LFA Termination
A router MUST limit the amount of time an alternate next-hop is used after the primary next-hop has become unavailable. This ensures that the router will start using the new primary next-hops.
LFA termination avoids a micro looping in topology, when particular network goes down, LFA backup path will be installed and if termination interval is configured, LFA backup will be still used till the interval and it is used in order to verify new primary path is loop free.
R1
Configure termination interval on R1 in router mode:
(config)#router isis 1 | Create an IS-IS routing instance for area 49 with instance 1 |
(config-router)#fast-reroute terminate-hold-on interval 100000 | Configure LFA termination interval |
(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 : 100000 ms
R1#show ip route
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, E - EVPN,
v - vrf leaked
* - candidate default
IP Route Table for VRF "default"
C 10.10.10.0/24 is directly connected, eth1, 01:19:46
C 10.12.30.0/24 is directly connected, eth0, 02:19:27
C 20.20.20.0/24 is directly connected, eth2, 01:19:46
C 30.30.30.0/24 is directly connected, eth3, 01:19:46
i L1 40.40.40.0/24 [115/10] via 10.10.10.141, eth1, 00:53:14
i L1 50.50.50.0/24 [115/15] via 20.20.20.143, eth2, 00:53:27
i L1 60.60.60.0/24 [115/15] via 10.10.10.141, eth1, 00:53:14
C 127.0.0.0/8 is directly connected, lo, 02:19:27
Gateway of last resort is not set
Shut down one of the primary nexthops, here eth2 of R1:
(config)#interface eth2 | Enter interface mode |
(config-if)#shutdown | Shutdown the interface |
(config-if)#exit | Exit interface mode |
(config)exit | Exit config mode |
Validation
R1#show clns neighbors
Total number of L1 adjacencies: 2
Total number of L2 adjacencies: 0
Total number of adjacencies: 2
Tag 1: VRF : default
System Id Interface SNPA State Holdtime Type Protocol
0000.0000.0002 eth1 5254.002a.230a Up 27 L1 IS-IS
0000.0000.0004 eth3 5254.00f5.35a4 Up 7 L1 IS-IS
Here, eth1 has become a primary path, which was originally a backup path:
R1#show ip route
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, E - EVPN,
v - vrf leaked
* - candidate default
IP Route Table for VRF "default"
C 10.10.10.0/24 is directly connected, eth1, 01:24:47
C 10.12.30.0/24 is directly connected, eth0, 02:24:28
C 30.30.30.0/24 is directly connected, eth3, 01:24:47
i L1 40.40.40.0/24 [115/10] via 10.10.10.141, eth1, 00:02:01
i L1 50.50.50.0/24 [115/20] via 10.10.10.141, eth1, 00:02:01
i L1 60.60.60.0/24 [115/15] via 10.10.10.141, eth1, 00:02:01
C 127.0.0.0/8 is directly connected, lo, 02:24:28
Gateway of last resort is not set
R1#show ip isis route
Codes: C - connected, E - external, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, D - discard, e - external metric
** - invalid
Tag 1: VRF : default
Destination Metric Next-Hop Interface Tag
C 10.10.10.0/24 5 -- eth1 0
C 30.30.30.0/24 15 -- eth3 0
L1 40.40.40.0/24 10 10.10.10.141 eth1 0
L1 50.50.50.0/24 20 10.10.10.141 eth1 0
L1 60.60.60.0/24 15 10.10.10.141 eth1 0
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 : 10.10.10.141, eth1
FRR Backup Path via : 30.30.30.144, eth3
FRR Metric : 30
Protection Provided : LP NP BP DP
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, 00:02:19
[FRR-NH] via 30.30.30.144, eth3
i L1 50.50.50.0/24 [115/20] via 10.10.10.141, eth1, 00:02:19
[FRR-NH] via 30.30.30.144, eth3
i L1 60.60.60.0/24 [115/15] via 10.10.10.141, eth1, 00:02:19
[FRR-NH] via 30.30.30.144, eth3