Lab 18: Default Route + Floating Static Backup
The dual-ISP pattern in miniature: a default route to the primary path and a floating static (higher administrative distance) that silently waits — then takes over the moment the primary dies. Difficulty: Intermediate · Time: ~30 min.
Lab objectives
- Configure a primary default route
- Add a floating static with AD 10
- Verify only the primary is installed
- Fail the primary and watch the backup activate
Topology & addressing
R1 with two upstream links: primary to ISP1 (10.0.1.2), backup to ISP2 (10.0.2.2). LAN 192.168.1.0/24 behind R1. A test target (loopback/server) reachable via both ISPs.
Step-by-step configuration
ip route 0.0.0.0 0.0.0.0 10.0.1.2 | Primary default (AD 1) |
ip route 0.0.0.0 0.0.0.0 10.0.2.2 10 | Floating backup — the trailing 10 is its AD |
show ip route | Only the AD-1 route installs; the backup floats invisibly |
Verification
Routing table shows a single S* via ISP1. Shut the primary interface: within a moment show ip route flips the S* to ISP2 — the floating route surfaced. Restore the link and the primary reclaims (lower AD always wins). Continuous ping across the failover makes it visceral.
Next lab: labs hub · test yourself: CCNA practice test.
Frequently asked questions
What makes a static route float?
A manually raised administrative distance — it loses to the primary while both exist, and installs only when the primary route disappears.
Why AD 10 and not 254?
Anything higher than the primary's AD works; keep it below dynamic protocols' ADs if you want the backup preferred over learned routes (or above, if not).
When does the primary come back?
As soon as its interface/next-hop returns, the lower-AD route reinstalls automatically — floats are self-managing.
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