Understand and Troubleshoot Flexible Algorithm in ISIS
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Contents
Introduction
This document describes operating Flexible Algorithm (Flex-Algo) in ISIS as well as, provides relevant commands for verification and troubleshooting.
Prerequisites
Requirements
- Cisco recommends that you have basic knowledge of ISIS Segment routing traffic engineering.
Components Used
- The information in this document is based on Device: Aggregation Services Router 9000 (ASR9K).
- The information in this document was created from the devices in a specific lab environment. All the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
Flex-Algo Overview
- Flexible Algorithm (Flex-Algo) is identified by a numerical value in the range 0–255; values 0–127 are reserved, while 128–255 are user-defined.
- A Flex-Algo is defined by a metric type (Interior Gateway Protocol (IGP) metric, delay metric, or TE metric) and a set of constraints (example link affinity, Shared Risk Link Group (SRLG)).
- By leveraging Flex-Algo, networks can implement network slicing, that is logical segmentation of the network, where path computation is restricted to the topology that satisfies the defined constraints.
Figure 1: Prefix-SID Sub-TLV Format (with Algorithm Field)
Figure 2: Flex-Algo Definition（FAD) TLV Format
Network Topology
Figure 3: Network Topology Diagram
Network
- Algorithm 128 → Defines a latency-optimized slice.
- Algorithm 129 → Defines a TE-optimized slice.
- ABR1 and ABR2 advertise their Flex-Algo Definitions (FAD TLVs) in IS-IS Link-State Protocol Data Unit (LSPs).
- A FAD originator can be any router in the IS-IS domain; a router does not need to participate in each Flex-Algo to advertise its definition.
Flex-Algo Configuration
router isis core
flex-algo 128
interface Loopback100
passive
address-family ipv4 unicast
prefix-sid index 11 explicit-null
prefix-sid algorithm 128 index 811 explicit-null
Note:
- index 11 - Prefix-SID bound to Flex-Algo 0 (default shortest path first (SPF)).
- index 811 - Prefix-SID bound to Flex-Algo 128.
- Flex-Algo Prefix-SIDs enable network slicing , For a given prefix (example Loopback100), multiple Prefix-SIDs can be advertised—each tied to a specific Flex-Algo.
- The first Prefix-SID (Index 11) is automatically associated with Flex-Algo 0 (default IGP shortest-path algorithm).
- The second Prefix-SID (Index 811) is explicitly associated with Flex-Algo 128, a user-defined algorithm.
Flex-Algo 129 Defined by Metric-type TE
Configuration
On ABR2:
router isis core
flex-algo 129
priority 50
metric-type te
advertise-definition
interface Loopback100
address-family ipv4 unicast
prefix-sid algorithm 129 index 92 explicit-null >> Prefix-SID bound to Flex-Algo 129
interface HundredGigE0/1/0/2
point-to-point
address-family ipv4 unicast
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa
te-metric flex-algo 20 level 2 >> 20 is the TE metric value for this link , need to be enabled on the links only between those routers participating in Flex-Algo 129
Verification
PE3#show isis flex-algo 129
IS-IS core Flex-Algo Database
Flex-Algo 129:
Level-2:
Definition Priority: 100
Definition Source: ASR9906-2-ABR-1.00
Definition Equal to Local: No
Definition Metric Type: TE
Definition Flex-Algo Prefix Metric: No
<snip>
FRR Disabled: No
Microloop Avoidance Disabled: No
UCMP Disabled: No
Data Plane Segment Routing: Yes
Data Plane IP: Yes
PE3#show isis database 55A1-2-PE4.00-00 verbose internal
IS-IS core (Level-2) Link State Database
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL Len Received From On Reason
55A1-2-PE4.00-00 0x00000149 0xfe2f 995 /1200 0/0/0 282 09:21:39 ASR9910-4-ABR-2 Hu0/1/0/0 Flooded
