Longest prefix match vs administrative distance (Network+)

Try 3 questions on this

Pick an answer to see why each option is right or wrong.

1. A packet's destination is covered by both a router's default route and a more specific route that has a worse metric. Which of the following decides which route is used?

   Right. In the game that’s Renown for your Network Implementation building.

   Not that one: C is right. In the game your Network Implementation building would lose a little Renown, and this question would come back later.

   Your answerA.MetricThe metric is compared only among routes that already match the destination equally well. A default route matches with a prefix length of zero, so a more specific route beats it before any metric is looked at.Your answerB.Administrative distanceAdministrative distance chooses between sources that offer the same prefix, such as a static route and an OSPF route to one network. Here the two routes cover different prefixes, so the longest match decides first.The answerC.Prefix lengthRFC 1812 defines the default route as the route to all networks for which there are no explicit routes, 'by definition the route whose prefix length is zero'.Your answerD.Hop countHop count is just a metric used by some routing protocols. Like any metric it only breaks a tie between routes of equal prefix length, and these two are not equal.

   Show the full explanation and doc quotes

   Routes are selected in order of decreasing prefix length, so any explicit route covering the destination is more specific than the default route and is used, whatever its metric.

   Written from www.rfc-editor.org/rfc/rfc1812.html

2. A router installs 192.168.0.0/16 from a source with a low administrative distance and 192.168.1.0/24 from a source with a higher one. Which of the following routes forwards a packet to 192.168.1.10?

   Right. In the game that’s Renown for your Network Implementation building.

   Not that one: C is right. In the game your Network Implementation building would lose a little Renown, and this question would come back later.

   Your answerA.The 192.168.0.0/16 routeAdministrative distance only decides between candidates for the same prefix when routes are installed. These two prefixes differ, so both are in the table, and the longer match wins for this packet.Your answerB.Both routes, sharing the loadRouters share load across equal-cost routes to the same prefix. A /16 and a /24 are different routes, and the packet takes only the more specific one.The answerC.The 192.168.1.0/24 routeCisco's administrative distance document says 'AD and routing metrics are not evaluated during forwarding' and 'AD does not override longest prefix match.' Both routes are installed because their prefixes differ, and 192.168.1.10 matches both, so the router forwards with the more specific 192.168.1.0/24 route.Your answerD.Whichever was installed firstThe order in which routes entered the table does not matter. For every packet, the router picks the most specific installed route that matches the destination.

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

3. A router has a static route with an administrative distance of 250 and an OSPF-learned route to the same prefix. Which of the following does the router install while both exist?

   Right. In the game that’s Renown for your Network Implementation building.

   Not that one: A is right. In the game your Network Implementation building would lose a little Renown, and this question would come back later.

   The answerA.The OSPF routeCisco's administrative distance document defines AD as the value used to rank the trustworthiness of routing information sources, with the lowest value installed.Your answerB.The static routeA static route is preferred over OSPF only at its default distance of 1. At 250 it ranks below OSPF's 110 and waits as a backup.Your answerC.Both the static and OSPF routesTwo sources for the same prefix are not combined. The router compares their distances and installs only the more trusted one.Your answerD.Whichever has the lower metricThe page notes that most routing protocols have metric structures that are not compatible with other protocols, so the router never weighs a static route against an OSPF cost by metric.

   Show the full explanation and doc quotes

   OSPF's default distance is 110, below the static route's 250, so the OSPF route is installed; the page's own example sets a static route to 250 so it stays out of the table until the OSPF route fails.

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