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Communication on Different VLAN IDs on Cisco vs Juniper

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  • 1.  Communication on Different VLAN IDs on Cisco vs Juniper

    Posted 10-24-2018 02:27

    Hi Guys,

    I have different VLANs on Juniper EX4200 and Cisco 6509 and they are connected via access port. All my ports are access( switch to switch and switch to servers).

    Servers in different VLANs are able to reach each other. Any one can explain this behavior please ?





    Swtich to switch link configurations:


    Cisco 6509

    interface GigabitEthernet1/48
     description to JNPR_es_4200_ge-0/0/38
     switchport access vlan 10
     switchport mode access
     no cdp enable
    show run vlan 10
    Building configuration...
    Current configuration:
    vlan 10
     name TEST_10

    Juniper EX4200

    show configuration interfaces ge-0/0/38
    description to_csco_6509:gi1/48;
    unit 0 {
        family ethernet-switching {
            vlan {
                members 20
    >>> show configuration vlans | match 20 | display set
    set vlans TEST vlan-id 20



  • 2.  RE: Communication on Different VLAN IDs on Cisco vs Juniper

    Posted 10-24-2018 02:43


    First things first - there appears to be an error on Your diagram : Your CSCO 6509 Juniper-facing port shows on Your diagram as Gi1/37 but in the printouts it is referred to as Gi1/48.

    Secondly, the behaviour You described is expected with inter-switch access ports. In more detail:

    1/ an untagged Ethernet frame from Server_04/_03 enters CSCO 6509

    2/ it exits as untagged from CSCO 6509 Gi1/48

    3/ JNPR EX4200 receives this frame as untagged on ge-0/0/38 access port

    4/ JNPR EX4200 passes this frame as untagged to access ports connected to Server_01/_02.

    The same happens in reverse direction as well.





  • 3.  RE: Communication on Different VLAN IDs on Cisco vs Juniper
    Best Answer

    Posted 10-24-2018 04:09

    Hi MYN,


    Whenever you send a frame out of an access port the vlan information is not carried along the frame. So the only way a switch classifies the vlan of an incoming frame on an interface is through the vlan of the port on which the frame was received. 


    On the other hand, frames sent out a trunk port carry vlan id information as a tag along with them which will help the receiving switch classify the frame to the corresponding vlan. 


    In your topology frames are sent out through access ports on both cisco and juniper devices and hence though the frames originated from different vlans, the receiving switches classified the frames based on the vlan configured on the ports where frames were recieved. 


    This is why it is recommended to always configure the interswitch links as trunk links instead of access links.