Stupid Mistake – Brings Up Question

In the excitement of finding out if my Ebay purchases were going to work out (may be an excuse ;--}), I made a stupid addressing mistake … But still don’t understand why the **local interface** did not respond to a **local ping**.

I connected two routers between their serial interfaces, assigned IP addresses, clock rate on DCE, no shutdown. The interfaces came up, but I could not ping anything. Did the basic troubleshooting commands … show int … sh cont … show ip int + brief, but everything looked normal. This drove me crazy for a while.

Then it clicked, I had the interfaces on two different networks:
Router 1 = 192.168.0.1 255.255.255.0
Router 2 = 192.168.1.1 255.255.255.0

OK easy fix, but why didn’t either interface respond to a ping when the interface is local to the router? How does an addressing error on the opposite router cause a ping not to respond from the router’s local interface. The line and protocol was up according to the show commands … ?

Sure I’m again missing something simple … does anyone have an explanation? and what “show” command would have indicated a problem under this scenario?

TIA,
RK

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Comments

networker050184

When you ping your own interface the router sends the echo request to the router on the other end and that router sends the echo reply back. So if they are on different subnets the distant router doesn't know how to send the echo reply back to you.

dtlokee

To ping the local serial interface it needs to cross th connection to the other end and come back.

korszo

Wow, didn't know that. I have not yet studied WAN technologies, so I assume it has to do with the serial interface?

I've really only worked with Ethernet, so I'm used to pinging the local interface first.

Let me tell you, this really screwed with my head.

I assume this will be covered in my future reading / studying?

Thanks!!!

RK

dtlokee

In the future don't forget about CDP, it will run on the serial interface without IP addressing

networker050184

Don't remeber ever learning it in exam study, but something learned from the same mistake you just made. This is why hands on is soooo important.

EDIT: I think I learned it learning about frame relay... not sure though

korszo

dtlokee wrote:

In the future don't forget about CDP, it will run on the serial interface without IP addressing

Believe I used some CDP commands, but don't think it helped the matter. No show commands I could find helped me determine the problem.

Coming from Ethernet, I assumed the local interface would respond to a ping, providing the protocol stack was bound properly.

Guess this turned out to be good experience after all.

Thanks again,
RK

Netwurk

Interface has to be up and up for you to ping it, even locally

I thought this was true just with serial interfaces, but your post got me thinking and I tried it with a fasteth int - same result

Not something you'd be tested on, but good to know for troubleshooting

korszo

Netwurk wrote:

Interface has to be up and up for you to ping it, even locally

I thought this was true just with serial interfaces, but your post got me thinking and I tried it with a fasteth int - same result

Not something you'd be tested on, but good to know for troubleshooting

In my scenario **both** router serial interfaces were “up and up”, but the local interface did not respond to an echo request, due to the adjacent interface being on a different subnet.

Ethernet does not act this way … in the same scenario the local Ethernet interface would respond.

Can’t wait to understand this!

RK

dtlokee

If you want to see something fun, change your encapsulation on the serial line to ppp and use different subnets at both ends and see what happens.

Something like

R1

int serial 0/0
encap ppp
clock rate 64000
ip address 10.1.1.1 255.255.255.0
no shut

R2

int serial 0/0
encap ppp
ip address 20.2.2.2 255.255.255.0
no shut

then try to ping from one to the other...

phantasm

The interface and line protocol are up, ping fails. I'm going to assume that the line protocol is up because of the encapsulation type?