LFA Tie-Breaker
Based on the index values configured, if inequalities are satisfied, protections will be provided:
• Lower the index will have the highest priority, The path which provides protection with highest priority will be selected. If there are multiple paths providing the highest priority protection then we will check which of the path provides the protection which has 2nd highest priority and so on.
• If all the paths provide same priority, then the LFA route is chosen on the basis of path cost.
• If none of the paths provides the protection with highest priority, then we will see which path provides the 2nd highest priority and so on.
The show command below displays default values for tie-breaker, by default maximum protection (link, node, broadcast, if ecmp, ecmp backup path) will be provided.
After configuring tie-breaker with index, values will be changed accordingly.
R1#show ip isis lfa-config level-1
TIE-Breaker Preference values
-------------------------------------------------
Primary Path : 20
Link Protecting : 60
Node Protecting : 30
Broadcast Interface Disjoint : 70
Secondary Path : 255
Downstream Path : 90
Termination Hold On Interval : 1000 ms
R1
To change index values, below configurations should be used, with the lower the index highest the priority.
(config)#router isis 1 | Create an IS-IS routing instance for area 49 with instance 1 |
(config-router)#fast-reroute per-prefix level-1 proto ipv4 all | Configure LFA-FRR to calculate the available backup path for all L1 ipv4 prefixes learned |
(config-router)#fast-reroute tie-break level-1 proto ipv4 interface-disjoint index 1 | Configure index value to change priority for link protection |
(config-router)#fast-reroute tie-break level-1 proto ipv4 node-protecting index 2 | Configure index value to change priority for node protection |
(config-router)#fast-reroute tie-break level-1 proto ipv4 broadcast-interface-disjoint index 3 | Configure index value to change priority for broadcast link protection |
(config-if)#commit | Commit candidate configuration to the running configuration |
(config-router)#exit | Exit router mode. |
(config)#exit | Exit config mode. |
Validation
R1#show ip isis lfa-config level-1
TIE-Breaker Preference values
-------------------------------------------------
Primary Path : 20
Link Protecting : 1
Node Protecting : 2
Broadcast Interface Disjoint : 3
Secondary Path : 255
Downstream Path : 90
Termination Hold On Interval : 1000 ms
R1#show ip isis route fast-reroute
Tag : 1 VRF : default
Codes : L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area,
D - discard, LP - Link Protecting, NP - Node Protecting,
BP - Broadcast Interface Disjoint, Pri - Primary Path,
Sec - Secondary Path, DP - Downstream Path
L1 40.40.40.0/24
Primary Path via : 10.10.10.141, eth1
FRR Backup Path via : 30.30.30.144, eth3
FRR Metric : 25
Protection Provided : LP NP BP
L1 50.50.50.0/24
Primary Path via : 20.20.20.143, eth2
FRR Backup Path via : 30.30.30.144, eth3
FRR Metric : 20
Protection Provided : LP NP BP
L1 60.60.60.0/24
Primary Path via : 10.10.10.141, eth1
FRR Backup Path via : 30.30.30.144, eth3
FRR Metric : 20
Protection Provided : LP NP BP DP
R1#show ip route fast-reroute
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area ,p - stale info
* - candidate default
IP Route Table for VRF "default"
i L1 40.40.40.0/24 [115/10] via 10.10.10.141, eth1, 00:45:16
[FRR-NH] via 30.30.30.144, eth3
i L1 50.50.50.0/24 [115/15] via 20.20.20.143, eth2, 00:45:29
[FRR-NH] via 30.30.30.144, eth3
i L1 60.60.60.0/24 [115/15] via 10.10.10.141, eth1, 00:45:16
[FRR-NH] via 30.30.30.144, eth3