<snip>
TLV code:242 length:28
Router Cap: 10.10.44.44 D:0 S:0
SubTLV code:2 length:9
Segment Routing: I:1 V:0, SRGB Base: 16000 Range: 8000
SubTLV code:23 length:2
Node Maximum SID Depth:
Label Imposition: 12
SubTLV code:19 length:3
SR Algorithm:
Algorithm: 0
Algorithm: 1
Algorithm: 129
SubTLV code:29 length:1
IP Algorithm:
Algorithm: 129
TLV code:22 length:117
Metric: 10 IS-Extended ASR9910-4-ABR-2.00
SubTLV code:4 length:8
Local Interface ID: 32, Remote Interface ID: 38
SubTLV code:6 length:4
Interface IP Address: 10.10.244.44
SubTLV code:8 length:4
Neighbor IP Address: 10.10.244.2
SubTLV code:9 length:4
Physical BW: 10000000 kbits/sec
SubTLV code:16 length:9
Application Specific Link Attributes:
L flag: 0, SA-Length: 1, UDA-Length: 1
Standard Applications: 0x10 FLEX-ALGO
User Defined Applications: 0x10
SubTLV code:18 length:3
Admin. Weight: 20 >>>>>>>>>> configured TE metric
PE3#show isis topology flex-algo 129
IS-IS core paths to IPv4 Unicast (Level-2) routers
System Id Metric Next-Hop Interface SNPA
ASR9910-4-ABR-2 20 ASR9910-4-ABR-2 Hu0/1/0/0 *PtoP* >> 1 hop away from PE3
55A1-2-PE4 40 ASR9910-4-ABR-2 Hu0/1/0/0 *PtoP* >> 2 hops away from PE3
Flex-Algo 128 Defined by Metric-type Delay
Configuration
router isis core
flex-algo 128
priority 100
metric-type delay
advertise-definition
performance-measurement
interface HundredGigE0/1/0/0
delay-measurement >>> this CLI needs to be enabled only on those interfaces between those routers participating in Flex-Algo 128
!
!
interface HundredGigE0/1/0/3
delay-measurement
Verification
PE1#show isis flex-algo 128
IS-IS core Flex-Algo Database
Flex-Algo 128:
Level-2:
Definition Priority: 100
Definition Source: ASR9906-2-ABR-1.00
Definition Equal to Local: No
Definition Metric Type: Delay
Definition Flex-Algo Prefix Metric: No
Note: On PE2, the performance-measurement interface HundredGigE0/1/0/1 >> link between PE2 and ABR 1 delay-measurement advertise-delay 100 >> delay is statically configured. This increases the delay metric by +100 in the direction from PE2 towards PE1, hence the delay to reach ABR 1 from PE2 is 100.
PE1#show isis database ASR-9904-5-PE2.00-00 verbose internal
IS-IS core (Level-2) Link State Database
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL LSP Length
ASR-9904-5-PE2.00-00 0x00000231 0x6084 1131 /1200 0/0/0 310
<snip>
TLV code:242 length:25
<snip>
SubTLV code:19 length:3
SR Algorithm:
Algorithm: 0
Algorithm: 1
Algorithm: 128
TLV code:22 length:144
Metric: 10 IS-Extended ASR9906-2-ABR-1.00
SubTLV code:4 length:8
Local Interface ID: 17, Remote Interface ID: 22
SubTLV code:6 length:4
Interface IP Address: 10.10.221.22
SubTLV code:8 length:4
Neighbor IP Address: 10.10.221.1
SubTLV code:9 length:4
Physical BW: 99999998 kbits/sec
SubTLV code:33 length:4
Link Average Delay: 100 us
SubTLV code:34 length:8
Link Min/Max Delay: 100/100 us
SubTLV code:35 length:4
Link Delay Variation: 0 us
SubTLV code:16 length:14
Application Specific Link Attributes:
L flag: 0, SA-Length: 1, UDA-Length: 1
Standard Applications: 0x10 FLEX-ALGO
User Defined Applications: 0x10
SubTLV code:34 length:8
Link Min/Max Delay: 100/100 us >>> delay calculated on this link due to the confoguration
PE2#show isis topology flex-algo 128
IS-IS core paths to IPv4 Unicast (Level-2) routers
System Id Metric Next-Hop Interface SNPA
ASR9906-2-ABR-1 100 ASR9906-2-ABR-1 Hu0/1/0/1 *PtoP*
ASR9906-1-PE1 112 ASR9906-2-ABR-1 Hu0/1/0/1 *PtoP*
When there is no manual advertise-delay configured on the interface, and using the default delay measurement profile enabled on the interface, probes are sent on the links to measure the delay on the interface.
