Switch port connections

[pcromwell]

We have an MPLS connection terminated on a cisco 1800 router. This is connected to our 2900 Router gig interface via a Trunk.

The below is the interface config on our 2900. Gig0/0 carries the Public Ip address and Gig0/0.323 is the MPLS link.

interface GigabitEthernet0/0
 ip address 99.99.99.99 255.255.255.224
 ip access-group 101 in
        
interface GigabitEthernet0/0.323
 bandwidth 10240
 encapsulation dot1Q 323
 ip address 172.16.X.X 255.255.255.252

We want to put a switch in between the 2 routers.

the switch we have is an HP 1810-8G.

I am trying to work out whether I need to setup VLAN 323 on this switch and have it tagged. or does the command "encapsulation dot1Q 323" mean Vlan 323 is that native vlan and is therefore untagged?  any help would be much appreciated
 

[Jan Hrnko · Accepted solution]

Hi,

I am trying to work out whether I need to setup VLAN 323 on this switch and have it tagged. or does the command "encapsulation dot1Q 323" mean Vlan 323 is that native vlan and is therefore untagged?

Short answer: No.

Long answer: encapsulation dot1Q 323 command means that this subinterface receives and sends TAGGED frames with vlan tag 323.

Is the output that you provided from the same interface? If yes, then everything that is placed on the physical interface gi0/0 (with public IP) should be untagged. I have seen this kind of behaviour when physical interface is configured along with subinterface(s).

Btw. there is a command that changes the native vlan: encapsulation dot1Q vlan-id native but I do not recommend you to use it as you already have untagged frames on the physical interface (with the public ip).

So I think that you need to configure the appropriate VLANs and trunks to carry them on that switch (Or at least native vlan and vlan 323).

Best regards,

Jan

[pcromwell]

many thanks for the reply

yes, the gig0/0 adn gig0/0.323 are the same interface.

so on the HP switch, I need it to be a trunk port as well, or can I just make it access port on Vlan 323?

[Jan Hrnko]

Hi,

so the connection will be something like

Cisco 1800 --- HP SW --- Cisco 2900

right?

If this is the case, you will need trunks on both sides with vlan 323 and some native vlan configured as well.

Making the ports access ports would not work, I can explain why later, if you want me to.

Best regards,

Jan
 

[Jon Marshall]

Jan

Apologies for posting as you were.

I've just logged on so am catching up with threads :-)

Jon

[pcromwell]

Thank you so much to the both of you.  I now have a clearer understanding

[Jan Hrnko]

Jon,

No need to apologise! Thanks for your input and have a great day :) .

Best regards,

Jan

[Jon Marshall · Accepted solution]

You need it to be a trunk port.

As noted by Jan you are passing untagged frames on the gi0/0 interface and tagged frames on the gi0/0.323 interface.

So the connection from the switch has to be a trunk and in addition to that the vlan for the public addresses must be configured as the native vlan on the trunk link.

Jon