Tejas Jain

The AD values of the protocols are generally different across different vendors and care must be taken to take this fact into account in case of multi-vendor environments. Cisco Directly Connected Interface = 0 Static route = 1 EIGRP summary route = 5 External Border Gateway Protocol (eBGP) = 20 Enhanced Interior Gateway Routing Protocol (EIGRP) - Internal = 90 Open Shortest Path First (OSPF) = 110 Intermediate System to Intermediate System = 115 Routing Information Protocol (RIP) = 120 Enhanced Interior Gateway Routing Protocol (EIGRP) - External = 170 Internal Border Gateway Protocol (iBGP) = 200 Juniper Directly Connected Interface = 0 Static route = 5 Intermediate System to Intermediate System - Level-1 Internal = 15 Intermediate System to Intermediate System - Level-2 Internal = 18 Routing Information Protocol (RIP) = 100 Summary Route (Aggregate route) = 130 Open Shortest Path First (OSPF) = 150 Intermediate System to Intermediate System - Level-1 External = 160 Intermediate Syst...

Most routing protocols have metric structure that are not comparable with each other, in case multiple protocols are being used in a network. For eg. RIP uses hop count and OSPF uses cost for best path selection to a particular route. In order to have a common parameter which can be used for computing the best path, the concept of Administrative Distance (AD) was introduced. AD is simply a number which helps identity a particular route learned from a particular protocol. For eg, if the route is learned from Routing Information Protocol (RIP), then it would have a default AD value of 120 (unless it has been explicitly modified). Similary, a route learned from OSPF would have the AD value of 110. Rule: A route with lower Administrative Distance (AD) value would always be preferred over the one with higher AD value. In the above scenario, if the route is being learned from both RIP as well as OSPF, then the route learned via OSPF would be preferred since OSPF has a lower AD value of 110 t...