ABR-1#show performance-measurement sessions
----------------------------------------------------------------------------------------------------------------------------------------------------------------
0/1/CPU0
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Transport type : Interface
Measurement type : Delay Measurement
Interface name : HundredGigE0/1/0/0
Nexthop : Unknown
Delay Measurement session:
Session ID : 4097
Profile Keys:
Profile name : default
Profile type. : Interface Delay Measurement
Last advertisement:
Advertised at: Sep 26 2025 14:00:36.179 (247623.282 seconds ago)
Advertised reason: Periodic timer, min delay threshold crossed
Advertised delays (uSec): avg: 12, min: 12, max: 13, variance: 0
A flag set: False
Next advertisement:
Threshold check scheduled in 2 more probes (roughly every 120 seconds)
Aggregated delays (uSec): avg: 12, min: 11, max: 12, variance: 1
Rolling average (uSec): 11
Current computation:
Started at: Sep 29 2025 10:47:17.373 (22.88 seconds ago)
Packets Sent: 8, received: 8
Measured delays (uSec): avg: 12, min: 11, max: 12, variance: 1 >>>>>>>>>>>> the delay measured using probes on this interface
Next probe scheduled at: Sep 29 2025 10:47:47.369 (in 7.908 seconds)
Next packet will be sent in 1.908 seconds
Packet sent every 3.0 seconds
Responder IP : 10.10.111.11
Number of Hops : 1
ABR-1#show isis database ASR9906-1-PE1.00-00 verbose internal
IS-IS core (Level-2) Link State Database
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL LSP Length
ASR9906-1-PE1.00-00 0x00000777 0x13ef 497 /1200 0/0/0 370
<snip>
TLV code:22 length:197
Metric: 10 IS-Extended ASR9906-2-ABR-1.00
SubTLV code:4 length:8
Local Interface ID: 40, Remote Interface ID: 21
SubTLV code:6 length:4
Interface IP Address: 10.10.111.11
SubTLV code:8 length:4
Neighbor IP Address: 10.10.111.1
SubTLV code:9 length:4
Physical BW: 99999998 kbits/sec
SubTLV code:33 length:4
Link Average Delay: 12 us >>>>>>>>>>>>>> the calculated delay is propagated via ISIS which is used by the Headend router to calculate the path with minimum delay
SubTLV code:34 length:8
Link Min/Max Delay: 12/13 us
SubTLV code:35 length:4
Link Delay Variation: 0 us
SubTLV code:16 length:14
Application Specific Link Attributes:
L flag: 0, SA-Length: 1, UDA-Length: 1
Standard Applications: 0x10 FLEX-ALGO
User Defined Applications: 0x10
SubTLV code:34 length:8
Link Min/Max Delay: 12/13 us
PE1#show isis topology flex-algo 128
IS-IS core paths to IPv4 Unicast (Level-2) routers
System Id Metric Next-Hop Interface SNPA
ASR9906-2-ABR-1 12 ASR9906-2-ABR-1 Hu0/1/0/0 *PtoP*
ASR-9904-5-PE2 24 ASR9906-2-ABR-1 Hu0/1/0/0 *PtoP*
Note:
- From PE1 to reach ABR 1 there is only 12 us delay
- From PE1 to reach PE2 there is 24 us delay
Topology-independent Loop-Free Alternate (TI-LFA) and Microloop Avoidance ( MLA ) on FLEX-ALGO 129
Figure 4: Network Topology Diagram Used to Demonstrate TI-LFA and MLA for Flex-Algo 129
ABR-1#show isis fast-reroute flex-algo 129 10.10.2.2/32 detail
L2 10.10.2.2/32 [20/115] Label: 16092, medium priority
Installed Sep 17 10:40:08.503 for 00:16:25
via 10.10.12.2, HundredGigE0/1/0/1, Label: Exp-Null-v4, ASR9910-4-ABR-2, SRGB Base: 16000, Weight: 0
Backup path: TI-LFA (link), via 10.10.221.22, HundredGigE0/1/0/3 ASR-9904-5-PE2, SRGB Base: 16000, Weight: 0, Metric: 60
