Changing Native Vlan in Cisco Switch

If I understand ,Cisco Switches have VLAN 1 as the default native Vlan.
Many resources say that leaving Native VLAN to VLAN 1, can be security breach.
1- Can someone explain why leaving VLAN 1 can be security breach  ?
2- If I need to change Native VLAN 1 to VLAN 777, I believe this is done at the Interface Level. Does that mean I need to go to each switch and change the interfaces that are in VLAN1 to VLAN 777 ?
3- if some switches Native VLAN is VLAN1 and some is VLAN 777, does that mean traffic coming from those Native Vlans will be forwarding to all ports on the switch...
in other words if a PC is connected to Native Vlan , will be able to ping another PC which is connected to VLAN 33 and the other way around is also True?

Thank you
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Don JohnstonInstructorCommented:
1) There is some management traffic that is carried by VLAN1 (DTP, VTP, CDP, etc.). Given the right circumstances, it is possible for a knowledgeable, hacker type person to get traffic onto the native VLAN of a trunk.  If the native VLAN is left to the default of VLAN 1, they could affect the traffic on that VLAN.  That's why it's recommended to change the native VLAN to something other than 1.  Preferably to a VLAN which is not used for anything else.
2) Correct. You have to go to every trunking interface on every switch to change the native VLAN.
3) Not sure that I'm following your scenario. All the native VLAN means is that is it not tagged. All other rules still apply.  Which means if you're connected to an access port in VLAN 8, you will only be able to communicate with other devices in VLAN 8.
jskfanAuthor Commented:
in the topology above, R1 e2/2 IP = and R2 e2/2 IP=
both interfaces can ping each other.
I changed Native Vlan for Switch2  e2/2:
interface Ethernet2/2
 switchport trunk native vlan 777

Still both interfaces on R1 and R2 can ping each other. However when I run : Show Vlan Brief, it still shows e2/2 on SW2 in VLAN1 and native VLAN 777, does not show up:
SW2#sh vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Et0/1, Et0/2, Et0/3, Et1/0
                                                Et1/2, Et1/3, Et2/0, Et2/1
                                                Et2/2, Et2/3, Et3/0, Et3/1
                                                Et3/2, Et3/3
55   VLAN0055                         active    
66   VLAN0066                         active    
77   VLAN0077                         active    
88   VLAN0088                         active    
1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup
Don JohnstonInstructorCommented:
The native VLAN is only a factor on trunks.

E2/2 is not a trunk. So issuing the command "switchport trunk native vlan 777" has no effect on this interface.
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jskfanAuthor Commented:
I made e2/2 on SW2 as Trunk then changed its native Vlan to 777
Now I cannot ping from R1 to R2.
I thought 2 interfaces in 2 different Native Vlans will still be able to Ping each other ?
Don JohnstonInstructorCommented:
I would stick to keeping the trunk links limited to inter-switch links until you've got a better handle on things. Otherwise you're just going to confuse yourself.

Lets call trunks to end stations an Advanced Topic for now. ;-)
jskfanAuthor Commented:
I agree..
I am trying to see when Switch Trunk interfaces are in different Native Vlans , whether the Hosts connected to those switches can ping each other.

in the topology above, I configured SW2 e0/0  connected to SW1 and e1/1 connected to SW4  both in Native Vlan 777

Now I cannot ping from R1 to R2 and back....
Don JohnstonInstructorCommented:
Well, it doesn't sound like you've got the trunks configured correctly.  The native VLAN should match on each end of the link.
jskfanAuthor Commented:
for Native VLAN 1 and Native VLAN 777 to talk to each other, do I need L3 device ?
Don JohnstonInstructorCommented:
trunksI think you're missing the concept of the native VLAN.

The native VLAN is the VLAN on a trunk that does not get tagged.   It only exists on a trunk between two switches (or a switch and a trunking capable device).

Best practice is to not have the native VLAN communicate with anything.

In the attached topology diagram, all inter-switch links are trunks.
jskfanAuthor Commented:
- In your Diagram above will R1 be able to ping R2 and vice-versa ?
- If R1 was in VLAN 777 and R2 in VLAN 55, will still be able to ping each other ?
Don JohnstonInstructorCommented:
- In your Diagram above will R1 be able to ping R2 and vice-versa ?
- If R1 was in VLAN 777 and R2 in VLAN 55, will still be able to ping each other ?
Of course not.  :-)

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jskfanAuthor Commented:
Thanks Don !
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