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.
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]
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 120. 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]
The following table lists the default administrative distances for various routing protocols used on Cisco routers. [3]
Routing protocol | Administrative distance |
---|---|
Directly connected interface | 0 [a] [5] |
Static route | 1 |
Dynamic Mobile Network Routing (DMNR) | 3 |
EIGRP summary route | 5 |
External BGP | 20 |
EIGRP internal route | 90 |
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 route | 170 |
Internal BGP | 200 |
Next Hop Resolution Protocol (NHRP) | 250 [6] |
Default static route learned via DHCP | 254[ citation needed ] |
Unknown and unused | 255 [b] |
The following table lists the default administrative distances for various routing protocols used on Juniper routers. [7]
Routing protocol | Administrative distance |
---|---|
Directly connected interface | 0 |
Static routes | 5 |
OSPF internal routes | 10 |
IS-IS Level 1 Internal | 15 |
IS-IS Level 2 Internal | 18 |
RIP | 100 |
Aggregate (route summary) | 130 |
OSPF external routes | 150 |
IS-IS Level 1 External | 160 |
IS-IS Level 2 External | 165 |
BGP | 170 |
The following table lists the default administrative distances used on ExtremeXOS / Switch-Engine.
Routing protocol | Administrative distance |
---|---|
Directly connected | 10 |
MPLS | 20 |
Blackhole | 50 |
Static | 1100 |
HostMobility | 1150 |
ICMP-Redirect | 1200 |
Fabric | 1699 |
eBGP | 1700 |
iBGP | 1900 |
OSPFintra | 2200 |
OSPFinter | 2300 |
IS-IS | 2350 |
IS-IS L1 | 2360 |
IS-IS L2 | 2370 |
RIP | 2400 |
OSPF AS Ext | 3100 |
OSPF Ext1 | 3200 |
OSPF Ext2 | 3300 |
IS-IS L1 Ext | 3400 |
IS-IS L2 Ext | 3500 |
Bootp | 5000 |
The following table lists the default administrative distances used on Extreme VOSS / Fabric-Engine.
Routing Protocol | Administrative distance |
---|---|
Local | 0 |
Static | 5 |
SPBm L1 | 7 |
OSPFintra | 20 |
OSPFinter | 25 |
eBGP | 45 |
RIP | 100 |
OSPF Ext1 | 120 |
OSPF Ext2 | 125 |
iBGP | 175 |
The following table lists the default administrative distances for various routing protocols used on Fortinet FortiGate routers. [8]
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. [9] [10] 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
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.
An Internet Protocol address is a numerical label such as 192.0.2.1 that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface identification, and location addressing.
A router is a computer and networking device that forwards data packets between computer networks, including internetworks such as the global Internet.
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.
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.
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.
A virtual local area network (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer. In this context, virtual refers to a physical object recreated and altered by additional logic, within the local area network. Basically, a VLAN behaves like a virtual switch or network link that can share the same physical structure with other VLANs while staying logically separate from them. VLANs work by applying tags to network frames and handling these tags in networking systems, in effect creating the appearance and functionality of network traffic that, while on a single physical network, behaves as if it were split between separate networks. In this way, VLANs can keep network applications separate despite being connected to the same physical network, and without requiring multiple sets of cabling and networking devices to be deployed.
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.
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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.
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Static routing describes a process by which routing is configured with fixed values that do not change at runtime unless manually edited. Static routes are used with and without dynamic Routing protocols and usually share the same routing table as those protocols. Routes require at least two attributes; the destination and the gateway, but may contain additional attributes such as a metric. Some implementations treat the network address and subnet mask as separate values, however in practice both of the values have to be considered for any given routing decision to determine the longest prefix match. Static routes together with connected routes and routes from configuration protocols such as DHCP or Router Advertisements provide the routes which are then redistributed using dynamic routing protocols. While static routes are entered into the system and remain there until removed or changed manually, dynamic routing protocols create and delete routes dynamically at runtime without intervention. Thus the term static here refers to the nature of remaining unchanged by the system itself. The most prominent example of a static route is a default route which is often used on devices with a statically configured IP address to provide the device with access to the rest of the network or the internet by default. In contrast to a so called connected route which is automatically generated upon address assignment based on the used subnet mask, a static route must be manually configured. Due to this the configuration may fail if there is no route to the provided gateway at the time of configuration, other than the connected route which will always succeed as it does not require a gateway. The gateway of a static route need not be an address, but can also specify an interface in most implementations.
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.
DNOS or Dell Networking Operating System is a network operating system running on switches from Dell Networking. It is derived from either the PowerConnect OS or Force10 OS/FTOS and will be made available for the 10G and faster Dell Networking S-series switches, the Z-series 40G core switches and DNOS6 is available for the N-series switches.
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.
Routing protocol | Administrative distance |
---|---|
Directly connected | 1 |
Static routes | 10 [1] |
External BGP | 20 [2] |
Open Shortest Path First (OSPF) | 110 [3] |
Routing Information Protocol (RIP) | 120 |
Internal & Local BGP | 200 [2] |
Kernel | 255 [4] |