Floating Static Routes

A floating static route is a static route configured with an administrative distance (AD) greater than that of the primary route. The primary route is typically learned via a dynamic routing protocol (e.g., OSPF, EIGRP) or another static route with a lower AD. The term 'floating' refers to the fact that this static route only appears in the routing table when the primary route is not present – it 'floats' in and out of the routing table as the primary route fails and recovers.

Floating static routes provide a simple, lightweight method for backup connectivity. They are often used on stub networks, branch offices, or in scenarios where a secondary link (e.g., a cellular backup or a low-bandwidth WAN link) should only be used if the primary link fails. Unlike dynamic routing protocols, floating static routes require no neighbor relationships, no hello timers, and no complex configuration. They are deterministic – the engineer knows exactly which path will be used as backup.

Consider a router R1 that connects to the corporate network via two paths:

Under normal conditions, R1's routing table contains the OSPF-learned route because its AD (110) is lower than the static route's AD (130). The static route is present in the configuration but not in the routing table. When the primary link fails (e.g., FastEthernet0/0 goes down), the OSPF route is removed from the routing table. The router then re-evaluates its routing table and finds the floating static route (AD 130) as the best path for 10.0.0.0/8. It installs this route in the routing table. Traffic now flows over the backup link.

When the primary link comes back up, OSPF re-converges and the OSPF route (AD 110) returns. The router compares ADs: 110 < 130, so the OSPF route is preferred. The floating static route is removed from the routing table (but remains in the configuration).

Cisco IOS uses default AD values for various route sources. For floating static routes, you set the AD to a number higher than the primary route's AD. Common defaults:

For example, to create a floating backup for an OSPF route, use AD 120 or higher (but less than 255). For an EIGRP internal route, use AD 100 or higher.

To configure a floating static route, add the distance keyword at the end of the ip route command:

ip route <destination-network> <subnet-mask> <next-hop-ip> <distance>

Example:

ip route 10.0.0.0 255.0.0.0 192.168.1.2 130

This creates a static route to 10.0.0.0/8 via next-hop 192.168.1.2 with AD 130.

To verify floating static routes, use:

Example output when primary route is up:

R1# show ip route 10.0.0.0
Routing entry for 10.0.0.0/8
  Known via "ospf 1", distance 110, metric 20
  Last update from 10.1.1.1 on FastEthernet0/0, 00:00:05 ago
  ...

Example output when primary route is down:

R1# show ip route 10.0.0.0
Routing entry for 10.0.0.0/8
  Known via "static", distance 130, metric 0
  Last update from 192.168.1.2 on Serial0/0/0, 00:00:02 ago
  ...

Floating static routes work seamlessly with dynamic routing protocols. The router always prefers the route with the lowest AD, regardless of whether it is static or dynamic. This allows you to use a static route as a backup for a dynamic route, or vice versa. However, be careful: if the primary route is learned via a protocol with a higher AD (e.g., RIP, AD 120) and you configure a static route with AD 1 (default), the static route will always be preferred – that's not a floating static route, it's a static route overriding the dynamic one.

The concept is identical for IPv6. Use the ipv6 route command with the distance parameter:

ipv6 route 2001:db8:1::/48 2001:db8:2::2 130

Verification uses show ipv6 route.

In enterprise networks, floating static routes are commonly used to provide backup connectivity for branch offices that have a primary MPLS connection and a secondary internet VPN or cellular link. For example, a branch router might learn the corporate network routes via OSPF over an MPLS circuit (AD 110). To ensure connectivity if the MPLS fails, the network engineer configures a floating static default route (0.0.0.0/0) with AD 130 pointing to a VPN concentrator or a cellular gateway. When the MPLS goes down, OSPF routes are removed, and the floating default route kicks in, sending traffic over the backup link. This is a simple, reliable solution without the need to run a dynamic routing protocol over the backup link (which might be bandwidth-constrained or metered).

Another scenario is in data center networks where you have a primary and a backup link to the core. Instead of using a dynamic protocol that might have slow convergence, you can use static routes with different ADs. For instance, a server farm router might have a primary static route to the default gateway with AD 1, and a floating static route with AD 10 via a backup link. This provides fast failover because the router immediately removes the static route when the interface goes down, and the floating route is already in the configuration.

Misconfiguration can lead to unexpected behavior. For example, if you set the floating static route's AD lower than the primary route's AD, the static route will always be used, defeating the purpose. Also, if you forget to specify the distance, the static route will have AD 1 and will override the dynamic route. Another common mistake is using the wrong next-hop IP or interface, causing the route to be unreachable when it should be used. Always verify with show ip route after configuration.

In production, floating static routes are often combined with IP SLA tracking to provide more intelligent failover. IP SLA can monitor the reachability of a remote host and adjust the administrative distance of the static route dynamically. For example, if the primary link is up but the remote end is unreachable, IP SLA can increase the AD of the primary static route so that the floating route takes over. This is beyond the CCNA scope but is a real-world extension.