P node: ASR9910-3-PE3.00 [10.10.33.33], Label: 16933 >>>>>>>>>> TI-LFA precomputed path for FRR
Prefix label: 16092
Backup-src: ASR9910-4-ABR-2.00
P: No, TM: 60, LC: No, NP: No, D: No, SRLG: Yes
src ASR9910-4-ABR-2.00-00, 10.10.2.2, prefix-SID index 92, R:0 N:1 P:1 E:1 V:0 L:0, Alg:129
Note:
- 16933 PE3 Flex-Algo 129 label
- 16092 ABR -2 Flex-Algo 129 label
- The TI-LFA path is also restricted with the logical network slice of flex algo 129
On ABR-1 when the link between ABR 1 and 2 is shut down MLA-tunnel is triggered:
ABR-1 #show isis microloop avoidance 10.10.2.2/32 flex-algo 129 detail
10.10.2.2/32 participated in 1 of 6 microloop avoidance events
number start spf duration cleanup duration pins: ipv4 ipv6
--- --- --- --- --- --- --- ---
5 restart 10:59:16.440 0 ms 10:59:16.643 3 0
trigger: Link down, near node: ASR9906-2-ABR-1.00, far node: ASR9910-4-ABR-2.00
via 10.10.221.22, Hu0/1/0/3, Labels: 16933
In both the TI-LFA and MLA , Flex-Algo 129 runs its own constrained SPF, precomputes a fast reroute tunnel that respects Algo 129 constraints, using only FLEX-ALGO 129 labels.
Link Affinity-Map Constraints
- Affinity is a 32-bit attribute associated with an IGP link, administratively assigned by the operator.
- Each bit position corresponds to a semantic meaning defined by the operator.
- These affinity bits are advertised in the IGP (OSPF/IS-IS) as part of the link’s TE attributes.
When a Flex-Algo computes its shortest path tree (SPF), it applies affinity constraints during path selection:
- Include-Any (IA): The computed path must traverse at least one link that has any of the specified affinity bits set.
- Include-All (IAll): The computed path must traverse only links that collectively contain all of the specified affinity bits.
- Exclude-Any (EA): The computed path must avoid all links that carry any of the specified affinity bits.
Configuration
On ABR 1and 2:
On ABR 1and 2
router isis core
affinity-map ALGO-129 bit-position 6
flex-algo 129
priority 100
metric-type te
advertise-definition
affinity exclude-any ALGO-129
Post this configuration on the FAD advertised.
Verification
PE4#show isis database ASR9906-2-ABR-1.00-00 verbose internal
IS-IS core (Level-2) Link State Database
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL Len Received From On Reason
ASR9906-2-ABR-1.00-00 0x0000023f 0xa89f 871 /1200 0/0/0 453 11:12:43 ASR9906-1-PE1 Hu0/0/0/5 Flooded
TLV code:242 length:44
Router Cap: 10.10.1.1 D:0 S:0
<snip>
SubTLV code:19 length:4
SR Algorithm:
Algorithm: 0
Algorithm: 1
Algorithm: 128
Algorithm: 129
SubTLV code:26 length:4
Flex-Algo Definition:
Algorithm: 129 Metric-Type: 2 Alg-type: 0 Priority: 100
SubTLV code:1 length:4
Flex-Algo Exclude-Any Ext Admin Group: >>> FAD to exclude any link with this bit position set
0x00000040
PE4#show isis flex-algo 129
IS-IS core Flex-Algo Database
Flex-Algo 129:
Level-2:
Definition Priority: 100
Definition Source: ASR9906-2-ABR-1.00
Definition Equal to Local: No
Definition Metric Type: TE
Definition Flex-Algo Prefix Metric: No
Exclude Any Affinity Bit Positions: 6
Path taken from PE4 to PE3 when the link affinity-map is not configured on any of the links:
PE4#traceroute sr-mpls labels 16933 lsp-end-point 10.10.33.33
Type escape sequence to abort.
