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IP SLA Questions

July 11th, 2019 in ROUTE 300-101 Go to comments

Question 1

Question 2


IP SLA PBR (Policy-Based Routing) Object Tracking allows you to make sure that the next hop is reachable before that route is used. If the next hop is not reachable, another route is used as defined in the PBR configuration. If no other route is present in the route map, the routing table is used.

An example of configuring PBR based on tracking object is shown below:

//Configure and schedule IP SLA operations
ip sla 1
ip sla schedule 1 life forever start-time now
// Configure Object Tracking to track the operations
track 1 ip sla 1 reachability
//Configure ACL
ip access-list standard ACL
permit ip
//Configure PBR policing on the router
route-map PBR
match ip address ACL
set ip next-hop verify-availability track 1
set ip next-hop verify-availability track 2 -> Track 2 is not shown here but it is used if track 1 fails
//Apply PBR policy on the incoming interface of the router.
interface ethernet 0/0
ip address
ip policy route-map PBR

Reference: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/6_x/nx-os/IPSLA/configuration/guide/b_Cisco_Nexus_7000_Series_NX-OS_IP_SLAs_Configuration_Guide_rel_6-x/b_Cisco_Nexus_7000_Series_NX-OS_IP_SLAs_Configuration_Guide_rel_6-x_chapter_01000.html

Question 3


The keyword “tcp-connect” enables the responder for TCP connect operations. TCP is a connection-oriented transport layer protocol -> C is correct.

Question 4


The “num-packets” specifies the number of packets to be sent for a jitter operation.

The “frequency” is the rate (in seconds) at which this IP SLA operation repeats. The “tos” defines a type of service (ToS) byte in the IP header of this IP SLA operation.

Question 5


When enabled, the IP SLAs Responder allows the target device to take two time stamps both when the packet arrives on the interface at interrupt level and again just as it is leaving, eliminating the processing time. At times of high network activity, an ICMP ping test often shows a long and inaccurate response time, while an IP SLAs test shows an accurate response time due to the time stamping on the responder.

An additional benefit of the two time stamps at the target device is the ability to track one-way delay, jitter, and directional packet loss. Because much network behavior is asynchronous, it is critical to have these statistics. However, to capture one-way delay measurements the configuration of both the source device and target device with Network Time Protocol (NTP) is required. Both the source and target need to be synchronized to the same clock source. One-way jitter measurements do not require clock synchronization.

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipsla/configuration/15-mt/sla-15-mt-book/sla_overview.html

Question 6

Question 7

Question 8


Depending on the specific Cisco IOS IP SLAs operation, statistics of delay, packet loss, jitter, packet sequence, connectivity, path, server response time, and download time are monitored within the Cisco device and stored in both CLI and SNMP MIBs.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_4/ip_sla/configuration/guide/hsla_c/hsoverv.html

Question 9


IP SLAs supports proactive threshold monitoring and notifications for performance parameters such as average jitter, unidirectional latency, bidirectional round-trip time (RTT), and connectivity for most IP SLAs operations. The proactive monitoring capability also provides options for configuring reaction thresholds for important VoIP related parameters including unidirectional jitter, unidirectional packet loss, and unidirectional VoIP voice quality scoring.

IP SLAs reactions are configured to trigger when a monitored value exceeds or falls below a specified level or when a monitored event, such as a timeout or connection loss, occurs. If IP SLAs measures too high or too low of any configured reaction, IP SLAs can generate a notification (in the form of SNMP trap) to a network management application or trigger another IP SLA operation to gather more data.

Cisco IOS IP SLAs can send SNMP traps that are triggered by events such as the following:
+ Connection loss
+ Timeout
+ Round-trip time threshold
+ Average jitter threshold
+ One-way packet loss
+ One-way jitter
+ One-way mean opinion score (MOS)
+ One-way latency

Question 10


Round-trip time (RTT), also called round-trip delay, is the time required for a packet to travel from a specific source to a specific destination and back again.

An ICMP Path Echo operation measures end-to-end (full path) and hop-by-hop response time (round-trip delay) between a Cisco router and devices using IP. ICMP Path Echo is useful for determining network availability and for troubleshooting network connectivity issues.

Note: ICMP Echo only measures round-trip delay for the full path.

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipsla/configuration/xe-3s/sla-xe-3s-book/sla_icmp_pathecho.html

Question 11


The default route command (at the last line) must include the “track” keyword for the tracking feature to work.

ip route track 10

Question 12

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  1. digitaltut
    July 31st, 2019

    Thanks for your detection. We have just updated Q.6

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