{"id":3788,"date":"2013-01-15T04:54:53","date_gmt":"2013-01-15T04:54:53","guid":{"rendered":"http:\/\/vskills.in\/certification\/tutorial\/?p=3788"},"modified":"2024-04-12T14:13:24","modified_gmt":"2024-04-12T08:43:24","slug":"administrative-distance","status":"publish","type":"page","link":"https:\/\/www.vskills.in\/certification\/tutorial\/administrative-distance\/","title":{"rendered":"Administrative Distance"},"content":{"rendered":"

Administrative Distance<\/strong><\/h4>\n

Administrative distance (AD) is how a router determines which source of routes it should use if it has two identical routes from different sources. In other words, the router needs to be able to determine which routes to trust if it’s receiving the same information from two different sources. For a better idea, consider trying to decide which local news program, all of which more or less cover the same events, is most trustworthy.<\/p>\n

If you only have one router with one routing protocol and one WAN circuit, or if you’re only using static routes, administrative distance doesn’t affect your situation. But that doesn’t mean you shouldn’t be familiar with its purpose.<\/p>\n

But if you have a slightly more complex network\u2014say you have two WAN circuits or you’re using two routing protocols (even if one of them is static routing)\u2014administrative distance takes on more importance.<\/p>\n

The sources of the routes aren’t only routing protocols, such as RIP, OSPF, or BGP. Possible sources can also be connected routes (i.e., the interfaces on the router) and static routes (which you entered as network administrator).<\/p>\n

The router determines which source is the most trustworthy (i.e., reliable) according to the administrative distance. The lower the administrative distance, the more trustworthy the routing source.<\/p>\n

To help make this decision, routers contain a preprogrammed table that lists all of the possible sources and their default administrative distances. Listing A offers an example of what this table looks like. (While administrators can change default administrative distances by using the distance command in Router Configuration Mode, this is usually not advisable.)<\/p>\n

Listing –<\/p>\n

SOURCE OF THE ROUTE\t                              DEFAULT DISTANCE\n\nConnected Interface or static route to an Interface   0\nStatic route to an IP Address\t                      1\nEIGRP Summary\t\t\t\t\t      5\nBGP External\t\t\t\t\t      20\nEIGRP Internal\t\t\t\t\t      90\nIGRP\t\t\t\t\t\t      100\nOSPF\t\t\t\t\t\t      110\nIS-IS\t\t\t\t\t\t      115\nRIP\t\t\t\t\t\t      120\nEIGRP External\t\t\t\t\t      170\nBGP Internal\t\t\t\t\t      200\nUnknown Source\t\t\t\t\t      255<\/pre>\n
<\/a>Building the Routing Table<\/span><\/h2>\n
The main considerations while building the routing table are:<\/span><\/p>\n