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IP SLA Questions 2

July 11th, 2017 in ROUTE 300-101 Go to comments

Question 1

Explanation

User Datagram Protocol (UDP) Jitter for VoIP is the most common operation for networks that carry voice traffic, video, or UDP jitter-sensitive applications. Requires Cisco endpoints.

Note: The ICMP jitter operation is similar to the IP SLAs UDP jitter operation but does not require a Cisco endpoint (maybe only Cisco router has been designated to reply to Cisco IOS IP SLA test packets).

The config below shows an example of configuring UDP Jitter for VoIP:

Router(config)# ip sla 10
//Configures the operation as a jitter (codec) operation that will generate VoIP scores in addition to latency, jitter, and packet loss statistics. Notice that it requires an endpoint.
Router(config-ip-sla)# udp-jitter 209.165.200.225 16384 codec g711alaw advantage-factor 10
//The below configs are only optional
Router(config-ip-sla-jitter)# frequency 30
Router(config-ip-sla-jitter)# history hours-of-statistics-kept 4
Router(config-ip-sla-jitter)# owner admin
Router(config-ip-sla-jitter)# tag TelnetPollServer1
Router(config-ip-sla-jitter)# threshold 10000
Router(config-ip-sla-jitter)# timeout 10000
Router(config-ip-sla-jitter)# tos 160

Reference: http://www.cisco.com/en/US/technologies/tk648/tk362/tk920/technologies_qas0900aecd8017bd5a.html & http://www.cisco.com/c/en/us/td/docs/ios/ipsla/configuration/guide/15_s/sla_15_0s_book/sla_udp_jitter_voip.pdf

Question 2

Explanation

There is no problem with the Fa0/0 as the source interface as we want to check the ping from the LAN interface -> A is not correct.

Answer B is not correct as we must track the destination of the primary link, not backup link.

In this question, R1 pings R2 via its LAN Fa0/0 interface so maybe R1 (which is an ISP) will not know how to reply back as an ISP usually does not configure a route to a customer’s LAN -> C is correct.

There is no problem with the default route -> D is not correct.

For answer E, we need to understand about how timeout and threshold are defined:

Timeout (in milliseconds) sets the amount of time an IP SLAs operation waits for a response from its request packet. In other words, the timeout specifies how long the router should wait for a response to its ping before it is considered failed.Threshold (in milliseconds too) sets the upper threshold value for calculating network monitoring statistics created by an IP SLAs operation. Threshold is used to activate a response to IP SLA violation, e.g. send SNMP trap or start secondary SLA operation. In other words, the threshold value is only used to indicate over threshold events, which do not affect reachability but may be used to evaluate the proper settings for the timeout command.

For reachability tracking, if the return code is OK or OverThreshold, reachability is up; if not OK, reachability is down.

Therefore in this question, we are using “Reachability” tracking (via the command “track 10 ip sla 1 reachability”) so threshold value is not important and can be ignored -> Answer E is correct. In fact, answer E is not wrong but it is the best option left.

This tutorial can help you revise IP SLA tracking topic: http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/813-cisco-router-ipsla-basic.html and http://www.ciscozine.com/using-ip-sla-to-change-routing/

Note: Maybe some of us will wonder why there are these two commands:

R1(config)#ip route 0.0.0.0 0.0.0.0 172.20.20.2 track 10
R1(config)#no ip route 0.0.0.0 0.0.0.0 172.20.20.2

In fact the two commands:

ip route 0.0.0.0 0.0.0.0 172.20.20.2 track 10
ip route 0.0.0.0 0.0.0.0 172.20.20.2

are different. These two static routes can co-exist in the routing table. Therefore if the tracking goes down, the first command will be removed but the second one still exists and the backup path is not preferred. So we have to remove the second one.

