why should i use both dns server isp and google dns or how to configure or which better ?

[amralrazzaz]

here u are the dhcp pool configuration

why should i use both dns servers isp and google?

can some one explain to me the below configuration ? each line need to know the explanation for it 

ip dhcp relay information trust-all
!
ip dhcp pool voice
import all
network 192.168.2.0 255.255.255.0
default-router 192.168.2.207
option 150 ip 192.168.2.207
dns-server 8.8.8.8 8.8.4.4
!
!
!
ip dhcp global-options
dns-server 163.121.128.134 163.121.128.135
!
ip name-server 8.8.8.8
ip name-server 8.8.4.4
ip cef

need to know the dns configuration explanation and the diffrence if i use google dns only or dns for isp only?

also ip dhcp relay information trust-all what does it mean?

also ip dhcp global-options
dns-server 163.121.128.134 163.121.128.135
!
ip name-server 8.8.8.8
ip name-server 8.8.4.4
ip cef

what does it mean ?and this is how to configure the dns and dhcp for all routers?

thanks

[Deepak Kumar]

Hi, 

Q:  ip dhcp relay information trust-all what does it mean?

Ans: This is an optional command for your DHCP server which is running on Cisco router.  Configures all interfaces on a router as trusted sources of the DHCP relay information option. By default, if the gateway address is set to all zeros in the DHCP packet and the relay agent information option is already present in the packet, the DHCP relay agent will discard the packet. Use the IP DHCP relay information trust-all command to override this behaviour and accept the packets.

This command is useful if there is a switch in between the client and the relay agent that may insert option 82. Use this command to ensure that these packets do not get dropped.

Q: ip name-server What is this?

Ans: This is giving DNS server IP address to your router. If your router will try to resolve any FQDN then where the query will route through the router itself. It's same like your laptop or Desktop DNS server IP address configuration.

Q: What is ip cef?

Ans: This is a Cisco technology. Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns.

More:

https://www.cisco.com/c/en/us/td/docs/ios/12_2/switch/configuration/guide/fswtch_c/xcfcef.html

Q: Which DNS server is better?

Ans: This is not an easy question because it depends on many factors. 

If you really want to know that which is better for your network then you have to make test measuring latency on DNS queries. Use below command on your laptop or desktop power shell:

Measure-Command {Resolve-DnsName google.com -DnsOnly -Type A -NoHostsFile -server x.x.x.x}

Regards,

Deepak Kumar

[amralrazzaz]

thanks alot for explanation really appreciate 

 ip dhcp relay information trust-all : is it better when i configure a router and dhcp pool to enable this feature even if no need for it ? just in case

why in this configuration use both dns ? or should i configure any router like that putting the google and isp dns on configuration as below: 

ip dhcp pool voice
import all
network 192.168.2.0 255.255.255.0
default-router 192.168.2.207
option 150 ip 192.168.2.207

dns-server 8.8.8.8 8.8.4.4
!
!
!
ip dhcp global-options
dns-server 163.121.128.134 163.121.128.135
!
ip name-server 8.8.8.8
ip name-server 8.8.4.4

[Deepak Kumar · Accepted solution]

Hi, 

If you are asking for security reasons then I will not advise making DHCP trust on all interfaces. It depends on your network design. If all interfaces are not required in the trusted area then you can configure an individual interface as a trusted source of the DHCP relay information option by using the ip dhcp relay information trusted interface configuration mode command.

Q:   dns-server 8.8.8.8 8.8.4.4:

This command is part of DHCP Pool configuration. This is offering DNS server information to DHCP client. 

Q: ip dhcp global-options ! dns-server 163.121.128.134 163.121.128.135

This is DHCP option. Some time some devices are not accepted DNS server information from direct "DNS-Server" command from the DHCP pool then it required. But in the normal case, it will overwrite the DNS-Server command.

