IP SLA — Cisco's Active Network Measurement + Track Object Integration

The one-sentence mental model

IP SLA is a synthetic probe engine built into IOS. Instead of waiting for user traffic to fail (or a routing-protocol hello to time out), the router itself sends test packets — ICMP echo, TCP connect, UDP jitter, HTTP GET, DNS query — and measures the result. Feed that into track objects and the router reacts to measurement, not just interface state.

What IP SLA can probe

Basic ICMP probe

ip sla 10
  icmp-echo 8.8.8.8 source-interface GigabitEthernet0/1
  frequency 10
  timeout 500
  threshold 200
ip sla schedule 10 life forever start-time now

• Send an ICMP echo to 8.8.8.8 every 10 seconds
• Timeout after 500 ms
• Consider it “over threshold” (unhealthy) at 200 ms
• Run forever, start now

Verify:

show ip sla statistics 10
show ip sla summary

UDP jitter with a responder

Real jitter measurement needs both endpoints. The far-end must be a Cisco device running the IP SLA Responder:

! On the responder (far end):
ip sla responder

! On the source (near end):
ip sla 20
  udp-jitter 198.51.100.10 5000 num-packets 20 interval 10
  frequency 30
  request-data-size 200
ip sla schedule 20 life forever start-time now

Now you get one-way latency + jitter + loss (both directions) with timestamps that account for the responder-side processing delay.

Track objects — where the value comes in

Raw IP SLA data is useful but the real power is coupling it to track objects that other features consume.

Track SLA reachability

track 100 ip sla 10 reachability
  delay down 5 up 20

Track 100 is “up” while IP SLA 10 is meeting its threshold, “down” when it isn’t. Delay smooths flapping.

HSRP with tracking — WAN-aware failover

interface Vlan10
  ip address 10.10.10.2 255.255.255.0
  standby 1 ip 10.10.10.1
  standby 1 priority 110
  standby 1 preempt
  standby 1 track 100 decrement 20

If track 100 goes down (WAN link failing SLA), the priority drops from 110 to 90 — the standby (which has priority 100) becomes active. Gateway follows the healthy WAN.

Floating static + track — dual-WAN failover

ip route 0.0.0.0 0.0.0.0 203.0.113.1 track 100        ! primary, active while WAN is healthy
ip route 0.0.0.0 0.0.0.0 198.51.100.1 200             ! floating static, AD 200

When track 100 (IP SLA 10 to the primary ISP) goes down, the primary default is removed and the AD-200 floating static becomes active.

PBR with tracking — reroute voice

route-map VOICE-BEST-PATH permit 10
  match ip address VOICE-ACL
  set ip next-hop verify-availability 10.1.1.1 10 track 100
  set ip next-hop 10.2.2.1

If track 100 (measuring the good WAN) is down, PBR falls back to the second next-hop.

Reasonable thresholds

• VoIP one-way latency: < 150 ms end-to-end, ideally < 100 ms on the WAN
• Jitter: < 30 ms for good voice quality
• Packet loss: < 1%
• Transactional apps: RTT < 200 ms typically fine

Match threshold on IP SLA to your app’s real requirements — too tight = false positives; too loose = late reaction.

Common exam / real-world mistakes

1. Probing 8.8.8.8 and being surprised when Google has a bad minute. Probe an FQDN you control, or use a responder at your hub.
2. Setting probe frequency too aggressive. 1-second probes × 50 IP SLA operations = 50 pps of extra load, and CPU cost on lower-end platforms.
3. Track object without delay. A flapping SLA → flapping track → HSRP flap. Use delay down N up M (typical: down 5, up 20-30) to smooth.
4. Skipping the responder for jitter. UDP jitter without a responder still runs but the timestamps aren’t reliable → jitter number is garbage.
5. Track SLA reachability vs threshold. reachability = up while probes succeed. threshold triggers on the threshold statement in the SLA config (usually average RTT). They’re not interchangeable.
6. Not verifying with show ip sla statistics. If probes aren’t reporting, your track object is stuck. Check first.

Cheat strip

Operations   icmp-echo | tcp-connect | udp-jitter | udp-echo | http | dns | ftp
Roles        source (initiates) + responder (accepts, timestamps for jitter)

Config shape ip sla N → operation → frequency + timeout + threshold
             ip sla schedule N life forever start-time now

Track        track N ip sla M reachability | state
             delay down T up T   (smooth flapping)

Consumers    HSRP standby X track N decrement Y
             ip route ... track N  (floating static failover)
             PBR: set ip next-hop verify-availability ... track N

Verify       show ip sla statistics N
             show track brief
             show ip sla summary