Router default source interface for own communication

[Nikolaos Milas]

Hello,

On a router (say R1) with multiple physical interfaces, subinterfaces and a Loopback interface, which is the (source) interface used by the router for router's own outgoing traffic (dns queries, pings, traceroute, ntp, etc.)?

It seems that it is the interface currently used for outgoing traffic. This may be a problem, if the traffic is not sent directly to the ISP but it is redirected through another router (e.g. R2, connected to the ISP) to which R1 communicates using a link with a private address-space subnet. Then, the private address of the respective interface will be used, and, if this address is not NAT'ed (which is not always desirable), it will not be able to see the Internet.

The question: Can we configure (force) the router (R1) to use another interface as the default for its own outgoing communications? For example, Loopback 0 in our case has a public IP Address. Can we instruct the router to always use Loopback 0 as source when doing DNS, ftp, ntp communications etc.?

I am on a 3825 router, with IOS v12.4(24)T8 Advanced Enterprise.

Please advise!

Thanks,
Nick

[Mark Malone · Accepted solution]

Yes you can set the source for most services on your switch/router as below examples

Ip ntp source L0

Ip tacacs source-interface L0

Ip ftp source L0

logging source-interface L0

Ip dns source-interface L0

[Nikolaos Milas]

Many thanks Mark,

It worked.

Note: The actual commands I used were:

ip ftp source-interface Lo0
ip domain lookup source-interface Lo0
ntp source Lo 0

All the best,
Nick