Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 164 Viewing the routing table The following command will allow you to view the routing table: Router# show ip route Gateway of last resort is 192.168.1.1 to network 0.0.0.0 C 192.168.1.0/24 is directly connected, Ethernet0 150.50.0.0/24 is subnetted, 1 subnets C 150.50.200.0 is directly connected, Loopback1 C 192.168.123.0 is directly connected, Serial0 C 192.168.111.0 is directly connected, Serial1 R 10.0.0.0 [120/1] via 192.168.123.1, 00:00:00, Serial0 [120/1] via 192.168.111.2, 00:00:00, Serial1 S* 0.0.0.0/0 [1/0] via 192.168.1.1 Routes are labeled based on what protocol placed them in the table: o C – Directly connected o S – Static o S* - Default route o D - EIGRP o R – RIP o I – IGRP o i – IS-IS o O - OSPF Notice the RIP routes contain the following field: [120/1]. This indicates both the administrative distance and the metric (the 120 is the AD, and the 1 is the hop-count metric). To clear all routes from the routing table, and thus forcing any routing protocol to repopulate the table: Router# clear ip route * * * * 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 165 Choosing the Best Route (Example) Assume the following routes existed to the following host: 192.168.111.5/24 O 192.168.111.0/24 [110/58] via 192.168.131.1, 00:00:00, Serial3 R 192.168.111.0/24 [120/1] via 192.168.123.1, 00:00:00, Serial0 R 192.168.111.0/24 [120/5] via 192.168.5.2, 00:00:00, Serial1 S 192.168.0.0/16 [1/0] via 10.1.1.1 We have two RIP routes, an OSPF route, and a Static route to that destination. Which route will be chosen by the router? Remember the three criteria the router considers: • Prefix-Length • Metric • Administrative Distance The static route has the lowest administrative distance (1) of any of the routes; however, its prefix-length is less specific. 192.168.111.0/24 is a more specific route than 192.168.0.0/16. Remember, prefix-length is always considered first. The second RIP route will not be inserted into the routing table, because it has a higher metric (5) than the first RIP route (1). Thus, our routing table will actually look as follows: O 192.168.111.0/24 [110/58] via 192.168.131.1, 00:00:00, Serial3 R 192.168.111.0/24 [120/1] via 192.168.123.1, 00:00:00, Serial0 S 192.168.0.0/16 [1/0] via 10.1.1.1 Thus, the true choice is between the OSPF route and the first RIP route. OSPF has the lowest administrative distance, and thus that route will be preferred. PLEASE NOTE: Calculating the lowest metric route within a routing protocol occurs before administrative distance chooses the route it “trusts” the most. This is why the order of the above “criteria” is prefix-length, metric, and then administrative distance. However, the route with the lowest administrative distance is always preferred, regardless of metric (assuming the prefix-length is equal). Thus, the metric is calculated first, but not preferred first over AD. * * * 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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Cisco CCNA Study Guide - Router Alley

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