0 10.10.244.44 MRU 1500 [Labels: 16933/explicit-null Exp: 0/0] >>>>>>>>>>>>>>>>>>>> PE4
L 1 10.10.244.2 MRU 1500 [Labels: explicit-null/explicit-null Exp: 0/0] 5 ms >>>>>>>>ABR2
! 2 10.10.32.33 4 ms >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>PE3
Figure 5: traceroute Between PE4 to PE3 with no Affinity Map Defined
Configuration on ABR-2 of the Affinity Map
router isis core
affinity-map ALGO-129 bit-position 6 >> define the bit position and associate it with a name
router isis core
interface HundredGigE0/1/0/2 >>> link between ABR-2 to PE3
affinity flex-algo ALGO-129 >>>>> link affinity map
point-to-point
address-family ipv4 unicast
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa
te-metric flex-algo 20 level 2
Verification
PE4#show isis database ASR9910-4-ABR-2.00-00 verbose internal
<snip>
TLV code:22 length:193
<snip>
SubTLV code:6 length:4
Interface IP Address: 10.10.32.2
SubTLV code:8 length:4
Neighbor IP Address: 10.10.32.33
SubTLV code:9 length:4
Physical BW: 99999998 kbits/sec, 12499999744 bytes/sec
SubTLV code:16 length:21
Application Specific Link Attributes:
L flag: 0, SA-Length: 1, UDA-Length: 1
Standard Applications: 0x10 FLEX-ALGO
User Defined Applications: 0x10
SubTLV code:14 length:4
Ext Admin Group:
0x00000040 >>>>>> link affinity flooded via IGP
SubTLV code:3 length:4
Affinity: 0x00000040
SubTLV code:18 length:3
Admin. Weight: 20
As per the FAD for Flex Algo 129 from PE4 towards PE3, it needs to exclude any link with affinity map bit position 6.
PE4#traceroute sr-mpls labels 16933 lsp-end-point 10.10.33.33
0 10.10.244.44 MRU 1500 [Labels: 16933/explicit-null Exp: 0/0]
L 1 10.10.244.2 MRU 1500 [Labels: 16933/explicit-null Exp: 0/0] 5 ms >>>>>>>>>>>>> ABR2
L 2 10.10.12.1 MRU 1500 [Labels: 16933/explicit-null Exp: 0/0] 4 ms >>>>>>>>>>>>>> ABR1
L 3 10.10.221.22 MRU 1500 [Labels: explicit-null/explicit-null Exp: 0/0] 15 ms >>> PE2
! 4 10.10.233.33 4 ms >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> PE3
Figure 6: Path Taken from PE4 to PE3 after Configuring the Affinity Map
Note: Similarly, using “include any” and “include all” in the FAD of a flex algo, we can influence the path calculation from the head end towards the tail end.
Conclusion
This document describes the use of Flexible Algorithm (Flex-Algo) to logically slice an IS-IS domain into multiple network slices. Each slice can define its own path computation rules and constraints, enabling differentiated treatment of traffic streams based on SLA requirements or traffic sensitivity.
Commands
show isis flex-algo <>
show isis flex-algo 129 neighbors
show isis topology flex-algo
show isis database <> verbose detail
show isis route <> flex-algo <> detail
show mpls forwarding labels <> detail
show isis microloop avoidance <> flex-algo <> detail
show isis fast-reroute flex-algo <> <prefix> detail
Revision History
| Revision | Publish Date | Comments |
|---|---|---|
2.0 |
10-Sep-2025
|
Initial Release |
1.0 |
08-Sep-2025
|
Initial Release |
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