Static routing metric

Both R1 and R3 advertise network into RIP domain

R3 is configured to offset all network updates via interface fa1/0 by 10

R2 receives R3 route with a metric of 11 hops

A debug ip rip events shows that only R1 route succeeds its entrance to rip database

In rip database only the best route is preserved

A debug ip routing on R2 shows that R3 route is not accepted by the routing table and returns code:19

As soon as we shutdown the lo13 interface on R1 the route gets poisoned and R3 route added

We bring up the lo13 interface on R1 and see that R3 route is removed because R2 receives now a better route metric from R1

In the R2 routing table the best protocol candidate route in terms of protocol metric (in our case hops) is added

The protocol metric is just copied to the structure the default routing table entry describing this route

This entry includes a new attribute that was not in the protocol table or database (rip database): distance (of 120 in case of RIP) which indicates the source of the route information

We configure static routing to the same destination and use a distance of 120

Now the static route is installed in the routing table

Not because of the administrative distance but rather the metric which is 0 for static routes

Static routing is a local routing information source to the router

It is different from dynamic routing because no routing information is exchanged between routers

Knowing that, why should I configure a metric for static routes?

Let’s add another static route to the same destination but that points the opposite direction

Both routes are installed because they have the same cost (metric)

In static routing metric and administrative distance are both local

There’s no case that may apply only to the metric in choosing the best route to put in the routing table

Using the metric in static routing is redundant

But the administrative distance is still needed to discriminate the routing information sources

Additionally, we could use different administrative distances per routes…


Learn More →

Leave a Reply

Translate »