# ISIS Timers

Load **isis.inter.area.cfg**

```
#IOS-XE
config replace flash:isis.inter.area.cfg

#IOS-XR
configure
load bootflash:isis.inter.area.cfg
commit replace
y
```



Set R1 and XR2 to use the following ISIS timers:

* Local LSPs should have a max lifetime of 600 seconds. These LSPs should be refreshed every 300 seconds.
* Locally-generated LSPs should be throttled 500 msec after a change, 750 msec if occuring after an LSP was just generated previously, and a max backoff of 3 seconds.
* SPF calculation should be throttled 250 msec after receiving an update, 500 msec if an SPF run has just occured, and a max wait time of 3 seconds.
* Partial route calculation should use the same timers as SPF calculation.
* During flooding, interfaces should pace LSPs at 20 msec intervals
* During flooding, the router should allow for up to 3 seconds before retransmitting the same LSP. If LSPs do need to be retransmitted, these should be paced at 40 msec intervals.













































## Answer <a href="#e7fa98cd-5cbd-4d4e-b6af-c6d2d38f7666" id="e7fa98cd-5cbd-4d4e-b6af-c6d2d38f7666"></a>

```
#R1
router isis
 max-lsp-lifetime 600
 lsp-refresh-interval 300
 spf-interval 3 250 500
 prc-interval 3 250 500
 lsp-gen-interval 3 500 750
!
int Gi2.12
 isis lsp-interval 20
 isis retransmit-interval 3
 isis retransmit-throttle-interval 40

#XR2
router isis 1
 lsp-max-lifetime 600
 lsp-refresh-interval 300
 lsp-gen-interval initial-wait 500 secondary-wait 750 maximum-weight 3000
 !
 address-family ipv4
  spf-interval initial-wait 250 second-wait 500 maximum-wait 3000
 !
 int Gi0/0/0/0.1920
  lsp-interval 20
  retransmit-interval 3
  retransmit-throttle-interval 40
```



## Explanation <a href="#id-14b1764d-1cba-4d37-a071-e3761cc2fd51" id="id-14b1764d-1cba-4d37-a071-e3761cc2fd51"></a>

The **lsp-max-lifetime** command sets the lifetime on locally generated LSPs. By default this appears to be 1200 seconds. This defines how long an LSP is valid in the LSDB. A longer interval allows for less frequent refreshes, which saves some bandwidth in a large network.

```
#IOS-XE
router isis
 lsp-max-lifetime <sec>

#IOS-XR
router isis 1
 lsp-max-lifetime <sec>
```



The **lsp-refresh-interval** controls how frequently locally-generated LSPs are refreshed. This timer cannot be larger than the lsp-max-lifetime timer.

```
#IOS-XE
router isis
 lsp-refresh-interval <sec>

#IOS-XR
router isis 1
 lsp-refresh-interval <sec>
```



When an IGP topology change is detected, either via a local change or a received LSP, SPF should be delayed incase subsequent changes occur. The more you delay this, the more you can “batch” multiple topology changes together, but the less quickly the IGP will converge. The first parameter is the maxmium wait time. The second parameter is the initial wait time. The last parameter is the wait time between the first and second SPF run. Subsequent SPF runs will continue to backoff until reaching the max wait time.

```
#IOS-XE
router isis
 spf-interval <max wait in seconds> <initial wait in msec> <min wait for 2nd run in msec>

#IOS-XR
router isis 1
 address-family ipv4
  spf-interval initial-wait <msec> second-wait <msec> maximum-wait <msec>
```



Partial route calculation is any change that does not affect the topology. For example, if a prefix is removed/added from a node, the IGP topology has not changed, so an SPF run is not needed. Instead, a partial rotue calculation is used to more quickly update the RIB. The timers for PRC is configured in a similar manner to the SPF timers. This does not appear to be configurable for IOS-XR.

```
#IOS-XE
router isis
 prc-interval <max wait in seconds> <initial wait in msec> <min wait for 2nd run in msec>
```



The interface-level command **isis lsp-interval** controls pacing during LSP flooding. During LSP flooding, you may want the router to send the LSPs at a slower pace to give the remote router a better chance of processing the LSPs. This command sets the delay between subsequent LSP transmissions on the interface.

```
#IOS-XE
int Gi1
 isis lsp-interval <msec>

#IOS-XR
router isis 1
 int Gi0/0/0/0
  lsp-interval <msec>
```



During flooding, it’s possible that some LSPs are lost and need to be retransmitted. The amount of time to wait for an ACK to an LSP is controlled with the **isis retransmit-interval** interface command.

```
#IOS-XE
int Gi1
 isis retransmit-interval <sec>

#IOS-XR
router isis 1
 int Gi0/0/0/0
  retransmit-interval <msec>
```



If many LSPs need to be retransmitted, these can be paced using a separate **retransmit-throttle-interval** setting. It may be the case that the router was pacing these LSPs too quickly at first, so retransmited LSPs can be paced at a slower timer.

```
#IOS-XE
int Gi1
 isis retransmit-throttle-interval <msec>

#IOS-XR
router isis 1
 int Gi0/0/0/0
  retransmit-throttle-interval <msec>
```
