Caption script:

Brief: VLAN ATP, Ping

Goal: Observe how ARP and Ping pacets are being to tagged/untagged, flooded, switched in a 2-switch LAN.
Topology: 6 hosts in 2 VLANs are connected by 2 switches. A VLAN trunk connnects switches S1 and S2. 
Steps: 1) H1 sends ARP request to  find H6's MAC. This frame is tagged, flooded in VLAN 2, untagged.
             2) H6 sends ARP reply to H1. This frame is tagged, switched in VLAN 2, untagged.
             3) VLAN 2 and VLAN 2 packets are being transmitted over trunk link at the same time.

Topology and configure

To configure a VLAN, we need to do the following on each switch: 
- Create VLAN, configure access ports for each VLAN. 
- configure trunk port. 
In this animation, VLAN 2 is blue. VLAN 3 is red. Switch command window lists detail configuration commands.
Note: See Appendix for VLN configuration commands

H1 ping H H6, Send ARP out

H1 wants to ping H6. They both are connected to VLAN 2..
But H1's ARP cache does not have H6's MAC. H1 sends an ARP Request to find out.
It adds an entry (IP.H6, Incomplete) in H1's ARP cache to indicate that H1 is looing for H6's MAC.)

S1: Tags and floods

When S1 receives the frame at F0/1, S1 inserts a VLAN header (VLAN ID 2) since F0/1 is a VLAN 2 access port. This is called VLAN tagging. S1 also discovers the frame's source MAC, MAC.H1, is a new address, and adds (MAC.H1, 2, F0/1) to MAC table. It means: MAC.H1 is in VLAN 2, it can be reached via F0/1.

S2: flood, untag (exit VLAN)

S2 floods ARP request to access ports F0/5, F0/6. Then it takes off VLAN header before transmits. This is called VLAN untagging. VLAN tag is inserted when a frame is entering a VLAN and dropped when existing the VLAN. Tags are used by switches to flood and forward frames within a VLAN.

H6: ARP reply switched to H1

Upon receiving ARP request, H6 sends ARP reply to tell H1 its MAC address. This frame is tagged at S2 access port F0/6 as VLAN 2, switched inside VLAN 2 by S2, S1 to access port F0/1, then untagged, and transmitted to H1. S2, S1 also discover MAC.H6 is a new MAC and add it to their Mac tables.

H1 ping H6, H6 echo H1

After receiving ARP reply, H1 updates its ARP table and sends ping to H6. S1 tags ping, switches it to S2. Then S2 switches it to F0/6, untags and transmits it to H6. Then H6 sends echo to H1. It is switched by S2, S1 inside VLAN 2 and reaches H1.

multi-VLAN frames on trunk

Now H1 ping H6 and H4 sends ARP request at the same time. H1, H6 are in VLAN 2, H4, H2 are in VLAN 3. Ping in VLAN 2 and ARP request in VLAN 3 are traversing on VLAN trunk link concurrently.

Appendix: VLAN configuration

S1#vlan database
S1(vlan)#vlan 2
S1(vlan)#vlan 2; VLAN 20 added:
S1(vlan)#apply
S1(vlan)#exit
S1#conf t 
S1(config)#interface f0/1
S1(config-if)#switchport access vlan 2
S1(config-if)#inter f0/2
S1(config-if)#switchport access vlan 3
S1(config-if)#inter f0/3
S1(config-if)#switchport access vlan 3
S1(config-if)#inter f0/7
S1(config-if)#switchport mode trunk
S1(config-if)#switchport trunk allowed vlan all
S1(config-if)#exit
S1(config)#exit
S1#
S2(vlan)#vlan 2
S2(vlan)#vlan 2; VLAN 20 added:
S2(vlan)#apply
S2(vlan)#exit
S2#conf t 
S2(config)#interface f0/4
S2(config-if)#switchport access vlan 3
S2(config-if)#inter f0/5
S2(config-if)#switchport access vlan 2
S2(config-if)#inter f0/6
S2(config-if)#switchport access vlan 2
S2(config-if)#inter f0/7
S2(config-if)#switchport mode trunk
S2(config-if)#switchport trunk allowed vlan all
S2(config-if)#exit
S2(config)#exit
S2#

Appendix: VLAN FAQ

Q1. How does LAN evolve into VLNA
Answer: LAN grows up in phrases.
   1) Cabling. In the 80s, LAN cabling was a mess. Each node (PC