7 - IPv4 Routing Troubleshooting

Question 1

What is the difference between ICMP in IPv4 and IPv6?

Answer 1

In IPv4 ICMP is implemented as a separate Layer 3 protocol. ICMPv6 is implemented as an integrated part of IPv6 and is carried after the basic header information as an extension header

Question 2

What does ping use to function?

Answer 2

ICMP Echo request and ICMP Echo reply messages

Question 3

True/False: ICMP relies on TCP and UDP

Answer 3

False. ICMP is a part of Layer 3 as a control protocol used to assist IP

Question 4

What layer protocol is ICMP?

Answer 4

Layer 3

Question 5

What does it usually mean when one of the first pings fails?

Answer 5

A device somewhere in the end to end roue is missing an ARP table entry

Question 6

What can we tell about a network and route if a ping works all the way end to end?

Answer 6

• Interfaces are working (up/up)
• No ACLs stopping traffic
• Port security does not filter frames
• Routes are working
• STP has placed correct ports in forwarding state

Question 7

What does ping use as the source IP for an echo request (on a router for example)?

Answer 7

The IP address of the outgoing interface

Question 8

Why is extended ping useful?

Answer 8

For testing the reverse route with ping

Question 9

How does extended ping work?

Answer 9

It lets ping use a source IP from an interface(/subnet) that is not on the outgoing interfaces subnet

Question 10

What could the failure of a ping mean (even for devices on the same subnet)?

Answer 10

• IP addressing problem
• DHCP problems
• VLAN trunking problems (e.g. router configured for trunking but not switch or vice versa)
• LAN issues

Question 11

What might be the issue when a standard ping to the host from a router works, but not an extended ping?

Answer 11

This might indicate an issue with the default gateway of the host

Question 12

Why can it be a mistake to assume everything is okay if a regular ping works end to end from a router when you’re trying to troubleshoot a host?

Answer 12

Because this doesn’t test the reverse route. Because when ping sends it uses the routers source IP, but the issue could lie in the reverse route back to the hosts subnet

Question 13

How is ping useful for testing if DNS is working?

Answer 13

You can try to ping a hostname to ensure it can be resolved to an IP address

Question 14

How do IPv4 routers defeat routing loops (not STP)?

Answer 14

By discarding packets based off the TTL field

Question 15

How does traceroute work?

Answer 15

By sending messages with low TTL fields to make receiving routers send back a TTL exceeded message, gradually increasing the TTL when sending sets of packets until they eventually reach the destination host

Question 16

What is the difference between traceroute on a host OS and on Cisco IOS? Why is this important to understand?

Answer 16

A host OS traceroute usually creates ICMP echo requests, but Cisco IOS instead creates IP packets with a UDP header.

Because an ACL may filter traffic from a hosts traceroute messages, but not the router traceroute command, or vice versa

Question 17

Why might Telnet/SSH fail with regards to routing?

Answer 17

Routing protocols failing to be exchanged