Administrative distance

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Administrative distance (AD) or route preference [1] is a number of arbitrary unit assigned to dynamic routes, static routes and directly-connected routes. The value is used in routers to rank routes from most preferred (low AD value) to least preferred (high AD value). [2] [3] When multiple paths to the same destination are available in its routing table, the router uses the route with the lowest administrative distance.

Contents

Router vendors typically design their routers to assign a default administrative distance to each kind of route. For example, on Cisco routers, routes issued by the Open Shortest Path First routing protocol have a lower default administrative distance than routes issued by the Routing Information Protocol. This is because, by default on Cisco routers, OSPF has a default administrative distance of 110 and RIP has a default administrative distance of 120. Administrative distance values can, however, usually be adjusted manually by a network administrator. [2]

Overview

The administrative distance (AD) value is assigned by the router on a per-protocol basis. Routers, by design, should not install multiple routes into the routing table as this has the potential to cause routing loops. [2] While a router may run multiple routing protocols on the same device, it is necessary for the router to implement a process to ensure that multiple routes, pointing to the same destination do not simultaneously exist in the routing table. Each process running on a router advertises its administrative distance value to the local router. The router uses this value to determine which route should be used. Once a route has been selected, the routing information database is updated. If two routes have the same administrative distance, the router uses its vendor-specific algorithm to determine which route should be installed. [2] Cisco routers simply ignore the values and fall back to the default values, which are never the same. [4]

The router will usually compare administrative distances to determine which protocol has the lowest value. The router prefers protocols that have a lower assigned administrative distance. For example, OSPF has a default distance of 110, so it is preferred by the router process, over RIP, which has a default distance of 190. The administrator can arbitrarily reconfigure the administrative distances, which affects the ranking of the preferred routes by the routing process. On Cisco routers, static routes have an administrative distance of 1, making them preferred over routes issued by a dynamic routing protocol. The administrative distance is a value that is always only referenced by the local router itself. The administrative distance is not advertised on the network. [2]

Default administrative distances

Cisco

The following table lists the default administrative distances for various routing protocols used on Cisco routers. [3]

Routing protocolAdministrative distance
Directly connected interface0 [lower-alpha 1] [5]
Static route1
Dynamic Mobile Network Routing (DMNR)3
EIGRP summary route5
External BGP 20
EIGRP internal route90
IGRP 100
Open Shortest Path First (OSPF)110
Intermediate System to Intermediate System (IS-IS)115
Routing Information Protocol (RIP)120
Exterior Gateway Protocol (EGP)140
ODR 160
EIGRP external route170
Internal BGP 200
Next Hop Resolution Protocol (NHRP)250 [6]
Default static route learned via DHCP254[ citation needed ]
Unknown and unused255 [lower-alpha 2]
  1. ↑ Only the interface itself has an administrative distance of 0, since a route cannot have a distance of less than 1.
  2. ↑ An administrative distance of 255 will cause the router to remove the route from the routing table and not use it.

Juniper

The following table lists the default administrative distances for various routing protocols used on Juniper routers. [7]

Routing protocolAdministrative distance
Directly connected interface0
Static routes5
OSPF internal routes10
IS-IS Level 1 Internal15
IS-IS Level 2 Internal18
RIP 100
Aggregate (route summary)130
OSPF external routes150
IS-IS Level 1 External160
IS-IS Level 2 External165
BGP170

Extreme Networks

The following table lists the default administrative distances used on ExtremeXOS / Switch-Engine.

Routing protocolAdministrative distance
Directly connected10
MPLS20
Blackhole50
Static1100
HostMobility1150
ICMP-Redirect1200
Fabric1699
eBGP1700
iBGP1900
OSPFintra2200
OSPFinter2300
IS-IS2350
IS-IS L12360
IS-IS L22370
RIP2400
OSPF AS Ext3100
OSPF Ext13200
OSPF Ext23300
IS-IS L1 Ext3400
IS-IS L2 Ext3500
Bootp5000

The following table lists the default administrative distances used on Extreme VOSS / Fabric-Engine.

