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Module 9Basic Router

Troubleshooting

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The Routing Table and show ip route Command

• Routers build routing tables to:– list available routes for forwarding data– select the best path to a destination

• show ip route displays routing table information about all known networks and subnetworks

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Example Routing Table

•When RTA receives a packet destined for 192.168.4.46, it looks for the prefix 192.168.4.0/24 in its table.

•RTA then forwards the packet out an interface (Ethernet0) based on the routing table entry.

•If RTA receives a packet destined for 10.3.21.5, it sends that packet out Serial 0

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Example Routing Table

•4 routes – directly connected

•The router will drop any packet destined for a network not listed in the routing table

•So what do you do with those packets that this router doesn’t have a route to?

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Adding Routes

• Static routing– Manually entered by the administrator:

• more secure• low processor overhead• have precise control of path selection

• Dynamic routing– router learns route from other routers, dynamically

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Gateway of Last Resort (Default Static Route)

Two ways to statically configure default routes:

• From global configuration mode– ip route 0.0.0.0 0.0.0.0 [next-hop interface|exit-interface]

• From router configuration mode– ip default network [network number]

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ip default-network

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ip route

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Routing Processes• Remember that the IP source and destination do not

change in the packet, but the MAC addresses do change.

• The packet is forwarded based on the MAC address of the next hop.

• Layer 3 address is always used in routing, but the MAC address has to change because the data is framed as it moves out on the data link layer.

Source to destination --- we are always talking about network addressing

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Route Metrics

• Routing protocols use metrics to determine the best route to a destination.

• The metric is a value that measures the desirability of a route.

• The smaller the metric number, the better the path.

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M

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Route Metrics

• The more factors that make up a metric, the greater the flexibility to tailor network operations to meet specific needs.

• By default, IGRP uses the static factors bandwidth and delay to calculate a metric value (so bandwidth and delay can actually be entered by the administrator).

• IGRP may also be configured to include the dynamic factors, load and reliability, in the metric calculation.

• Remember, IGRP uses the metric

• Metric = bandwidth + delay

Be sure to do the Interactive Media Activity in 9.1.6

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Troubleshooting

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Testing by OSI Layers

• Layer 1 – check obvious first (lights on a NIC)– Broken cables – Disconnected cables – Cables connected to the wrong ports – Intermittent cable connection – Wrong cables used for the task at hand (must use

rollovers, crossover cables, and straight-through cables correctly)

– Transceiver problems – DCE cable problems – DTE cable problems – Devices turned off

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Testing by OSI Layers

• Layer 2 errors:– Improperly configured serial interfaces – Improperly configured Ethernet interfaces – Improper encapsulation set (HDLC is default for

serial interfaces) – Improper clockrate settings on serial interfaces – Network interface card (NIC) problems

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Testing by OSI Layers

• Layer 3 errors:– Routing protocol not enabled – Wrong routing protocol enabled – Incorrect IP addresses – Incorrect subnet masks

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ping Utility (Layer 3 testing)

Maybe the host isn’t available

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Telnet application (Layer 7 testing)

• Actually verifies all seven layers of the OSI• If telnet is not working, then that means that there are

upper-layer application layer problems.

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show interfaces serial

•Used to discover Layer 1 & Layer 2 problems with the router

•When the line is down, the protocol is always down, because there is no useable media for the Layer 2 protocol.

•If the interface is up and the line protocol is down, a Layer 2 problem exists (no keepalives, no clock rate, encap different)

•You would never see serial 0/0 is down, line protocol is up

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show interfaces serial

Interpreting the Show Interfaces Serial CommandOne of the most important elements of the show interfaces serial command output is the display of the line and data link protocol status.

The line status is triggered by a Carrier Detect signal, and refers to the physical layer status. However, the line protocol, triggered by keepalive frames, refers to the data link framing.

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show interfaces serial

Know allof these!

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show ip protocols

The show ip protocols command shows the following:• Sources of routing updates• The networks being advertised• Update timer values

– Ex. If a network is not being included in the routing updates and should be, an administrator could issue this command to see which networks are being advertised

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debug

• Be Careful!!!• Debug will give you live, changing information.• Therefore it will use processor time that may disrupt

normal router operation• Especially should not use “debug all”, but be more

specific, like – debug ip rip (only live info on rip routing ip)

• To turn off debug, use undebug all or u all