Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 162 Routing Table Basics Section 13 - The Routing Table - Routing is the process of sending a packet of information from one network to another network. Thus, routes are usually based on the destination network, and not the destination host (host routes can exist, but are used only in rare circumstances). To route, routers build Routing Tables that contain the following: • The destination network and subnet mask • The “next hop” router to get to the destination network • Routing metrics and Administrative Distance The routing table is concerned with two types of protocols: • A routed protocol is a layer 3 protocol that applies logical addresses to devices and routes data between networks. Examples would be IP and IPX. • A routing protocol dynamically builds the network, topology, and next hop information in routing tables. Examples would be RIP, IGRP, OSPF, etc. To determine the best route to a destination, a router considers three elements (in this order): • Prefix-Length • Metric (within a routing protocol) • Administrative Distance (between separate routing protocols) Prefix-length is the number of bits used to identify the network, and is used to determine the most specific route. A longer prefix-length indicates a more specific route. For example, assume we are trying to reach a host address of 10.1.5.2/24. If we had routes to the following networks in the routing table: 10.1.5.0/24 10.0.0.0/8 The router will do a bit-by-bit comparison to find the most specific route (i.e., longest matching prefix). Since the 10.1.5.0/24 network is more specific, that route will be used, regardless of metric or Administrative Distance. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 163 Administrative Distance vs. Metric A “metric” allows a router to choose the best path within a routing protocol. Distance vector routing protocols use “distance” (usually hop-count) as their metric. Link state protocols utilize some sort of “cost” as their metric. Only routes with the best metric are added to the routing table. Thus, even if a particular routing protocol (for example, RIP) has four routes to the same network, only the route with the best metric (hop-count in this example) would make it to the routing table. If multiple equal-metric routes exist to a particular network, most routing protocols will load-balance. If your router is running multiple routing protocols, Administrative Distance is used to determine which routing protocol to trust the most. Lowest administrative distance wins. Again: if a router receives two RIP routes to the same network, it will use the routes’ metric to determine which path to use. If the metric is identical for both routes, the router will load balance between both paths. If a router receives a RIP and an OSPF route to the same network, it will use Administrative Distance to determine which routing path to choose. The Administrative Distance of common routing protocols (remember, lowest wins): Connected 0 Static 1 EIGRP Summary 5 External BGP 20 Internal EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 External EIGRP 170 Internal BGP 200 Unknown 255 A route with an “unknown” Administrative Distance will never be inserted into the routing table. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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