Some interesting thing about PING our interface

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Dec 10th, 2002
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Hi when ever i try to ping my routers(client side) serial interface I am unable to do .after debugging I came to know how ping to our own interface is working.Whenever we are pinging our own interface the ICMP request goes to the other end of serial interface on point to pint link.the other router when see the destination is on the same network as on the interface it recieved so it sends ICMP redirect message to the client side router to use default gateway as its own client interface and forwards the first icmp packet recieved to the client side also.

Here is the debug i used on server side which sends icmp redirect message.


Client side ip -> 1.1.1.2 (ping 1.1.1.2)

Server side has ip of 1.1.1.1

Debug issued here on server side

00:33:19: IP: s=1.1.1.2 (Serial0), d=1.1.1.2 (Serial0), len 100, redirected

00:33:19: ICMP type=8, code=0

00:33:19: IP: s=1.1.1.1 (local), d=1.1.1.2 (Serial0), len 56, sending

00:33:19: ICMP type=5, code=1

00:33:19: IP: s=1.1.1.2 (Serial0), d=1.1.1.2 (Serial0), g=1.1.1.2, len 100, forw

ard

00:33:19: ICMP type=8, code=0


The ICMP type 5 contains a redirect message to send data packets on alternative route.ICMP Redirect is a mechanism for routers to convey routing information to hosts.The IP address of the gateway and the internet header plus the first 8 bytes of the original datagram's data is returned to the sender. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first 64 data bits of the original datagram's data.

Now another thing which i observed if i give clients side interface address on the remote side to some interface and that interface is shut down.It is unable to ping its own address.

Example

Client side

int s0,ip address 1.1.1.1,no shut

remote side

int s0,ip address 1.1.1.2,no shut

int s1,ip address 1.1.1.1,shut

client side ---> ping 1.1.1.1 unable to ping.

Can somebody explain.

Similarly thats see pinging our own interface requires the packet to go through the link to the other side,thats why in case of frame relay or ATM ckts we are unable to ping since our own interface address is not mapped to the pvc/dlci and we need to do static mapping.Moreover when i issue show ip redirects it does not show any things and gives the cache is empty.

Moreover Please do comment on all of the above, and correct me i am wrong

Regards,

Munit


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ROBERT WATSON Tue, 12/10/2002 - 13:38
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Packet response is type 5 icmp redirect but it is not actually redirected routers automatically send redirect replies when they recieve packets from a host on a network to which the gateway is attached ( which is the case in your scenario) refer to rfc 792 page 13: in this case source .1 d .2





Example

Client side

int s0,ip address 1.1.1.1,no shut

remote side

int s0,ip address 1.1.1.2,no shut

int s1,ip address 1.1.1.1,shut

client side ---> ping 1.1.1.1 unable to ping.


you must be using host base subnet masks due to the fact you cant configure 2 separate interfaces on separate router with the same subnet ip addresses. provided you have ip routing enable( default) and you can't ping 1.1.1.1 b/c when the rtr sees the d as 1.1.1.1 it assumes it's own interface not the address on the other end of the s link.


and the command sho ip redirects on the router means packet redirects not icmp type 5 messages (icmp redirects)

msingla Tue, 12/10/2002 - 16:26
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Hi,

I am still not clear about this I have gone through RFC also but infact could not get out.

Yes I agree that we cannot assign the same address of that subnet to another interface but if interface is shut down i am able to assign as shown in above configurations .I am not using any host base subnet masks as u reffered.Can u be little clear .Can u clear this line "Packet response is type 5 icmp redirect but it is not actually redirected routers automatically send redirect replies "

Thanx for u reply

prafuljaded Sat, 12/14/2002 - 10:05
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Plz check the discussion " Ping response on a serial link" on same discussion room on December 3rd or 4th . It also refers to same question.

msingla Sat, 12/14/2002 - 10:18
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thanxxxxxxxxxxxxxxxxxxxxxxxxxx for the reply I will check it out

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