Cisco: IP Policy Routing with IP SLA and EEM

Considering the same environment like in the post Cisco: Policy Routing with IP SLA, there is another way to achieve the same behavior using again IP SLA and EEM (Embedded Event Manager).

For those of you who are not so familiar with EEM please read http://www.cisco.com/en/US/products/ps6815/products_ios_protocol_group_home.html. You will find a nice explanation and some examples how to use EEM to achieve the desired result.

Now, going back to our example, please conside the same topology like in the previous post:

We start by configuring again the IP SLA (explanation in this post):

ip sla 5
icmp-echo 172.82.100.1 source-interface GigabitEthernet0/0
timeout 1000
frequency 2
ip sla schedule 5 life forever start-time now

We have the path measured. Instead of tracking this and applying the route based on tracking, we have now a different approach. We use EEM to check the conditions of IP SLA, and according to the result we configure the necessary IP routing. For EEM to work we need to know and Object name and the OID associated with it. In my example I will use the SNMP Object name rttMonCtrlOperTimeoutOccurred with OID value: 1.3.6.1.4.1.9.9.42.1.2.9.1.6

According to Cisco’s explanation “This object is set to true when an operation times out, and set to false when an operation completes under rttMonCtrlAdminTimeout. When this value changes, a reaction may occur, as defined by rttMonReactAdminTimeoutEnable”

As a summary, we will check the IP SLA with EEM using a certain SNMP Object. When a change occur in the monitored IP SLA, EEM will apply a certain configuration defined by us:

event manager applet IP-SLA-5-TIMEOUT
event snmp oid 1.3.6.1.4.1.9.9.42.1.2.9.1.6.5 get-type exact entry-op eq entry-val 1 exit-op eq exit-val 2 poll-interval 5
action 1.0 syslog msg “172.82.100.1 not reachable – primary line NOK”
action 1.1 cli command “enable”
action 1.2 cli command “configure terminal”
action 1.3 cli command “ip route 0.0.0.0 0.0.0.0 10.10.10.1″

EEM is based on a SNMP event. It is monitoring the OID value explained above. You may notice that at the end of the OID value, has been added another value .5 This is important as it defines the relation between EEM and IP SLA. In my case this number is 5, as the IP SLA session is defined, but in your case it may be different. This is checking if the TruthValue is 1 (true) or 2(false), on a 5 second interval and it’s applying the defined configuration. The EEM triggers on value 1 (true), so when the timeout occurs in IP SLA.

You might wonder, what will happen when the primary line is working. Well nothing in this conditions, because EEM is not configure for the case when the primary line is OK. In other words, EEM will not retract the backup default IP route. For this we need another EEM to be configured with a small modification:

event manager applet IP-SLA-5-OK
event snmp oid 1.3.6.1.4.1.9.9.42.1.2.9.1.6.5 get-type exact entry-op eq entry-val 2 exit-op eq exit-val 1 poll-interval 5
action 1.0 syslog msg “172.82.100.1 is reachable – primary line OK”
action 1.1 cli command “enable”
action 1.2 cli command “configure terminal”
action 1.3 cli command “no ip route 0.0.0.0 0.0.0.0 10.10.10.1″

Now the EEM is triggered on the value 2 (false), so when no timeout occurs in IP SLA.

You might be interested in another EEM configuration, which send an e-mail notification when a certain condition occur. Check it here.