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Administrative Distance (AD) and Autonomous System (AS)

Administrative Distance (AD)

Administrative Distance (AD) is helpful in evaluating the reliability of routing information obtained from a neighboring router. The optimum route to reach the remote target network will be determined based on which route has the least AD, and that route will be added to the routing table. It identifies a routing protocol's dependability. It is an integer value between 0 and 255, where 255 indicates that the route will never be added to the routing table, and 0 indicates that it is the most trusted route.

Default Distance Value Table - The protocols that Cisco supports are listed in this table with their administrative distance default values:

Route Sources Default AD
Connected Interface 0
Static Route 1
External BGP 20
OSPF 110
RIP 120
External EIGRP 170
Internal BGP 200
Unknown 255 (Not used route)

The routing protocol is more reliable for the lower value of AD. For example, suppose a router receives an advertised route from OSPF and EIGRP to a distant destination network. In that case, EIGRP's advertised route will be considered as the preferred route and added to the routing table because EIGRP has a lower AD.

Sometimes you need to change a protocol's administrative distance while using route redistribution so that it takes precedence. For example, to choose the RIP-learned route with 120 as the default value instead of the IGRP-learned route with 100 as the default value, you need to decrement the AD value of RIP to less than 100 or increment the Ad value of IGRP greater than 120.

In the routing process sub-configuration mode, the distance command allows you to change the administrative distance of a protocol. This command directs that the administrative distance be provided for routes found using a certain routing protocol. This process is often required when switching a network's routing protocol from one with a low administrative distance to one with a higher administrative distance. However, a shift in the administrative distance can result in routing snarls and voids. So, if you modify the administrative distance, proceed with caution.

Best path selection by a Dynamic Protocol - If a router receives the same advertised routes from several sources for a faraway network, the first AD value is analysed. The promoted route with the lowest AD value will be given preference. If the AD value of the advertised routes is the same, the metrics of those routes are checked. In the routing table, there will be the advertised route with the fewest metrics. If both the AD and the metric are the same, load balancing is carried out, which causes the traffic to travel through several routes. Either equal or unequal load balancing is possible. Under equal load balancing, the same amount of traffic will flow through both routes concurrently as opposed to the different volumes that do so under uneven load balancing.

Autonomous System (AS)

An autonomous system is a collection of routers and networks managed by a single administrative domain (AS). A 16-bit number specifies the routers' routing domain; the range is 1 to 65535.

  1. Public Autonomous System Number - These values are 16-bit values that range from 1 to 64511. The service provider will offer a public AS if a consumer has many ISP connections, such as through multihoming. When customers request that their BGP routes be propagated between two ISPs, they are given a global autonomous number that will be distinct.
  2. Private Autonomous System number - The 16-bit Private Autonomous System Numbers fall between 64512 and 65535. The service provider will issue the user with a private autonomous system number if the customer only wants multiple connections to one ISP (single house or dual home network) and not to several ISPs. To preserve the autonomous system numbers, these are offered.
  3. Assigning of AS numbers - The autonomous numbers are initially assigned to the relevant regional registries by IANA (Internet Assign Number Authority). Additionally, the regional registry gives these autonomous numbers (from the IANA-provided block of autonomous numbers) to entities inside their defined territory.

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