Routing ProtocolAdministrative distance
Local0
Static5
SPBm L17
OSPFintra20
OSPFinter25
eBGP45
RIP100
OSPF Ext1120
OSPF Ext2125
iBGP175

    Configuration

    Cisco IOS

    The network administrator may modify the administrative distance to change the desired ranking of router protocols. This may be necessary in cases where routing redistribution has to be used, otherwise, routing loops could occur. [3] The Cisco Internetwork Operating System enables network administrators to modify the distance by changing the distance value in sub-router configuration mode. In the example below, RIP's administrative distance is changed to 89 so that it used in preference to OSPF. [3] 

    R1#enable  R1#configure terminal  R1(config)#router rip  R1(config-router)#distance 89

    Manually configuring the administrative distance is also required when configuring a floating static route. Floating static routes are used to provide an alternate path when a primary link fails. In order for static routes to be configured as a backup, the static route's administrative distance would need to be adjusted. Otherwise, it will take precedence over all routing protocols and routes issued from a routing protocol will not be inserted into the routing table. [3] The example below shows how to configure the administrative distance to 254 to specify that it should only be used as a last resort.

    R1(config)# ip route 10.0.0.0 255.0.0.0 backupLink 1 254

    In the event that two routing protocols are configured with the same administrative distance, the Cisco router will ignore the configured values and instead use the default values. [4]

    Verifying the configuration of the administrative distance is done on Cisco equipment using the show ip route command in privileged exec mode on the console of the Cisco router. [8] [9] In the example shown below, the administrative distance is 1. The letter "S" indicates that the route is a static route that has, for all intents and purposes, been added manually to the router process by the administrator and installed into the routing table.

    Router#enable  Router#configure terminal  Router(config)#ip route 1.1.1.0 255.255.255.0 fastEthernet 0/0  Router(config)#do show ip route

    The do show ip route command will display the following, confirming that a static route has an administrative distance of 1.

    S 1.1.1.0/0 [1/0] via 172.31.0.1

    See also

    Related Research Articles

    Interior Gateway Routing Protocol (IGRP) is a distance vector interior gateway protocol (IGP) developed by Cisco. It is used by routers to exchange routing data within an autonomous system.

    Internetwork Packet Exchange (IPX) is the network layer protocol in the IPX/SPX protocol suite. IPX is derived from Xerox Network Systems' IDP. It also has the ability to act as a transport layer protocol.

    <span class="mw-page-title-main">Router (computing)</span> Device that forwards data packets between computer networks

    A router is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions between networks and on the global Internet. Data sent through a network, such as a web page or email, is in the form of data packets. A packet is typically forwarded from one router to another router through the networks that constitute an internetwork until it reaches its destination node.

    Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone network (PSTN), and computer networks, such as the Internet.

    <span class="mw-page-title-main">Border Gateway Protocol</span> Protocol for communicating routing information on the Internet

    Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator.

    Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).

    The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

    <span class="mw-page-title-main">Routing table</span> Data table stored in a router that lists the routes to network destinations

    In computer networking, a routing table, or routing information base (RIB), is a data table stored in a router or a network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. The routing table contains information about the topology of the network immediately around it.

    Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers. In 2013, Cisco permitted other vendors to freely implement a limited version of EIGRP with some of its associated features such as High Availability (HA), while withholding other EIGRP features such as EIGRP stub, needed for DMVPN and large-scale campus deployment. Information needed for implementation was published with informational status as RFC 7868 in 2016, which did not advance to Internet Standards Track level, and allowed Cisco to retain control of the EIGRP protocol.

    The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include backup links providing fault tolerance if an active link fails.

    A distance-vector routing protocol in data networks determines the best route for data packets based on distance. Distance-vector routing protocols measure the distance by the number of routers a packet has to pass; one router counts as one hop. Some distance-vector protocols also take into account network latency and other factors that influence traffic on a given route. To determine the best route across a network, routers using a distance-vector protocol exchange information with one another, usually routing tables plus hop counts for destination networks and possibly other traffic information. Distance-vector routing protocols also require that a router inform its neighbours of network topology changes periodically.

    Dynamic Multipoint Virtual Private Network (DMVPN) is a dynamic tunneling form of a virtual private network (VPN) supported on Cisco IOS-based routers, and Huawei AR G3 routers, and on Unix-like operating systems.

