About Administrative Distance
Administrative distance (admin distance) is a value that influences which source of routes should be used for two identical routes from different sources. The lower the administrative distance value, the more trusted the route.
The admin distance, when set, is only used by the ZebOS components when choosing which routes to:
- Populate into PBR
- Redistribute to other routing protocols when a static route competes with a route received from a particular routing protocol.
The admin distance is not used for prioritizing routes within PBR itself, so unless dynamic routing is in use, the admin distance set for a static route has no effect. When dynamic routing is being used, the admin distance provides a mechanism by which static routes defined in PBR can be compared to otherwise equivalent dynamic routes possibly received from protocols such as OSPF, RIP, or BGP. By default, the admin distance of a PBR static route inserted into the network services module (NSM) is equal to the metric defined for the PBR route. The admin distance of each static route may optionally be set to a different value when a custom value is entered for Admin Distance.
For example, if a simple (destination only) static route (for example, destination =
18.104.22.168/24) is defined with a metric of 10 and the admin distance set to its default of Auto, that route is populated into NSM with an admin distance and metric of 10.
Now assume the same
22.214.171.124/24 route is received from both RIP and OSPF. RIP routes have a default admin distance of 120 and OSPF routes 110, so the static route, with a default admin distance (== the metric) of 10 would be preferred over both routes, and NSM would not populate either the OSPF or RIP route into PBR. If the admin distance of the static route had been set to 115 (keeping the metric at 10), however, then the OSPF route (at 110) would be preferred over the static route, but the RIP route would not. If the OSPF route disappeared, NSM would withdraw the OSPF route and would not populate the RIP route as its 120 AD is greater than the static route's 115 AD.
In either of the above cases, the static route is still preferred in PBR because all non-default routes populated into PBR from NSM are added with a 110 metric, which is greater than the metric of 10 for the static route.
If an admin distance of 110 and a metric > 110 are used for the static routes, the metric value passed to NSM would be used by OSPF when it compares the metric of the static route to the OSPF metric (or cost) of any competing OSPF route.
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