) hop available
E = Inactive best-external BGP route
k = RIB-API or Forwarding Policy backup hop
===============================================================================
A:Dut-A#
Example: Detailed tunnel-table command output
A:Dut-A# show router tunnel-table 10.20.1.2/32 detail
===============================================================================
Tunnel Table (Router: Base)
===============================================================================
Destination : 10.20.1.2/32
NextHop : 10.10.10.2
Tunnel Flags : entropy-label-capable
Age : 18h21m35s
CBF Classes : (Not Specified)
Owner : isis (0) Encap : MPLS
Tunnel ID : 524296 Preference : 11
Tunnel Label : 20002 Tunnel Metric : 10
Tunnel MTU : 1560 Max Label Stack : 1
-------------------------------------------------------------------------------
Destination : 10.20.1.2/32
NextHop : 10.10.10.6
Tunnel Flags : entropy-label-capable
Age : 02h01m32s
CBF Classes : (Not Specified)
Owner : isis (0) Encap : MPLS
Algorithm : 128
Tunnel ID : 524306 Preference : 11
Tunnel Label : 20012 Tunnel Metric : 200
Tunnel MTU : 1560 Max Label Stack : 1
-------------------------------------------------------------------------------
Number of tunnel-table entries : 2
Number of tunnel-table entries with LFA : 0
===============================================================================
A:Dut-A#
Example: Route table output with and without the Flex-Algorithm context
A:Dut-A# show router isis routes
===============================================================================
Rtr Base ISIS Instance 0 Route Table
===============================================================================
Prefix[Flags] Metric Lvl/Typ Ver. SysID/Hostname
NextHop MT AdminTag/SID[F]
-------------------------------------------------------------------------------
10.10.10.0/30 10 1/Int. 65 Dut-A
0.0.0.0 0 0
10.10.10.4/30 10 1/Int. 42 Dut-A
0.0.0.0 0 0
10.10.10.8/30 20 2/Int. 65 Dut-B
10.10.10.2 0 0
10.20.1.1/32 0 1/Int. 42 Dut-A
0.0.0.0 0 0/1[NnP]
10.20.1.2/32 10 2/Int. 65 Dut-B
10.10.10.2 0 0/2[NnP]
10.20.1.3/32 10 2/Int. 42 Dut-C
10.10.10.6 0 0/3[NnP]
-------------------------------------------------------------------------------
No. of Routes: 6 (6 paths)
-------------------------------------------------------------------------------
Flags : L = LFA nexthop available
SID[F] : R = Re-advertisement
N = Node-SID
nP = no penultimate hop POP
E = Explicit-Null
V = Prefix-SID carries a value
L = value/index has local significance
===============================================================================
A:Dut-A#
A:Dut-A# show router isis routes flex-algo 128
===============================================================================
Rtr Base ISIS Instance 0 Flex-Algo 128 Route Table
===============================================================================
Prefix[Flags] Metric Lvl/Typ Ver. SysID/Hostname
NextHop MT AdminTag/SID[F]
-------------------------------------------------------------------------------
10.20.1.2/32 200 2/Int. 82 Dut-C
10.10.10.6 0 0/12[NnP]
10.20.1.3/32 100 2/Int. 82 Dut-C
10.10.10.6 0 0/13[NnP]
-------------------------------------------------------------------------------
No. of Routes: 2 (2 paths)
-------------------------------------------------------------------------------
Flags : L = LFA nexthop available
SID[F] : R = Re-advertisement
N = Node-SID
nP = no penultimate hop POP
E = Explicit-Null
V = Prefix-SID carries a value
L = value/index has local significance
===============================================================================
A:Dut-A#
Example: Detailed route table output, with and without the Flex-Algorithm context
A:Dut-A# show router isis routes 10.20.1.2 detail
===============================================================================
Rtr Base ISIS Instance 0 Route Table (detail)
===============================================================================
Prefix : 10.20.1.2/32
Status : Active Level : 2
NextHop : 10.10.10.2
Metric : 10 Type : Internal
SPF Version : 65 SysID/Hostname : Dut-B
MT : 0 AdminTag : 0
SID : 2 SID-Flags : NnP
-------------------------------------------------------------------------------
No. of Routes: 1 (1 path)
-------------------------------------------------------------------------------
SID[F] : R = Re-advertisement
N = Node-SID
nP = no penultimate hop POP
E = Explicit-Null
V = Prefix-SID carries a value
L = value/index has local significance
===============================================================================
A:Dut-A#
A:Dut-A# show router isis routes 10.20.1.2 flex-algo 128 detail
===============================================================================
Rtr Base ISIS Instance 0 Flex-Algo 128 Route Table (detail)
===============================================================================
Prefix : 10.20.1.2/32
Status : Active Level : 2
NextHop : 10.10.10.6
Metric : 200 Type : Internal
SPF Version : 82 SysID/Hostname : Dut-C
MT : 0 AdminTag : 0
SID : 12 SID-Flags : NnP
-------------------------------------------------------------------------------
No. of Routes: 1 (1 path)
-------------------------------------------------------------------------------
SID[F] : R = Re-advertisement
N = Node-SID
nP = no penultimate hop POP
E = Explicit-Null
V = Prefix-SID carries a value
L = value/index has local significance
===============================================================================
A:Dut-A#