Routing Fundamentals And Static Routes: Floating Static Routes + On Demand Routing

Floating Static Routes

In a hub and spoke network topology, we can use a single dynamic routing protocol to have 3 routers advertise all of their known networks. However we can also introduce a secondary way to reach a particular destination via a floating static route. We know that by default the AD on a static route is ‘1’. Therefore this is lower than all known IGPs AD values, in fact the nearest AD to this is EIGRP summary routes with an AD of 5. So we need to change the AD of the floating static route to a higher value so that it doesn’t better the IGP by being ‘best’ and therefore ending up in the routing table.

For example, an internal EIGRP route will have an AD of 90 and therefore will NOT beat the static route AD of 1 and be populated into the routing table. If we change the AD of the static route to something HIGHER than 90, then the IGP route will of course win and be in the routing table and the static route is in effect ‘floating’ in the running config with a higher AD value.

The real world usage for such a command is in the hub and spoke style topology, with an IGP setup across all routers, however one of the spoke routers having an additional backup link to the hub which doesn’t participate in the IGP process. When the primary WAN link fails to the hub via the chosen IGP (EIGRP), the static route will then be populated into the routing table as it is now the only path to the intended destination with the EIGRP route being removed with the failure of the primary WAN link.

Syntax for floating route with a custom AD value:

R1(config)#ip route 172.12.23.0 255.255.255.224 210.1.1.3 100

100 being the custom AD value for this static route and therefore not superior to the current IGP of EIGRP, however when the EIGRP link changes to down state, the static route will then be the best and only active path to the destination prefix and will then populate into the routing table.

-Dean

On Demand Routing

In a nutshell: ‘The spokes are going to use ODR to send directly connected network prefixes to the hub. The spoke will use the IP address of the hub on the common link as its default gateway. By using only a single default route, the spoke routers conserve their resources’

  • ODR requires your network to run CDP.
  • ODR is designed for use only in a hub-and-spoke network.
  • ODR supports VLSM.
  • ODR is mainly aimed at networks with bandwidth limitations.
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Routing Fundamentals And Static Routes: Static Routes

One major advantage of using Static Routes: ‘One major reason is when a route has one IP address as the next-hop address for every single route in its table’ Why keep a full dynamic routing table when a single static route will do?

*This strategy is actually built in to these dynamic routing protocols such as EIGRP and OSPF.

A static route can often provide a ‘quick fix’ to a routing problem with a dynamic protocol.

A static route can also serve as a backup to a dynamic routing protocols – a floating static route. A floating static route is assigned an administrative distance higher than that of any dynamic protocol running on the router, ensuring that the only way the static route can be used is if all dynamic routes leave the table.

A default static route serves as a router’s gateway of last resort. Remember that a default static route isn’t the path packets will take first, it’s the path they’ll take if there is no other match in the routing table at all.

I (DB) always remember static route syntax as ‘Where do you want to go? How do you get there?