7.4 STATIC ROUTING

Static routing provides a means of explicitly defining the next hop from a router for a particular destination. A router SHOULD provide a means for defining a static route to a destination, where the destination is defined by a network prefix. The mechanism SHOULD also allow for a metric to be specified for each static route.

A router that supports a dynamic routing protocol MUST allow static routes to be defined with any metric valid for the routing protocol used. The router MUST provide the ability for the user to specify a list of static routes that may or may not be propagated through the routing protocol. In addition, a router SHOULD support the following additional information if it supports a routing protocol that could make use of the information. They are:

• TOS,

• Subnet Mask, or

• Prefix Length, or

• A metric specific to a given routing protocol that can import the route.

DISCUSSION

We intend that one needs to support only the things useful to the given routing protocol. The need for TOS should not require the vendor to implement the other parts if they are not used.

Whether a router prefers a static route over a dynamic route (or vice versa) or whether the associated metrics are used to choose between conflicting static and dynamic routes SHOULD be configurable for each static route.

A router MUST allow a metric to be assigned to a static route for each routing domain that it supports. Each such metric MUST be explicitly assigned to a specific routing domain. For example:

           route 10.0.0.0/8 via 192.0.2.3 rip metric 3

           route 10.21.0.0/16 via 192.0.2.4 ospf inter-area metric 27

           route 10.22.0.0/16 via 192.0.2.5 egp 123 metric 99

DISCUSSION

It has been suggested that, ideally, static routes should have preference values rather than metrics (since metrics can only be compared with metrics of other routes in the same routing domain, the metric of a static route could only be compared with metrics of other static routes). This is contrary to some current implementations, where static routes really do have metrics, and those metrics are used to determine whether a particular dynamic route overrides the static route to the same destination. Thus, this document uses the term metric rather than preference.

This technique essentially makes the static route into a RIP route, or an OSPF route (or whatever, depending on the domain of the metric). Thus, the route lookup algorithm of that domain applies. However, this is NOT route leaking, in that coercing a static route into a dynamic routing domain does not authorize the router to redistribute the route into the dynamic routing domain.

For static routes not put into a specific routing domain, the route lookup algorithm is:

1. Basic match

2. Longest match

3. Weak TOS (if TOS supported)

4. Best metric (where metric are implementation-defined)

The last step may not be necessary, but it's useful in the case where you want to have a primary static route over one interface and a secondary static route over an alternate interface, with failover to the alternate path if the interface for the primary route fails.