Both /30 on serial link and /32 on router - presenting address?

[payman0176]

If I have a numbered serial link as well as a loopback on the router, what Ip will it present it self as?

Is there a something you can do to control this like true address...?

Rgds

Patrick Arkley

www.mci.com

[Kevin Dorrell]

The router does not have a "true address" as such, and different addresses will be used for different things. For example:

- Ping will be sourced from the address of the interface it comes out of,

- Routing protocols will use the loopback address as the router ID, unless configured otherwise,

- Telnet the router will be sourced from the exit interface address, unless configured otherwise

- Telnet to the router can be addressed to either address, BUT if addressed to the IP of an interface that is "down", the router will not respond

- NTP packets will be sourced from an interface specified in the config

It depends in what context you mean by "what IP will it present itself as?" The various applications inside the router will use various addresses by default, but most are individually configurable.

Hope this helps

Kevin Dorrell

Luxembourg

[payman0176]

Thanks for a swift response. In this particular issue it is a FW that needs the correct IP in it's ACL's for the router.

So the question is; what will be the source IP in the TCP packet from that router.

I would imagine it would be the IP on the physical interface and not the virtual IP of the loopback.

Do you know?

Rgds

Patrick Arkley

[olufemi.amao]

use the loopback ip address.

Longer prefixes are always preferred over shorter ones when forwarding a packet. it is also considered more stable

[Kevin Dorrell]

Again, I'm afraid it depends on which protocol you are talking about.

For most protocols, for example telnetting from the router, the source is indeed the interface address. But you can pin the telnet down to the loopback interface by ip telnet source-interface. You can do something similar with TFTP, SNMP, etc. The advantage of doing this is that the interface is always "up".

You have to decide what sort of traffic will come from the router itself, rather than through the router. The through traffic will use its own source address and the ruter's IP addresses will not enter into the equation. Then I would configure each of these individually in the FW, and make sure that the source address of that protocol is pinned down to the loopback interface of the router.

In short, I think you need to look at the traffic in more detail rather than treating the router as a generic IP address. What protocols do you want to run to/from the router itself.

Kevin Dorrell

Luxembourg