Administrative distance

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The administrative distance ( english administrative distance ) is a fixed value that in Cisco - Router , a path is selected -networks using prioritization, if different routing protocols provide for the same target different paths.

background

Since different routing protocols often run on a router and therefore several sources of path information are available, this method is used to select a path in a deterministic way if the various protocols contradict each other. For example, OSPF could determine the interface X for a certain IP prefix , whereas internal BGP would provide interface Y for the same prefix and a manually configured static route the interface Z. In this case, it must be clearly regulated which of these contradicting information the router is used as the basis should take for his forwarding decisions. In this example, the static route “wins” because it has the lowest administrative distance (see the following section).

Determination of the administrative distance

The metric is not always sufficient as a decision criterion for a route. If a router learns two routes for a destination, e.g. B. because the administrator has configured a static route and the dynamic routing protocol offers an alternative, he must select which path to enter in the routing table. The administrative distance, which reflects the trustworthiness of the route, is used to resolve this decision-making situation. According to this criterion, directly connected networks enjoy the greatest trust, followed by static routes and then, graded according to routing protocol, the dynamic concepts. For example, an OSPF route with an administrative distance of 110 is preferred over a RIP route with an administrative distance of 120.

The following table shows the default settings for the administrative distances of various protocols for Cisco routers:

protocol administrative distance
Directly connected 0
Static route 1
EIGRP summary route 5
External BGP 20th
Internal EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EGP 140
ODR 160
External EIGRP 170
Internal BGP 200
Floating Static Route (e.g. standard route via DHCP ) 254
Unknown 255

literature

  • Peer Kohlstetter: Now I'm learning Cisco routing and switching. Mart + Technik Verlag, Munich 2002, ISBN 3-8272-6395-6 .
  • Rick Graziani, Allan Johnson: Routing Protocols and Concepts. CCNA Exploration Companion Guide, Addison-Wesley Verlag, Munich 2009, ISBN 978-3-8273-2719-2 .
  • Wolfgang Riggert: Computer Networks. Basics - Ethernet - Internet, 5th updated edition, Carl Hanser Verlag, Munich 2014, ISBN 978-3-446-44204-7 .

Web links