Troubleshooting Network Services

Question 1

What is a Logical Domain Manager (LDM)

Answer 1

A logical Domain Manager listens to multicast messages on a network and keeps track of the MAC addresses being used.

Question 2

How to identify duplicate MAC adresses and what could prevent it?

Answer 2

Either you will have intermittent connecivity issues on the 2 devices having the same MAC address or one works and the other one doesnt.
Also Wireshark can help.

Port security will prevent it.

Question 3

What can cause duplicate IP adresses? (3)

Answer 3

• Static assigned IPs
• DHCP configuration issue
• A rogue DHCP server on the network

Question 4

What is the default lease time of DHCP IPs?

Answer 4

86400 seconds (1day) but can also be more. That can cause adress exhaustion.

Question 5

What are the 3 typical routing issues & how to prevent them?

Answer 5

Multicast Flooding: Happens when no host is associated with the multicast MAC address inside the CAM table of the switch, causing the multicast traffic to be flooded through the entire LAN or VLAN. / Block multicasting on the port or interface.

Asymmetrical routing: When packets leave through one route and return through another route, which can cause stateful fire walls to block the returning traffic / Put the the firewalls as close as possible to the systems they are protecting.

Missing routes: When theres no route to a specific destination in the routing table. Mostly caused by static routing. / Use show route on a switch or route print on windows.

Question 6

How to check if STP is enabled on a switch?

Answer 6

show spanning-tree

Question 7

What is a Hold Down Timer? And how long is it?

Answer 7

Hold down Timers are used by Distance Vector protocols. When they detect a bad/failed route, they are held down for usually 180s, so they cannot be passed to other routers.

Question 8

What can prevent routing loops besides split horizon and route poisoning?

Answer 8

Distance Vector protocols use a TTL in the packet headers, so they wont go around the network infinitely.

Question 9

How to show ACL on a switch?

Answer 9

show access-lists

Question 10

What 3 situations cause firewall related connectivity issues?

Answer 10

1. Access to protected ressources from unprotected networks isnt working.
2. Access to unprotected ressources from protected networks isnt working
3. ACcess to the firewall & its configurations isnt working.

Question 11

How to approach firewall issues?

Answer 11

Layer 1: Is there a physical connection between firewall and network, is that confirmed by the Activity Light?
Layer 2: Verify communication between router and firewall through ARP and the MAC addresses.
Layer 3: Verify Firewall has valid IP, Subnet Mask & Default Gateway
Layer 3/4: Control the firewalls config and ACLs

Question 12

What are causes for VLAN issues?

Answer 12

1. Devices in the same VLAN have to be in the same subnet
2. The device you cant connect to is in another VLAN
3. VLANs are enabled but not properly used meaning all traffic defaults back to VLAN 1, creating one giant broadcast domain.

Question 13

How to troubleshoot DNS client issues.

Answer 13

1. Check if only one device has the issue, if yes: check if the right DNS is configured on that device and verify connectivity.
2. If DNS can be reached, flush DNS cache of the device or change DNS configuration to another DNS, if that works, troubleshoot the DNS itself.

Question 14

How to troubleshoot DNS server issues.

Answer 14

1. Verify the A and the CNAME records –> nslookup
2. Check TTL, if its too high, the cached records could be outdated.
3. DNS Latency if DNS server is far away from its users