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ROUTE TRACKING USING IP SLA

i have some confusion with IP SLA feature, in this topology i have two links from R1 to R2 and R3, and two default route one primary another one backup with higher AD value (R1-ip route 0.0.0.0 0.0.0.0 2.2.2.2- R1-ip route 0.0.0.0 0.0.0.0 3.3.3.3 10) 

Without IP SLA if one link goes down R1 will automatically switch to backup link. 

what if th link between R2 via Internet wnt down ? is it possible to configure on R1 to check reachability to 8.8.8.8.if R1 not able to ping via R2 to 8.8.8.8, it should automatically switch to R1 to R3 link.

 

2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

Hello,

Hello,

The more proper way of using IP SLA here is not about pinging the 8.8.8.8 which is in neither ISP's address space, but rather ping some specific IP address inside the particular ISP's address space. If that address is responding, the ISP is considered usable. If not, the ISP is unusable and you should move over to the other ISP.

Assume that ISP1 was using 22.22.22.22 on its own link between R2 and the Internet. You should be pinging that IP address instead of 8.8.8.8, and the configuration would be similar to this:

ip sla 1
 icmp-echo 22.22.22.22
 timeout 2000
 threshold 1000
 frequency 10
!
ip sla schedule 1 life forever start-time now
!
track 1 rtr 1 ! In newer IOSes, this is track 1 ip sla 1 state
!
ip route 22.22.22.22 255.255.255.255 Serial1/0 2.2.2.2
ip route 22.22.22.22 255.255.255.255 Null0 2
ip route 0.0.0.0 0.0.0.0 2.2.2.2 track 1
ip route 0.0.0.0 0.0.0.0 3.3.3.3 2

The point of this configuration is to make sure that to reach the 22.22.22.22, you only use the link to ISP1, or drop the packets right away (i.e. you do not try to reach 22.22.22.22 mistakenly over ISP2 which could possibly work on real internet).
 

Does this make sense? Please feel welcome to ask further!

Best regards,
Peter

HelloWithout IP SLA if one

Hello

Without IP SLA if one link goes down R1 will automatically switch to backup link  -  Not necessary depends on were the link goes down

If R1 interace went down then yes the secondary static would come up but if the link failed at the isp end then the primary static would still be in the rib and balckhole you traffic unless you track the reachablity of the link to the isp
 

 

what if th link between R2 via Internet wnt down ? is it possible to configure on R1 to check reachability to 8.8.8.8.if R1 not able to ping via R2 to 8.8.8.8, it should automatically switch to R1 to R3 link - A wee bit harder as something like 8.8.8.8 would be reachable from either isp so you may still get reachbality and the tracking/ip sla would not get initacted - I would use something in the isp own address space to track instead to validate its availablity and remove the primary staitic is connection thorugh this isp isnt applicable

 

 

res

Paul

Please don't forget to rate any posts that have been helpful. Thanks.
6 REPLIES
Cisco Employee

Hello,

Hello,

The more proper way of using IP SLA here is not about pinging the 8.8.8.8 which is in neither ISP's address space, but rather ping some specific IP address inside the particular ISP's address space. If that address is responding, the ISP is considered usable. If not, the ISP is unusable and you should move over to the other ISP.

Assume that ISP1 was using 22.22.22.22 on its own link between R2 and the Internet. You should be pinging that IP address instead of 8.8.8.8, and the configuration would be similar to this:

ip sla 1
 icmp-echo 22.22.22.22
 timeout 2000
 threshold 1000
 frequency 10
!
ip sla schedule 1 life forever start-time now
!
track 1 rtr 1 ! In newer IOSes, this is track 1 ip sla 1 state
!
ip route 22.22.22.22 255.255.255.255 Serial1/0 2.2.2.2
ip route 22.22.22.22 255.255.255.255 Null0 2
ip route 0.0.0.0 0.0.0.0 2.2.2.2 track 1
ip route 0.0.0.0 0.0.0.0 3.3.3.3 2

The point of this configuration is to make sure that to reach the 22.22.22.22, you only use the link to ISP1, or drop the packets right away (i.e. you do not try to reach 22.22.22.22 mistakenly over ISP2 which could possibly work on real internet).
 

Does this make sense? Please feel welcome to ask further!

Best regards,
Peter

HelloApologies peter had my

Hello

Apologies peter had my browser window open to long and didnt see your responce..

res

Paul.

Please don't forget to rate any posts that have been helpful. Thanks.
Cisco Employee

Paul,Absolutely no need to

Paul,

Absolutely no need to apologize - you are welcome any time.

Best regards,
Peter

New Member

i will check your answer and

i will check your answer and tell you the result once i reach my room, thanks :)

New Member

Thanks Peter and Paul both

Thanks Peter and Paul both explained well the use of ip sla. 

HelloWithout IP SLA if one

Hello

Without IP SLA if one link goes down R1 will automatically switch to backup link  -  Not necessary depends on were the link goes down

If R1 interace went down then yes the secondary static would come up but if the link failed at the isp end then the primary static would still be in the rib and balckhole you traffic unless you track the reachablity of the link to the isp
 

 

what if th link between R2 via Internet wnt down ? is it possible to configure on R1 to check reachability to 8.8.8.8.if R1 not able to ping via R2 to 8.8.8.8, it should automatically switch to R1 to R3 link - A wee bit harder as something like 8.8.8.8 would be reachable from either isp so you may still get reachbality and the tracking/ip sla would not get initacted - I would use something in the isp own address space to track instead to validate its availablity and remove the primary staitic is connection thorugh this isp isnt applicable

 

 

res

Paul

Please don't forget to rate any posts that have been helpful. Thanks.
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