Question 3

Explanation

A primary benefit of Cisco IOS IP SLAs is accuracy, embedded flexibility, and cost-saving, a key component of which is the Cisco IOS IP SLAs responder enabled on the target device. When the responder is enabled, it allows the target device to take two timestamps: when the packet arrives on the interface at interrupt level and again just as it leaves. This eliminates processing time. This timestamping is made with a granularity of sub-millisecond (ms). The responder timestamping is very important because all routers and switches in the industry will prioritize switching traffic destined for other locations over packets destined for its local IP address (this includes Cisco IOS IP SLAs and ping test packets). Therefore, at times of high network activity, ping tests can reveal an inaccurately large response time; conversely, timestamping on the responder allows a Cisco IOS IP SLAs test to accurately represent the response time due.

Reference: http://www.cisco.com/en/US/technologies/tk648/tk362/tk920/technologies_white_paper0900aecd8017f8c9_ps6602_Products_White_Paper.html

Note: The ICMP echo operation is used to cause ICMP echo requests to be sent to a destination to check connectivity

Question 4

Explanation

The “show ip sla statistics” command displays the current operational status and statistics of all IP SLAs operations or a specified operation so the answer “operation availability” is the best choice here.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/ipsla/command/reference/sla_book/sla_04.html

Question 5

Explanation

You can configure a tracked list of objects with a Boolean expression, a weight threshold, or a percentage threshold.

The example configures track list 1 to track by weight threshold.

Switch(config)# track 1 list threshold weight
Switch(config-track)# object 1 weight 15
Switch(config-track)# object 2 weight 20
Switch(config-track)# object 3 weight 30
Switch(config-track)# threshold weight up 30 down 10

If object 1, and object 2 are down, then track list 1 is up, because object 3 satisfies the up threshold value of up 30. But, if object 3 is down, both objects 1 and 2 must be up in order to satisfy the threshold weight.

This configuration can be useful if object 1 and object 2 represent two small bandwidth connections and object 3 represents one large bandwidth connection. The configured down 10 value means that once the tracked object is up, it will not go down until the threshold value is equal to or lower than 10, which in this example means that all connections are down.

The below example configures tracked list 2 with three objects and a specified percentages to measure the state of the list with an up threshold of 70 percent and a down threshold of 30 percent:

Switch(config)# track 2 list threshold percentage
Switch(config-track)# object 1
Switch(config-track)# object 2
Switch(config-track)# object 3
Switch(config-track)# threshold percentage up 51 down 10

This means as long as 51% or more of the objects are up, the list will be considered “up”. So in this case if two objects are up, track 2 is considered “up”.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/blades/3020/software/release/12-2_58_se/configuration/guide/3020_scg/swhsrp.pdf

Question 6

Question 7

Question 8

Question 9

Explanation

Maybe this question wants to ask “which location IP SLAs are usually used to monitor the traffic?” then the answer should be WAN edge as IP SLA is usually used to track a remote device or service (usually via ping).

Comments
  1. sallu
    August 8th, 2017

    please share me the questions for the same

  2. ali
    August 22nd, 2017

    @Anonymous which dump did you follow??

  3. Anonymous
    August 27th, 2017

    @Anonymous , are the dumpZ ( Digitaltut ) valid ?

  4. tukul
    September 23rd, 2017

    are the dump digitaltut valid? uptodate until now?

  5. cl
    September 29th, 2017

    For Q2 the answer E is not correct. The correct answer should is A,C. Normally when configuring source interface for a IP SLA we choose the directly connected interface (which ensures that it is always reachable). No ISP will have the LAN address of R1 in its routing table.

  6. mago
    October 27th, 2017

    why in q2 the threshold value is wrong?
    the formula is frequency>timeout>threshold, so the threshold 500 is right

  7. mago
    November 4th, 2017

    in Q2 the source interface is wrong, should be FastEthernet1/0

  8. Csaba
    November 8th, 2017

    Q2. The explanation says:

    “..we are using “Reachability” tracking (via the command “track 10 ip sla 1 reachability”) so threshold value is not important and can be ignored ..”

    If it can be ignored in this case, how could it be a reason for IP SLA tracking failure?

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