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Webcast-Catalyst9k
New Member

ping to point-to-point (serial) interface

Hi,

in the simple Lab 

R1--------------------R2

the routers are linked together by serial interfaces (s0/0). I suspected that a ping from R1 for its s0/0 configured ip address is actually physically sent on the wire to R2 and received back by R1.

So, to see this, I've configured same ip address on both i/f (10.0.0.1/30) and turned on debugs

R1#ping 10.0.0.1 rep 1

Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:

!

Success rate is 100 percent (1/1), round-trip min/avg/max = 120/120/120 ms

R1#

R2#debu ip pack det

IP packet debugging is on (detailed)

R2#

R2#

*Mar  1 00:34:47.767: IP: tableid=0, s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), routed via RIB

*Mar  1 00:34:47.771: IP: s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), len 100, rcvd 3

*Mar  1 00:34:47.775:     ICMP type=8, code=0

*Mar  1 00:34:47.775: IP: tableid=0, s=10.0.0.1 (local), d=10.0.0.1 (Serial0/0), routed via RIB

*Mar  1 00:34:47.779: IP: s=10.0.0.1 (local), d=10.0.0.1 (Serial0/0), len 100, sending

*Mar  1 00:34:47.783:     ICMP type=0, code=0

R2#

Others p2p interface type exhibit same behaviour different from, for example, ethernet interfaces

Someone can explain me why these differences between L2 medias ?

Thanks.

2 REPLIES
Hall of Fame Super Silver

ping to point-to-point (serial) interface

Hello Carlo,

your understanding is correct.

p2p L2 media have the behaviour that you have shown in the debug output commands.

ethernet controllers implementation is different and a ping to the IP address of a local LAN based interface is not sent out on the wire:

ethernet is a broadcast and multicast  capable L2 media, other media like a FR or ATM network may be multi access / multi point but they are not broadcast and multicast capable by themselves (they can use replication of a frame over different PVCs to emulate it, but it is a different matter).

years ago it was possible to use a PA 4 Eth or a a PA 8 eth to do tests on a C7500 or C7200 by simply disabling the keepalive function ( instead of creating additional loopback interfaces) I did it some times in some functional tests.

things are different for GE or 10GE ports that are not so easy to foul

Hope to help

Giuseppe

New Member

ping to point-to-point (serial) interface

Thanks Giuseppe,

Clearing the ip fast-switching cache on R2 (c3725) and configuring now 2 different subnet ip address for R1 and R2 s0/0 (10.0.0.1 e 10.0.0.2), I see

R2#cle ip cache

R2#sh ip cache

IP routing cache 0 entries, 0 bytes

   11 adds, 11 invalidates, 0 refcounts

Minimum invalidation interval 2 seconds, maximum interval 5 seconds,

   quiet interval 3 seconds, threshold 0 requests

Invalidation rate 0 in last second, 0 in last 3 seconds

Last full cache invalidation occurred 00:00:03 ago

Prefix/Length           Age       Interface       Next Hop

R2#

R1#ping 10.0.0.1 rep 1

Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:

!

Success rate is 100 percent (1/1), round-trip min/avg/max = 44/44/44 ms

R1#

R2(config-if)#do deb ip pack det

IP packet debugging is on (detailed)

R2(config-if)#

*Mar  1 00:28:23.607: IP: tableid=0, s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), routed via RIB

*Mar  1 00:28:23.611: IP: s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), len 100, redirected

*Mar  1 00:28:23.615:     ICMP type=8, code=0  <--------------------------------------------------------------------- ICMP echo rqst

*Mar  1 00:28:23.615: IP: tableid=0, s=10.0.0.2 (local), d=10.0.0.1 (Serial0/0), routed via RIB

*Mar  1 00:28:23.619: IP: s=10.0.0.2 (local), d=10.0.0.1 (Serial0/0), len 56, sending

*Mar  1 00:28:23.623:     ICMP type=5, code=1  <--------------------------------------------------------------------- ICMP redirect

*Mar  1 00:28:23.623: IP: s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), g=10.0.0.1, len 100, forward

*Mar  1 00:28:23.631:     ICMP type=8, code=0

R2(config-if)#

*Mar  1 00:28:23.683: IP: tableid=0, s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), routed via RIB

*Mar  1 00:28:23.687: IP: s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), len 100, redirected

*Mar  1 00:28:23.691:     ICMP type=0, code=0  <--------------------------------------------------------------------- ICMP echo reply

*Mar  1 00:28:23.691: IP: s=10.0.0.1 (Serial0/0), d=10.0.0.1 (Serial0/0), g=10.0.0.1, len 100, forward

*Mar  1 00:28:23.695:     ICMP type=0, code=0

R2(config-if)#

R2#sh ip cache

IP routing cache 1 entry, 176 bytes

   12 adds, 11 invalidates, 0 refcounts

Minimum invalidation interval 2 seconds, maximum interval 5 seconds,

   quiet interval 3 seconds, threshold 0 requests

Invalidation rate 0 in last second, 0 in last 3 seconds

Last full cache invalidation occurred 00:02:23 ago

Prefix/Length           Age       Interface       Next Hop

10.0.0.1/32             00:00:03  Serial0/0       10.0.0.1

R2#

So next ping pkt will be fast-switched and thus no msg will be logged in the debug.....

However It's not clear to me if exist a specific reason for a different implementation between p2p and multiaccess/multipoint media.....

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