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CCNA 3.3.a: Admin distance

Overview:

Admin distance is what is used to select the best path when a router has two different paths to the same destination via two different routing protocols.

 

Study Notes:

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  • Most routing protocols are not compatible with other protocols.
  • In a network with multiple routing protocols, being able to select the best path between multiple protocols is critical.
  • Admin distance defines the reliability of a routing protocol.
  • Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an admin distance value.
  • IPv6 uses the same distances as IPv4.
  • Admin distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination.
  • Admin distance is a measure of the trustworthiness of the source of the routing information.
  • Admin distance has only local significance, and is not advertised in routing updates.
  • The smaller the admin distance value, the more reliable the protocol.
  • For example, if a router receives a route to a certain network from both Open Shortest Path First (OSPF) (default administrative distance - 110) and Interior Gateway Routing Protocol (IGRP) (default administrative distance - 100), the router chooses IGRP because IGRP is more reliable. This means the router adds the IGRP version of the route to the routing table

 

Default Admin Distances:

Route Source
Default Admin Distance Values
Connected interface 0
Static route 1
External BGP 20
Internal EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EDP 140
ODR 160
External EIGRP 170
Internal BGP 200
Unknown* 255

* If the admin distance is 255, the router does not believe the source of that route and does not install the route in the routing table.

 

Links:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html


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