What is IP SLA, and how do you use it to fail over a default route?
Show the answer
Say this first
IP SLA is Cisco's built-in probe engine. The router sends synthetic traffic on a schedule (ICMP echo, UDP jitter, TCP connect, HTTP, DNS and more) and records reachability, latency, jitter and loss. Paired with object tracking it drives decisions: a track object follows the probe result, a static route tied to that track is withdrawn when the probe fails, and a floating static route to a backup ISP takes over.
Go deeper
Configuration
ip sla 1
icmp-echo 198.51.100.200 source-interface GigabitEthernet0/0/0
timeout 2000
threshold 1000
frequency 10
ip sla schedule 1 life forever start-time now
!
track 1 ip sla 1 reachability
delay down 10 up 30
!
ip route 0.0.0.0 0.0.0.0 203.0.113.1 track 1
ip route 0.0.0.0 0.0.0.0 192.0.2.9 250
ip route 198.51.100.200 255.255.255.255 203.0.113.1- The probe target 198.51.100.200 sits beyond ISP1's first hop. ISP1's router at 203.0.113.1 can answer while its upstream is broken.
timeoutandthresholdare in milliseconds,frequencyin seconds. The defaults are 5000 ms, 5000 ms and 60 seconds.delay down 10 up 30stops a flapping path from flapping the route.- The host route keeps the probe on ISP1. Without it, after failover the probe would follow the backup default, succeed, and bring the primary route back: a loop of flaps.
- The backup default to ISP2 (192.0.2.9) has distance 250, so it only installs when the tracked route is gone.
Other probes
udp-jitter measures one-way jitter, loss and latency for voice and needs ip sla responder on the
far Cisco device. With a codec option it also estimates a MOS score.
Verify
show ip sla statistics 1, show track 1, show ip route static. On Junos the equivalent is RPM
(real-time performance monitoring) probes with ip-monitoring.
They will ask next
Why not just ping the ISP's next hop?
The next hop is on a directly connected link. It can answer happily while everything behind it is broken. Probe something further upstream that the ISP agrees will answer.
What does a udp-jitter probe need on the far end?
ip sla responderon a Cisco device. The responder timestamps the probes, so the sender can remove processing time and measure jitter and loss in each direction.
Traps
- Forgetting
ip sla schedule. Without it the probe is defined but never runs. - Probing the ISP next hop on the directly connected link.