08-18-2014 01:15 AM - edited 03-04-2019 11:33 PM
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.
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08-18-2014 06:26 AM
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
08-18-2014 06:58 AM
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
08-18-2014 06:26 AM
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
08-18-2014 07:00 AM
Hello
Apologies peter had my browser window open to long and didnt see your responce..
res
Paul.
08-18-2014 07:02 AM
Paul,
Absolutely no need to apologize - you are welcome any time.
Best regards,
Peter
08-18-2014 07:07 AM
i will check your answer and tell you the result once i reach my room, thanks :)
08-18-2014 11:59 AM
Thanks Peter and Paul both explained well the use of ip sla.
08-18-2014 06:58 AM
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
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