Tuesday, November 22, 2011

Floating Static Routes

Floating static routes static routes that appear in the routing table only when the primary route fails. They are often being used to configure backup routes or conditionally advertise a route.

A floating static route is configured with an administrative distance that is higher than the default administrative distance of a dynamic routing protocol; hence it can be overridden by the dynamically learned route. A floating static route creates a path of last resort which is being used when the dynamically learned route is unavailable and removed from the routing table.

Floating Static Route

The default AD of EIGRP is 90. The static routes are configured with an AD of 100 to make sure that the backup routes never override the primary routes. As long as the primary serial link is up, EIGRP will be used to exchange routing information. The static routes (backup routes) will only be inserted into the router routing tables when the serial link goes down and disables EIGRP.

Below lists the default administrative distance values of different routing protocols.
The lower the value, the higher the rating of trust of a routing information source – an inverse rating.

Route Type Default AD
Directly connected 0
Static route out an interface 1 *
Static route to a next-hop address 1
EIGRP summary route 5
External BGP (EBGP) 20
EIGRP (internal) 90
IGRP 100
OSPF 110
IS-IS 115
RIPv1 and RIPv2 120
Exterior Gateway Protocol (EGP) 140
ODR 160
External EIGRP 170
Internal BGP (IBGP) 200
Unknown 255

* – It is not 0 as claimed by many sources!

Based on the table above, routers believe static routes over any dynamically learned route. However, there are cases where this default behavior is not the desired behavior, eg: a static route that acts as the backup route to a dynamically learned route (floating static routes).
The creation of floating static routes is an example of changing the administrative distance. Extra caution must be taken when changing the default AD values for routing protocols.

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