This practice is continued from previous Practice 1 and Practice 2. Essentially, we must configure the two "Inter-switch Links" manually as "Trunk Link".
But that's not enough. One thing I have to point out is we also have to declare VLAN 10 and VLAN 20 in the middle switch: "CoreSwitch". We often forget about this when we implement LAN switching.
Why should we do this? This is because of the design of a hardware switch. It cannot "guess" how many VLANs could be there. Switches only separate and forward frames tagged as certain VLANs when it knows those VLANs exist. If we forget to declare them, then the "CoreSwitch" will not recognize frames tagged as VLAN 10 and VLAN 20, and thus drops all frames of them. We lose connectivity for both VLAN 10 and VLAN 20 in the end.
Then why we do not have to "declare" VLAN 10 and VLAN 20 in previous examples? This is because when we configure some switch port as access link of "VLAN 10", Cisco IOS add "VLAN 10" automatically for us. The same thing happens for VLAN 20, too. However, because we do not configure any switch ports in "CoreSwitch" as neither VLAN 10 nor VLAN 20, we have to declare them by ourselves.
CoreSwitch(config)#vlan 10 CoreSwitch(config-vlan)#exit CoreSwitch(config)#vlan 20 CoreSwitch(config-vlan)#exit CoreSwitch(config)#
We cannot know from "running-config" about which VLANs are added. We have to issue "show vlan" command to display the VLANs we successfully declared.
Before we add VLANs:
CoreSwitch#show vlan
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Fa0/24
Gig1/1, Gig1/2
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active
VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 0 0
1002 enet 101002 1500 - - - - - 0 0
1003 enet 101003 1500 - - - - - 0 0
1004 enet 101004 1500 - - - - - 0 0
1005 enet 101005 1500 - - - - - 0 0
CoreSwitch#
After we declared VLANs, we will see the differences:
CoreSwitch#show vlan
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/13, Fa0/14
Fa0/15, Fa0/16, Fa0/17, Fa0/18
Fa0/19, Fa0/20, Fa0/21, Fa0/22
Fa0/23, Fa0/24, Gig1/1, Gig1/2
10 VLAN0010 active
20 VLAN0020 active
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active
VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 0 0
10 enet 100010 1500 - - - - - 0 0
20 enet 100020 1500 - - - - - 0 0
1002 enet 101002 1500 - - - - - 0 0
1003 enet 101003 1500 - - - - - 0 0
1004 enet 101004 1500 - - - - - 0 0
1005 enet 101005 1500 - - - - - 0 0
CoreSwitch#
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