IPv4 and IPv6 Static Routing

What it is

A static route is a route that an administrator types in by hand, and it does not change unless someone changes it.

It is simple, predictable and uses no bandwidth or CPU to maintain, which makes it a good fit for small networks and for specific jobs in larger ones.

The downside is that it does not react to changes. If a link fails, a static route does not find another path by itself unless you plan for it.

The administrative distance of a static route is 1 by default, so it is preferred over almost every dynamic routing protocol.

How it works

You give the router a destination and a way to reach it, and the router installs the route if that way is usable.

IPv4 syntax

The command takes the destination network, its mask and the way to reach it:

ip route DESTINATION MASK NEXT-HOP
ip route DESTINATION MASK EXIT-INTERFACE
ip route DESTINATION MASK EXIT-INTERFACE NEXT-HOP

On Ethernet links, giving the next-hop address, or both the interface and the next hop, is the usual choice. An exit interface alone makes the router ARP for every destination. On a point-to-point serial link, the exit interface alone is common.

The four types of static route

Network route. The mask matches the destination subnet. It teaches the router the path to one remote network.

Default route. The destination and mask are both zeros, so it matches any address. It is the route of last resort and is commonly used at the edge of a network, pointing at the internet provider. It appears in the table as S*.

Host route. A /32 mask matches a single address. It is useful for reaching one server through a specific path. Because it is the longest possible prefix, it always wins over broader routes.

Floating static route. This is a static route with a deliberately higher administrative distance, so it does not get used while a better route exists. The number goes at the end of the command.

Suppose the primary path to the internet is learned by OSPF (distance 110). A floating static route with a distance of 200 stays hidden.

If the OSPF route disappears, the floating route enters the table and takes over. When the OSPF route comes back, the floating route steps aside.

IPv6 syntax

First, IPv6 routing must be enabled:

ipv6 unicast-routing

Then the routes use a prefix length instead of a mask:

ipv6 route 2001:db8:20::/64 2001:db8:12::2
ipv6 route ::/0 2001:db8:12::2
ipv6 route 2001:db8:99::1/128 2001:db8:12::2
ipv6 route ::/0 2001:db8:13::2 200

The four lines show a network route, a default route (::/0), a host route (/128) and a floating static route.

If the next hop is a link-local address, you must also name the exit interface, because the same link-local prefix exists on every link:

ipv6 route 2001:db8:20::/64 GigabitEthernet0/0 FE80::2

When a static route is installed

A static route enters the routing table only if the router can use it.

If the next hop address is not reachable, or the exit interface is down, the route is not installed. The router also needs a route for the next hop itself, which is usually satisfied by a connected network.

Verifying

These commands show what is installed and whether it works.

show ip route static
show ip route
show ipv6 route static
show running-config | include ip route
ping 192.168.20.1
traceroute 192.168.20.1

show ip route static lists only the static routes. The show running-config | include ip route command shows exactly what was typed.

Always test with ping or traceroute. Also remember that traffic needs a return path. If the remote router has no route back to the sender, the ping fails even though the forward route is perfect.