The difference between state (also the default) and reachability option has to do with the acceptance of the OverThreshold return code (as indicated on the upper link), and not whether the route is or not in the table.
If you configure a threshold on the ip sla operation:
Reachability --> Track up if the return code is OK or over threshold
--> Track down otherwise
State --> Track up if the return code is OK
--> Track down otherwise
Therefore, if you configure a threshold, the reachability option only takes into account if you can reach the destination IP address of the Echo IP sla operation. The state option also takes into account the RTT of the Echo IP sla operation.
However, if you configure a timeout for instance, bot the reachability and state options take into account the RTT of the Echo IP sla operation.
To track the state of a Cisco IOS IP Service Level Agreements (SLAs) operation and to enter tracking configuration mode, use the track ip sla command in global configuration mode. To remove the tracking, use the no form of this command.
track object-number ip sla operation-number [state | reachability]
no track object-number ip sla operation-number [state | reachability]
Syntax Description
object-number
|
Object number representing the object to be tracked. The range is from 1 to 1000.
|
operation-number
|
Number used for the identification of the IP SLAs operation you are tracking.
|
state
|
(Optional) Tracks the operation return code.
|
reachability
|
(Optional) Tracks whether the route is reachable.
|
Command Default
IP SLAs tracking is disabled.
Command Modes
Global configuration (config)
Command History
|
|
12.4(20)T
|
This command was introduced. This command replaces the track rtr command.
|
12.2(33)SXI1
|
This command was integrated into Cisco IOS Release 12.2(33)SXI1. This command replaces the track rtr command.
|
Cisco IOS XE Release 2.4
|
This command was integrated into Cisco IOS XE Release 2.4. This command replaces the track rtr command.
|
12.2(33)SRE
|
This command was integrated into Cisco IOS XE 12.2(33)SRE. This command replaces the track rtr command.
|
15.1(3)T
|
This command was modified. The valid range of the object-number argument increased to 1000.
|
15.1(1)S
|
This command was modified. The valid range for the object-number argument increased to 1000.
|
Usage Guidelines
Every IP SLAs operation maintains an operation return-code value. This return code is interpreted by the tracking process. The return code may return OK, OverThreshold, and several other return codes. Different operations may have different return-code values, so only values common to all operation types are used.
Two aspects of an IP SLAs operation can be tracked: state and reachability. The difference between these aspects relates to the acceptance of the OverThreshold return code. Table 1 shows the state and reachability aspects of IP SLAs operations that can be tracked.
Table 1 Comparison of State and Reachability Operations
|
|
|
State
|
OK
(all other return codes)
|
Up
Down
|
Reachability
|
OK or over threshold
(all other return codes)
|
Up
Down
|
As of Cisco IOS Release 15.1(3)T, a maximum of 1000 objects can be tracked. Although 1000 tracked objects can be configured, each tracked object uses CPU resources. The amount of available CPU resources on a router is dependent upon variables such as traffic load and how other protocols are configured and run. The ability to use 1000 tracked objects is dependent upon the available CPU. Testing should be conducted on site to ensure that the service works under the specific site traffic conditions.
You can go through the following link also:
http://www.cisco.com/c/en/us/td/docs/ios/ipsla/command/reference/sla_book/sla_05.html#wp1093733
