3.3.d Implement, optimize and troubleshoot administrative distance

Routing decision criteria

1) Valid next hop

upon update receipt, the router verifies a valid next hop

2) Metric

the router then attempts to install the best routing metric (OSPF=cost, ie) into the table

3) Administrative Distance

If multiple routing protocols are running, the route with the lowest AD is used

4) Prefix

Longest match preferred; trumps AD

Administrative Distances (default)

Connected interface 0

Static route 1

EIGRP summary route 5

External BGP 20

Internal EIGRP 90

IGRP 100

OSPF 110

IS-IS 115

RIP 120

Exterior Gateway Protocol 140

On-demand routing 160

External EIGRP 170

Internal BGP 200

Unknown 255

A floating static route is a route artificially increased from it’s default, ie. for EIGRP a floating static could be added with an AD of 93, which would be used in the event of a dynamic EIGRP route (AD 90) not found.

Routing Table

Routing’s classic model, hop-by-hop, keeps a list of destinations with their reachable next hops in the routing table. In recent years the hop-by-hop paradigm has given way to more advanced methods such as MPLS, which enables label lookup to dictate the next hop determination. Traffic engineering considerations have become popular as well, such as QOS, that are not limited to routing table only criteria.