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New Member

reaching dns on vlan


if we have say 2 vlans (vlanA, vlanB) on a layer 3 switch connected to the internet, how is dns server configured on the switch.

vlanA,vlanB -------router-----internet

ipA-ip address of vlan A

ipB-ip address of vlan B

ipDNS-ip address of DNS server on vlan A

thanks a lot.


Re: reaching dns on vlan

i am not 100% sure what you are asking for, but if you want to do dns lookups when you telnet or ssh from the switch, use the "ip domain lookup", "ip domain-name" and "ip name-server" commands.



New Member

Re: reaching dns on vlan

what am sayin is

Ip_DNS is the ip address of the DNS server on VlanA.

IP_A is the ip address of vlan_A

IP_B is the ip address of vlan_B

notice that the dns server is residing on vlan_A.

If a host on vlan_B wants to go to internet by using the dns server that resides on vlan_A, what configuration do i have to do in the switch.

Do i need to use the ip helper address in this case..Am not so clear about it...

hope you got a clearer picture on the question now



Re: reaching dns on vlan

there is no switch config required. You have to tell the PC where the DNS server is. There is nothing for the switch/router to do. So long as you have network connectivity from the PC on vlan B to the DNS port on the DNS server , then the rest of the config needs to be put on the PC.

New Member

Re: reaching dns on vlan

What confuses me is the discussion on the IP helper address thing. Because one of the 8 port by default is the DNS.Another is the DHCP..And in the case of DHCP, we are to use the ip helper address, so why not on a DNS.

Kindly clarify


Re: reaching dns on vlan

IP Helper address feature is used to relay certain kinds of broadcast traffic directly to a server. For example, a DHCP client sends out a broadcast packet requesting an IP address. The IP helper feature can forward the DHCP request directly to a DHCP server on another LAN.

To be honest, I have no idea why a PC would broadcast a DNS query, so I would just ignore

that. All PCs or network devices, that use DNS,

that I have ever configured always know the IP

address of the DNS server, either through DHCP

or by manually configuring it.

I think you shouldn't focus on the fact that the IP helper feature can forward broadcast DNS queries to a server. You should configure the PCs properly so that the pc knows how to directly communicate with the DNS server.

New Member

Re: reaching dns on vlan

thats a nice explanation..but lets go back to what you were sayin on your first reply..

You've stated there the ip name server etc...could u elaborate on those pls...


New Member

Re: reaching dns on vlan

I am having a problem pinging the DNS setup on my server. I have a VLAN setup on several switch ports and one computer can ping the DNS server but the server cannot. When I do a TRACERT on the PC, I see all the hops it takes to the DNS server. From the server, I cannot ping any of these hops. It's as if the server can't find the appropriate route to the DNS. Any ideas of what is stopping this?

New Member

Re: reaching dns on vlan

"ip name-server" is used for the router/switch to know the DNS server ip. Incase you ping or trace from the router/switch to say, the website address gets resolved to the IP address through the DNS server specified.