IP Service Level Agreement (IP SLA) in Networking

Introduction

IP Service Level Agreement (IP SLA) is also called as Cisco Response Time Reporter (RTR) and Service Assurance Agent (SAA). This is a very important topic of CCNA and CCNP which are very important Networking Certification in current IT industry. Expertise in it will help in clearing Cisco certifications in Delhi of CCNP route and switch exams.

Once a Network is built it is hard to analyze its performance from an end-user perspective. In some cases, factors like actual traffic loads and link conditions can impact the time-critical applications leading to slow convergence or network to be down.

IP SLA feature evaluates and examines traffic flow from source to destination across a network by running a preconfigured test and generating traffic destined for a far-end destination device. After the destination device receives the packet, depending on the type of IP SLAs operation, it responds with time-stamp information for the source to make the calculation on performance metrics and to assess how well the source and destination were able to communicate.

IP SLA collects below mentioned performance metrics

a) Delay (both round trip and one way)

Network delay describes how long it takes packets to travel the network to reach to the destination. In asymmetric routing design where all packets traverse the same hops for both directions. round-trip delay measurement could be easy to measure. If asymmetric routing is applied, packets from source to destination take a different path than the return traffic. This can be measured with one-way delay operations.

b) Jitter (directional)

Jitter means interpacket delay variance. If the destination receives a packet with the arrival delay of more or less than 10ms from source then they are considered to be non-zero delays. Ideally for a network to work well to have a zero delay destination should receive packets from a source within an interval of 10ms apart.

c) Packet loss (directional)

Packet loss happens when a network element drops packets instead of forwarding them. This could occur because of overload situations when a router or switch cannot accept any incoming data.

d) DHCP and DNS Response Time

Requesting an IP address through DHCP is usually limited to one operation per user session whereas DNS requests occur multiple times per session because web pages are usually designed to retrieve information from multiple servers.

e) Connectivity (directional)

TCP connect describes how long it takes a TCP request to be served at the destination server.

Objective

The aim is to use IP SLA to continuously check the network ICMP echo connectivity from source (R1) to destination (R4) router. To make source router have 2 links one fast Ethernet as primary and serial as secondary to reach R4 loopback. Finally to track the R1’s (source) primary ink using IP SLA and switch to secondary in case primary link fails.

The various IP SLA operations can be classified as follows:

ICMP-based operations for Echo, Path Echo, and Path Jitter.
UDP-based operations, such as echo, jitter, DNS, and DHCP.
TCP-based operations, such as TCP Connect, FTP, HTTP

IP SLA Advantages

1) IP SLA is used as a simulation tool for generating traffic like VOIP, ICMP, DNS, HTTP etc.

2) It is also used as object tracking used in HSRP, VRPP, GLBP and static routes. IP SLA tracks routes and if an interface fails or if the ping fails, IPSLA removes the route from the routing table.

3) Troubleshooting of network operation by providing consistent, reliable measurement live tests that immediately identifies problems and saves troubleshooting time.

4) Gather voice quality measurements from all over a network

Configuration

Tasks

1. Configure IP addresses on all the routers as shown in the topology

2. Create a loopback 4.4.4.4/24 to the loopback 4

3. Make sure you achieve full connectivity both ways from R1 to R4 in the topology using the static routes.

4. Make sure fast Ethernet link of R1 is used to reach R4 loopback 1 4.4.4.4/24 network as a primary link and serial link of R1 is used as a secondary link to reach R4 loopback1. Make sure only in the case when fast-Ethernet link being down, then the only serial link is used.

5. Generate continuous ICMP echo test packets from R1 to R4 to make sure connectivity is intact and verify the same using the “ip sla statistics” command.

Steps

Step 1) Configure IP addresses shown in the topology and static routes on all the routers to enable full to and fro connectivity reachability between R1 to R4.

Step2)

Fist "show ip sla application" command in the privilege mode to verify the type of operation supported by your software image.

As can be seen from above ICMP echo, dhcp, dns , http, ftp, udp jitter, path jitter etc operations are supported by the software image.

Step 3) configure IP SLA configuration on R3

R1 commands seen from the above screenshot are explained below

a) To make f0/0 primary link and s2/0 secondary link

Configure serial link static route ad value to 10 and let the ad value to of static route of fast Ethernet link to remain 1 by default. By doing this route having more ad value (via serial link) will be less preferable and route having less ad value (of f0/0 link) will be more preferable making it the primary link.

b) IP SLA Configuration

The IP SLA operation number can be anywhere from 1 to 2147483647. After that, Configure the ip sla operation type to be ICMP echo with the destination address configured to be the R4 lo1 interface 4.4.4.4 causing the icmp echo test packets to be sent to the destination router R4 network 4.4.4.4.

After that hit enter to enter the ICMP echo configuration mode where the frequency is specified which indicates after how many seconds the IP SLA operation repeats. The frequency range can be configured from 1 to 604800 by default 60 secs is used. Here we have used frequency of 9 seconds. After that exit ICMP echo configuration mode.

You can then enable the SLA test by using the schedule “ip sla schedule 1” command and configuring the parameters required to define the duration of the test.

Parameter “start-time” defines when to start the test, possible values used for it can be “now” meaning start right away (as used in our case) or after some specified hours using “ after hh::mm::ss”. Parameter life defines till what time to keep the test activated. Possible values for it can be forever as in keep the test alive for whole life (as used in our case) or for a specific number of seconds which can range from 0 to 2147483647 seconds and default value of 3600 seconds or 1hr is used.

Observation

From the first screenshots it can be seen that the R1(IP SLA source device) is sending ICMP echo packets to R4 loopback1 4.4.4.4 after every 9 sec and getting a response ICMP echo reply back from R4 lo1 which can be confirmed from

a) the number of successes having positive value in “show ip sla statistics” command and

b) the reachability being up from the “show track” command.

IP SLA is tracking the static route using its fast ethernet link that can be seen by the track keyword placed in front of R1 fast ethernet static route in the “show run | sec ip route” command

R1 who has another serial link available besides fast Ethernet link, is not using it as serial link has a higher ad value making lower ad value of fast Ethernet more preferable to reach the lo1 of R4 as can be verified from show ip route command.

From the above screenshot, it can be seen that fast Ethernet of R1 goes down using no shut command. IP SLA is continuously tracking the static route of the R1 f0/0 link destined to 4.4.4. which then goes down, IP SLA immediately switches over to another available serial static route of R1. IP SLA also removes the f0/0 link and adds the serial link to the R1routing table (as can be seen from “show ip route”) and does not cause any convergence issue (as can be seen from show “ip sla statistics”).

Conclusion

An IP SLAs operation performs a network measurement from the source device to a destination in the network using a specific protocol such as UDP. It is verified from the above lab that IP SLA continuously tracks routes by generating traffic for network troubleshooting. It can also switch over to the redundant link in case of the primary link goes down to avoid convergence issues.

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