What is a floating static route, and how do you choose its administrative distance?
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It is a backup static route with an AD higher than the primary route to the same prefix, so it stays out of the routing table until the primary disappears. The AD must be higher than whatever source provides the primary: above 1 to back up another static, above 110 to back up OSPF, above 170 if the primary is an EIGRP external route. Values like 200, 250 or 254 are common.
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Two defaults, primary over fiber, backup over LTE
ip route 0.0.0.0 0.0.0.0 203.0.113.1 name ISP1
ip route 0.0.0.0 0.0.0.0 198.51.100.1 250 name LTE-BACKUPWhile both are valid, only the AD 1 route is in show ip route. You see the backup only in the running
config, or with show ip static route on IOS-XE.
Backing up a dynamic route
If the branch learns its routes to HQ from OSPF over MPLS, a backup over a VPN tunnel looks like this:
ip route 10.0.0.0 255.0.0.0 172.31.0.1 115AD 115 is above OSPF's 110, so the static only appears when the OSPF route is withdrawn.
The catch
The primary static above stays in the table as long as Gi0/0/0 is up and 203.0.113.1 is in a connected subnet. If the ISP fails upstream while the Ethernet link to their router stays up, the primary never leaves and the backup never floats in. The fix is IP SLA tracking on the primary.
They will ask next
Why not use AD 255 for the backup?
255 means "never install". A floating static at 255 would never be used.
Does the other end need anything?
Yes. The far side needs its own way back over the backup path, or traffic leaves on the backup and returns on the dead primary. Failover has to work in both directions.
Traps
- Choosing an AD below the primary's. A backup static with AD 100 beats OSPF and steals all the traffic.
- Assuming a static on Ethernet fails over when the ISP fails. The link to the ISP's router usually stays up.