Monday, April 23, 2018

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Routers and Routing Basics - Basic Router Troubleshooting

Time for ‘Show and Tell’!

• show commands display various aspects of the current router configuration.

– show running-config: Displays the contents of the currently running configuration file.

• show running-config interface fast ethernet 0/1: Displays current configuration of specified interface.

– show startup-config: Displays the saved configuration located in NVRAM.

– show interfaces: Statistics for all router interfaces.

• show interfaces fastethernet 0/1: Displays statistics for the specific interface on that slot/port number.

– show ip interfaces brief: Shows configured IP information and status for all router interfaces.

– show protocols: Displays the global and interface-specific status of any configured Layer 3 protocols.

– show arp: Displays the ARP table of the router.

Displaying All Routes

• show ip routedisplays the routing table

– All dynamically discoveredand connected static networks are displayed.

– Routing table also shows the default route

Default Routes

• If the router receives a packet destined for a network not displayed in this routing table, it forwards the packet to the default route.

• To set the default route:

– Router(config)# ip default-network [network number]

– Router(config)# ip route 0.0.0.0 0.0.0.0 [interface]

Return of the MAC

Admin Distances and Metrics

• A router uses administrative distances to determine the best route to a destination.

• Admin distances calculated by metrics.

– RIP metric used is hop count, max of 15.

– IGRP uses composite metric built from bandwidth, load, delay, reliability

• Display IP routing protocol information:

– Router# show ip protocols

Balancing Act

• RIP supports load balancing via multiple paths to the same destination.

– Router(config-if)# no ip route-cache

• Forces router to look up routing table for every packet received, and not cache previous lookups.

• Additionally, IGRP supports unequal load balancing.

– Router(config)# router igrp [AS-number]

Router(config-router)# variance

• Check load balancing with show ip route

Tasks for Troubleshooting

• Important to utilise an orderly processto troubleshoot network.

• When trying to localise problem in Step 2 (see below), recommended to start from OSI Layer 1 and then work up the OSI stack.

Network Issues by OSI Layers

• OSI Layer 1 issues –Physical Layer:

– Usually related to cabling.

– Use show interfacesto detail connectivity.

• OSI Layer 2 issues –Data Link Layer:

– Usually related to physical interfaces on devices, includes serial encapsulation and clocking function.

– Use show interfacesand show cdp neighbors for details.

• OSI Layer 3 issues –Network Layer:

– Usually incorrectly configured routed or routing protocols.

– Use show cdp neighborsand pingto test routed protocols, and traceroute to test routing protocols.

• OSI Layer 7 issues –Application Layer:

– DNS not active, or OSI Layer 4 port not configured.

– Use telnet to test connectivity.

Issues with Interfaces

• show interfacescan help troubleshoot Layer 1 and Layer 2.

Good (Cisco) Neighbours

• show cdp neighborsis a Layer 2 protocol.

– Displays connected interfaces, hence Layer 2 connectivity.

– Displays IP addresses, hence Layer 3 connectivity.

Be a Serial Control Freak

• show controllersqueries the router’s internal hardware controller for serial interfaces.

– Highlights type of cable attached to serial interfaces.

– May indicate wrong or no serial cable used.

• Usually possible to connect to router via Ethernet or other interface to check if no serial cable is attached.

Something bugging you?

• Where showcommands provide a snapshot of router operations at a specific moment, debug commands provide real-time information of events.

– Constant updates are a burden on router.

• Use debug all ONLY as a last resort!

– debug is most useful when messages are prefixed with timestamps.

• Router(config)# service timestamps debug uptime

– Timestamps relative to router uptime.

• Router(config)# service timestamps debug datetime localtime

– Timestamps use local time, if system clock is set.

– Router# clock set 23:46:00 10 October 2004

– Disable all debug output with undebug all.

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