IS-IS Graceful Restart Configuration
The Intermediate System to Intermediate System (IS-IS) routing protocol is a link state intra-domain routing protocol. Normally, when an IS-IS router is restarted, temporary disruption of routing occurs due to events in both the restarting router and the neighbors of the restarting router.
ISIS provides graceful restart, in which the adjacency and routes are maintained in the routing table for the grace period. In this way, the data flow is not affected, and there is no packet loss during the restart phase.
With ISIS GR, the ISIS router should be able to restart gracefully with non-stop forwarding during the recovery. And the Helper ISIS router should be able to help restarting router by maintaining the adjacency.
ISIS Grace Restart Functionality applies to:
• ISIS broadcast network
• ISIS point-to-point network
• IPv4 domain
• IPv6 domain
Topology
In this example, R1 is the L1/L2 router, and R2 is the L1/L2 restart-helper router.
IS-IS Graceful Restart
Configuration
The following configuration is given only for R2, assuming that the adjacency with R1 is already up and the route tables with the appropriate routes are already populated.
R1
#configure terminal | Enter configure mode. |
(config)#isis restart grace-period 300 | Set the grace period to 300 seconds. The restarting router should come up before 300 seconds, otherwise, the adjacency and routes will be deleted. |
(config)#commit | Commit candidate configuration to the running configuration |
R2
#configure terminal | Enter configure mode. |
(config)#isis restart helper | Configure this router as a restart helper. |
(config)#isis restart grace-period 100 | Set the grace period to 100 seconds. The restarting router should come up before 100 seconds, otherwise, the adjacency and routes will be deleted. |
Note: The IS-IS daemon in the restarting router must be manually restarted using the restart IS-IS graceful command; it does not restart automatically.
Note: The unplanned GR is not supported.
Note: In the restarting node,
• restart the IS-IS daemon before it’s LSP's remaining lifetime expires in Helper nodes.
• the restarting node might take higher graceful restart time interval than what it advertised (before GR) as 'Remaning Lifetime' in its LSP.
• in such cases, the LSP's received from the restarting node are not refreshed in helper nodes. These timed-out LSP's are deleted from Helper nodes and disrupt the traffic, though IS-IS adjacency is unaffected. To avoid this, the administrator has to ensure that GR time window is always lesser than the 'Remaning Lifetime' in Helper nodes.
Validation
R1
#restart isis graceful
% Warning :ISISD process will stop and needs to restart manually,
You may loose ISIS configuration, if not saved
Proceed for graceful restart? (y/n):y
Or
R1
#restart isis graceful grace-period 300
% Warning :ISISD process will stop and needs to restart manually,
You may loose ISIS configuration, if not saved
Proceed for graceful restart? (y/n):y
R2
R2#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 13.1.1.0/24 is directly connected, eth1, 04:08:20
i ia 20.0.0.0/6 [115/11] via 13.1.1.2, eth1, 00:10:44
i L1 33.0.0.0/24 [115/20] via 13.1.1.2, eth1, 00:10:44
C 127.0.0.0/8 is directly connected, lo, 04:10:59
C 192.168.52.0/24 is directly connected, eth0, 04:10:55
R2#show clns neighbors
Tag 1: VRF : default
System Id Interface SNPA State Holdtime Type Protocol
0000.0000.0001 xe15 00e0.4bbe.ca02 Up 300 L2 IS-IS
R2#show ip route database
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
> - selected route, * - FIB route, p - stale info
IP Route Table for VRF "default"
C *> 13.1.1.0/24 is directly connected, eth1, 04:10:56
i L1 13.1.1.0/24 [115/10] is directly connected, eth1, 01:58:50
i ia *> 20.0.0.0/6 [115/11] via 13.1.1.2, eth1, 00:13:20
i L1 *> 33.0.0.0/24 [115/20] via 13.1.1.2, eth1, 00:13:20
C *> 127.0.0.0/8 is directly connected, lo, 04:13:35
C *> 192.168.52.0/24 is directly connected, eth0, 04:13:31
Gateway of last resort is not set