    A routing protocol specifies how routers communicate with each other to distribute information that enables them to select paths between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets are forwarded through the networks of the internet from router to router until they reach their destination computer. Routing algorithms determine the specific choice of route. Each router has a prior knowledge only of networks attached to it directly. A routing protocol shares this information first among immediate neighbors, and then throughout the network. This way, routers gain knowledge of the topology of the network. The ability of routing protocols to dynamically adjust to changing conditions such as disabled connections and components and route data around obstructions is what gives the Internet its fault tolerance and high availability.

    In computer networking, CDP spoofing is a technique employed to compromise the operation of network devices that use Cisco Discovery Protocol (CDP) for discovering neighboring devices. CDP spoofing is a network security threat that can be mitigated by taking precautionary measures.

    In network routing, the control plane is the part of the router architecture that is concerned with establishing the network topology, or the information in a routing table that defines what to do with incoming packets. Control plane functions, such as participating in routing protocols, run in the architectural control element. In most cases, the routing table contains a list of destination addresses and the outgoing interface(s) associated with each. Control plane logic also can identify certain packets to be discarded, as well as preferential treatment of certain packets for which a high quality of service is defined by such mechanisms as differentiated services.

    Static routing is a form of routing that occurs when a router uses a manually-configured routing entry, rather than information from dynamic routing traffic. In many cases, static routes are manually configured by a network administrator by adding in entries into a routing table, though this may not always be the case. Unlike dynamic routing, static routes are fixed and do not change if the network is changed or reconfigured. Static routing and dynamic routing are not mutually exclusive. Both dynamic routing and static routing are usually used on a router to maximise routing efficiency and to provide backups in case dynamic routing information fails to be exchanged. Static routing can also be used in stub networks, or to provide a gateway of last resort.

    route (command) Computer operating system command

    In computing, route is a command used to view and manipulate the IP routing table in Unix-like and Microsoft Windows operating systems and also in IBM OS/2 and ReactOS. Manual manipulation of the routing table is characteristic of static routing.

    IP routing is the application of routing methodologies to IP networks. This involves not only protocols and technologies but includes the policies of the worldwide organization and configuration of Internet infrastructure. In each IP network node, IP routing involves the determination of a suitable path for a network packet from a source to its destination in an IP network. The process uses static configuration rules or dynamically obtained from routing protocols to select specific packet forwarding methods to direct traffic to the next available intermediate network node one hop closer to the desired final destination, a total path potentially spanning multiple computer networks.

    Routing and Remote Access Service (RRAS) is a Microsoft API and server software that makes it possible to create applications to administer the routing and remote access service capabilities of the operating system, to function as a network router. Developers can also use RRAS to implement routing protocols. The RRAS server functionality follows and builds upon the Remote Access Service (RAS) in Windows NT 4.0.

    In a router, route redistribution allows a network that uses one routing protocol to route traffic dynamically based on information learned from another routing protocol.

    References

    1. ↑ "Route Preferences". Juniper Networks. Retrieved 2018-06-18.
    2. 1 2 3 4 5 Franck Le; Geoffrey G. Xie; Hui Zhang, Understanding Route Redistribution (PDF)
    3. 1 2 3 4 5 Cisco Systems (2013), What is Administrative Distance?, retrieved 14 September 2013
    4. 1 2 Cisco Systems(n.d.), Information About Routing, Cisco Systems Inc, retrieved 0109 2014
    5. ↑ Cisco, Default AD
    6. ↑ Cisco, NHRP
    7. ↑ Juniper, Default AD
    8. ↑ Cisco Systems (n.d), Configuring Static Routing, Cisco Systems Inc., retrieved 14 September 2013
    9. ↑ Cisco Systems (n.d), Show Commands, Cisco Systems Inc., retrieved 14 September 2013
    10. ↑ "Administrative Distance and Metric" . Retrieved 2021-12-23.
    11. ↑ "Understand the significance of administrative distance and metrics when working with routers" . Retrieved 2021-12-23.
    12. ↑ "Administrative distance & metric" . Retrieved 2021-12-23.
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