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Computer Networks 6 Layer 3 troubleshooting

Halmstad University

Olga Torstensson

035-167575 olga.torstensson@ide.hh.se

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Network Layer Overview

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Interior IP Routing

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Troubleshooting Static Routes

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Troubleshooting Static Routes

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Debug ip routing

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Common Routing Problems

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Isolating the Problem

• The first step is to isolate the problem.

Identify and document the symptoms

• Some of the things to consider include:

Has anything in the network changed recently?

Is there anyone currently working on the network infrastructure?

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Troubleshooting Commands

Commands used in this example and some of the commands that can be used for other routing protocols include:

• show ip interface

• show ip interface brief

• show ip eigrp interfaces

• show ip ospf interface

• debug ip routing

• debug ip rip

• debug ip igrp transactions

• debug ip eigrp

• debug isis adj packets

• debug isis update-packets

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RIP

• Incompatible RIP Version Types

• Mismatched Authentication Key

• RIP Hop Count Limit

• Discontinuous Networks

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Troubleshooting EIGRP

Mismatched K Values (K1 for bandwidth , K2 for load , K3 for delay , K4 for reliability, K5

for reliability)

• Mismatched AS Numbers

• Stuck in Active

• Duplicate Router ID’s

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Common EIGRP Errors

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Troubleshooting EIGRP

‘‘Big show’ & ‘Big D’ for EIGRPBig show’ & ‘Big D’ for EIGRP sh ip eigrp neighbors [ as_number | interface ]sh ip eigrp neighbors [ as_number | interface ]

sh ip eigrp topology [as_number | active | sh ip eigrp topology [as_number | active | pending | summary ]pending | summary ]

show ip protocolsshow ip protocols

show ip routeshow ip route

debug ip eigrp debug ip eigrp

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• Down

• Attempt

• Init

• 2Way

• Exstart / Exchange

• Loading

• Full

• No info from neighbour

• Hello sent – no reply

• 1 side blocking hello packet

• Normal on broadcast networks

• Access list, MTU size

• Neighbour sending bad packet

Troubleshooting OSPF - Neighbours

OSPF Neighbour stateOSPF Neighbour state Possible reason for stuck in state Possible reason for stuck in state

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• Layer 2 down

• OSPF no enabled

• Passive interface used

• Mismatched subnets

• Mismatched Hellos/Dead interval

• Mismatched authentication

• Mismatched Area Ids

• Mismatched Area types

Troubleshooting OSPF

Adjacencies not coming up?Adjacencies not coming up?

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Troubleshooting OSPF – missing routes

• Routing protocols

Redistribution

• Stub vs NSSA vs Totally stub

Default routes

Summarisation

• Route filters

access-lists

• Virtual Links

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OSPF Mismatched Parameters

• Mismatched Hellos/Dead interval

• Mismatched authentication

• Mismatched Area Ids

• Mismatched Area types

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External Route Installation Problems

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Route Summarisation Problems

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• show ip ospf database

• show ip ospf neighbors

• show ip ospf interface

• show ip ospf

• show ip route

• debug ip ospf adj

• debug ip ospf events

Troubleshooting OSPF

Big show’ & ‘Big D’ for OSPFBig show’ & ‘Big D’ for OSPF

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Common BGP Neighbour Problems

• Directly connected EBGP neighbours not initializing

• Non-directly connected EBGP neighbours not initializing

• IBGP neighbours not initializing

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Directly Connected EBGP Neighbor Problems

• Directly connected EBGP neighbors not initializing

Layer 2 is down

incorrect neighbor IP address

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Directly Connected EBGP Session

AS109 AS110

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Nondirectly Connected

• Common causes for nondirectly connected EBGP neighbors not initializing are:

route to the nondirectly connected peer address is missing from the routing table.

ebgp-multihop command is missing

update-source interface command is missing

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• show ip bgp

• show ip bgp neighbors

• show ip bgp summary

• debug ip bgp [ A.B.C.D | dampening | events | in | keepalives | out | updates | vpnv4 ]

Troubleshooting BGP

Big show’ & ‘Big D’ for BGPBig show’ & ‘Big D’ for BGP

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Documentation reviewDefine Problem

Gather Facts

Consider Possibilities

Create Action Plan

Implement Action Plan

Observe results

Fixed ?

Problem Diagnosis How Resolved

Document ProcessYesNo

Troubleshootingtools

'show ...''debug ...''logging ...' SiSi

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Documentation review

Define Problem

Gather Facts

Consider Possibilities

Create Action Plan

Implement Action Plan

Observe results

Fixed ?

Problem Diagnosis How Resolved

Document ProcessYesNo

Troubleshootingtools

'show ...''debug ...''logging ...' SiSi

RouterA - interfaceS0/0 is shutdown

sh ip interface briefshow running-configshow protocols

No shutdown oninterfaceS0/0.