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2.1.b Virtual machine

4 min read ENCOR 350-401 v1.2 Updated

Aligned to Cisco's 350-401 ENCOR v1.2 exam topics.

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What this objective tests

This is a describe objective. The exam wants you to explain what a virtual machine (VM) is and how its virtual NICs attach to the network. It also wants you to troubleshoot VM connectivity in the correct order.

A VM is a software-defined computer. It runs a guest OS and receives virtual hardware from the hypervisor: CPU, memory, disk, and network interfaces.

VM basics

A VM usually has:

  • virtual CPU
  • virtual memory
  • virtual disk
  • one or more virtual NICs (vNICs)
  • a guest OS
  • optional snapshots for rollback

The guest OS thinks it has real hardware. The hypervisor maps the virtual hardware to physical host resources.

For network engineers, the vNIC is the most important part. Each vNIC connects to a virtual switch, port group, or virtual network. If the vNIC attaches to the wrong place, a perfect IP configuration still fails.

The network path

Guest OS IP stack -> vNIC -> virtual switch / port group -> host uplink -> physical switch -> default gateway

Start inside the guest and move outward:

  1. Does the guest OS have an IP address?
  2. Is the vNIC up and connected?
  3. Is the vNIC on the correct virtual network?
  4. Does the virtual network map to the correct VLAN?
  5. Does the host uplink carry that VLAN?
  6. Does the gateway answer ARP?

Note: A VM interface can show up/up inside the guest while the vNIC attaches to the wrong virtual switch. Always check the attachment, not only the guest.

Snapshots are not backups

A snapshot preserves VM state at a point in time. Snapshots are useful in labs because you can roll back fast. They are not production backups.

The practical lesson for ENCOR: a VM can be reverted, cloned, moved, or reattached to a different network without any change to the physical switch.

Lab networking modes

Type 2 hypervisors offer common networking modes. Names vary by platform.

ModeMeaningGood for
BridgedVM appears on the same LAN as the hostReaching the physical network
NATVM reaches out through host translationInternet access from a lab VM
Host-onlyVM talks to the host and host-only VMsIsolated labs
InternalVM talks to VMs on the same internal networkRouter and switch practice

Example: two virtual routers on one LAN

Scenario: two virtual routers connect to the same internal virtual switch named LAN-A. The subnet is 10.10.10.0/24.

R1:

hostname R1
interface GigabitEthernet0/0
 description LAN-A to R2
 ip address 10.10.10.1 255.255.255.0
 no shutdown

R2:

hostname R2
interface GigabitEthernet0/0
 description LAN-A to R1
 ip address 10.10.10.2 255.255.255.0
 no shutdown

Expected verification output on R1:

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.10.1      YES manual up                    up

R1# ping 10.10.10.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)

R1# show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.10.1              -   5000.0001.0000  ARPA   GigabitEthernet0/0
Internet  10.10.10.2              1   5000.0002.0000  ARPA   GigabitEthernet0/0

The ARP entry for 10.10.10.2 proves the frames crossed the virtual switch. The routers behave exactly like physical routers on a physical LAN.

Lab: break the vNIC attachment

Use the same topology from the example above.

Topology:

R1 Gi0/0 10.10.10.1/24 -- virtual switch LAN-A -- R2 Gi0/0 10.10.10.2/24

Do these steps:

  1. Verify R1 pings 10.10.10.2 with 100 percent success.
  2. In the hypervisor, move the R2 vNIC to a different virtual switch.
  3. Ping 10.10.10.2 from R1 again.
  4. Check show ip interface brief on R2.
  5. Check show arp on R1.
  6. Move the R2 vNIC back to LAN-A and ping again.

Expected results:

  • After step 3, the ping fails.
  • After step 4, R2 still shows Gi0/0 as up/up.
  • After step 5, the ARP entry for 10.10.10.2 shows as incomplete.
  • After step 6, the ping succeeds again.

The lesson: the failure was in the VM-to-virtual-switch attachment, not in the router config. The guest OS could not see the problem.

Troubleshooting checklist

Inside the VM:

show ip interface brief
ping 10.10.10.1
show arp

On the physical switch:

show mac address-table interface gi1/0/10
show interfaces trunk
show vlan brief

In the hypervisor:

  1. Check the VM is powered on.
  2. Check the vNIC is connected.
  3. Check the virtual network or port group.
  4. Check the VLAN tag.
  5. Check the networking mode on Type 2 platforms.

Exam traps

  • A VM is not only an IP address. It has a vNIC attached to a specific virtual network.
  • A guest interface can look up while the vNIC sits on the wrong virtual switch.
  • Bridged, NAT, and host-only modes change what the VM can reach.
  • A snapshot rollback can undo network changes inside the guest.
  • The VM MAC address must appear somewhere: the virtual switch, the host uplink, or the physical MAC table.

Pass check

You are ready for this objective when you can do these things:

  • List the virtual hardware a VM receives from the hypervisor.
  • Draw the path from the guest IP stack to the physical switch.
  • Explain the difference between bridged, NAT, and host-only modes.
  • Explain why a ping can fail while both router interfaces show up/up.
  • Say why a snapshot is not a backup.

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