Regards,

Deepak Kumar

[amralrazzaz]

dear

thanks alot for all helps

just need to know whats the bold lines explanation 

ip dhcp relay information trust-all
!
ip dhcp pool voice
import all
network 192.168.2.0 255.255.255.0
default-router 192.168.2.207
option 150 ip 192.168.2.207
dns-server 8.8.8.8 8.8.4.4
!
!
!
ip dhcp global-options
dns-server 163.121.128.134 163.121.128.135
!
ip name-server 8.8.8.8
ip name-server 8.8.4.4
ip cef

about ip dhcp global : is it better to put the dns server for isp also under global dhcp as bckup dns incase of some clients not accepting the google dns?

also 2nd question is about ip name-server 8.8.8.8 : this configuration under config mode or which mode and what is the aim fro this in case if  its already dns configured under dhcp pool data ?

thanks

[Deepak Kumar · Accepted solution]

Q: about ip dhcp global : is it better to put the dns server for isp also under global dhcp as bckup dns incase of some clients not accepting the google dns?

Ans: Yes, It is better. Becuase some time public DNS server is having a delay due to wan routing. 

Q:2nd question is about ip name-server 8.8.8.8 : this configuration under config mode or which mode and what is the aim fro this in case if  its already dns configured under dhcp pool data ?

Ans: This is your router itself for internal use and will use to forward your query to DNS server. If your router will be required to resolve any FQDN then where DNS query will send to resolve the DNS name.

Regards,

Deepak Kumar

[amralrazzaz]

how to measure or i mean i have dns for google and dns for isp 

when should i put the ip ?

dns-server 8.8.8.8 8.8.4.4

dns-server 163.121.128.134 163.121.128.135

Measure-Command {Resolve-DnsName google.com -DnsOnly -Type A -NoHostsFile -server x.x.x.x}

[Deepak Kumar]

Measure-Command {Resolve-DnsName google.com -DnsOnly -Type A -NoHostsFile -server 8.8.8.8}

Replace X.X.X.X with your DNS server IP.

Regards,

Deepak Kumar

[amralrazzaz]

here is the output but i dunt know which is better ?

from which line i can know which is faster response time?

PS C:\Users\Administrator> Measure-Command {Resolve-DnsName google.com -DnsOnly -Type A -NoHostsFile -server 163.121.128.134}

Days : 0
Hours : 0
Minutes : 0
Seconds : 0
Milliseconds : 8
Ticks : 84077
TotalDays : 9.73113425925926E-08
TotalHours : 2.33547222222222E-06
TotalMinutes : 0.000140128333333333
TotalSeconds : 0.0084077
TotalMilliseconds : 8.4077

PS C:\Users\Administrator> Measure-Command {Resolve-DnsName google.com -DnsOnly -Type A -NoHostsFile -server 8.8.8.8}

Days : 0
Hours : 0
Minutes : 0
Seconds : 0
Milliseconds : 63
Ticks : 631624
TotalDays : 7.31046296296296E-07
TotalHours : 1.75451111111111E-05
TotalMinutes : 0.00105270666666667
TotalSeconds : 0.0631624
TotalMilliseconds : 63.1624

[Deepak Kumar]

Hi,

Here I see the fasted DNS server is 163.121.128.134

Days : 0
Hours : 0
Minutes : 0
Seconds : 0
Milliseconds : 8
Ticks : 84077
TotalDays : 9.73113425925926E-08
TotalHours : 2.33547222222222E-06
TotalMinutes : 0.000140128333333333
TotalSeconds : 0.0084077
TotalMilliseconds : 8.4077

Regards,

Deepak Kumar

[amralrazzaz]

SO IS THERE IS ANY CHANGED SHOULD I MAKE ON THE FOLLOWING CONFIGURATION TO MAKE THE ISP DNS IS WORKING STEAD OF GOOGLE DNS ?

ip dhcp relay information trust-all
!
ip dhcp pool voice
import all
network 192.168.2.0 255.255.255.0
default-router 192.168.2.207
option 150 ip 192.168.2.207
dns-server 8.8.8.8 8.8.4.4
!
!
!
ip dhcp global-options
dns-server 163.121.128.134 163.121.128.135
!
ip name-server 8.8.8.8
ip name-server 8.8.4.4
ip cef
no ipv6 cef