Pinging your local interface

EdTheLad

I configure an ip address on a local serial interface which is physically up with protocol up.This serial is connected back to back with another router in the same state. When the ip addresses are in the same subnet all is good as expected.Now when i change one of the ip addresses to a different subnet i am unable to ping my local interface.
This seems a little stange for me as now im wondering what is exactly happening when you ping your local interface.Is the router sending your ping to the remote router which sends it back?
Internally how does the router behave in this situation?
For me the router should check the arp cache, see the mac is a local interface and process the packet, why does the remote device effect this?

QUIX0TIC

Im assuming that you have Router A and the S0 has an 30.1 adx and Router B S1 has a 30.2 adx. When you change Router A S0 to 40.1 adx... you cannot ping the 40.1 or you cannot local ping 127.0.0.2 the 30.2? And of course, I will assume you are on the router that you are trying to ping?

Have you checked your ARP cache or even gone further and checked your routing table? Is the address missing? Is this in a simulator or actual cisco routers?

I just tried on the simulator (Boson) and I can do a local ping on both of the routers and I just recently changed a connection on my home lab and I am able to perform a local ping of the router.

So I can only ask.. can you explain in more detail in what steps you are trying to perform?

EdTheLad

QUIX0TIC wrote:

Im assuming that you have Router A and the S0 has an 30.1 adx and Router B S1 has a 30.2 adx. When you change Router A S0 to 40.1 adx... you cannot ping the 40.1 or you cannot local ping 127.0.0.2 the 30.2? And of course, I will assume you are on the router that you are trying to ping?

Correct, i'm using a lab.Ports are hdlc so im not sure exactly how the arp works for hdlc.Since ive no eth configured when i do sh arp the tables empty.Routing table is good.Loopback address 127.0.0.1 doesnt ping but i didnt expect it would as how would the router know which interface you are referring to.As u siad above i have 2 2500 connected routerA on a .30.1 and RouterB on .40.1, logged in on RouterA console i cannot ping 30.1.When i change RouterB from .40.1 to .30.2 log back into RouterA i can ping .30.1, im pretty sure if these were eth interfaces i wouldnt have a problem.I have no transcievers here to check.I'm just curious why its not working on the serials.But you said it works for you?
I'm running a 12.0 release as these routers are my companies scraps.Maybe its a bug? I will check on my 2600s later.

QUIX0TIC

I once had an issue of changing the "other" interface to a diff subnet. (Like what you are explaining here.) From what I found out... it had something to do with the interface going down down which brought the "RTR A" side interface unpingable even though I was in RTR A. When I did the debug commands... it just showed the interface was not there.

When I get home... Ill check some of my older routers that I dont have hooked up. I think they have the 11.x or 12.0 or 12.1 IOS on them. I have the pleasure of working for a NOC at midnight to afternoon (dripping sarcasm) so I will not be able to answer until tomorrow.

QUIX0TIC

BTW... if you unplug the cable... can you ping the interface?

EdTheLad

QUIX0TIC wrote:

BTW... if you unplug the cable... can you ping the interface?

Nope, cant ping, which is normal as the physical int is down.Also changed IP disconnected and reconnected cable to see if resting the int would do anything but still same result.

johnnynodough

what about an extended ping?

EdTheLad

johnnynodough wrote:

what about an extended ping?

Extended ping doesnt work.I've just tried this again between 2 2600's with 12.3 and the same result.Can someone else try this with a lab and tell me the results?Connect 2 routers back to back using serial ports and hdlc, have the ip addresses setup incorrectly i.e. in different subnets and try ping your local interface.

QUIX0TIC

When you change your interface to a different subnet. The interfaces are still showing up up right?

EdTheLad

Yeah

Mr.Noe

Hey Ed, I just tried what you did on Network Visualizer and I was able to ping my local interface s0/0 of router1 but not s0/1 of router2 and vice-versa when I change them to different subnet. That's just a simulator, are you using a real router?

Also it makes sense that when I enable a routing protocol on both router, and specify the network I was able to ping the remote interfaces.

QUIX0TIC

I would understand not pinging the local int if you are showing "up down" or "down down" but up up... Im able to ping mine. I will have to check more when I get back home again. Maybe Im missing something that I just havent fully looked at.

EdTheLad

Hey Mr Noe, shouldnt that be Dr Noe
Im using real routers,but thanx anyway.Looks like im gonna have to figure this one out myself.At least from the responses ive received i know there seems to be a problem rather than a Cisco feature.
Thanks Q???????? , im too lazy to go back and check your name again

QUIX0TIC

Hahhaha... Im looking into it too. I just cant seem to replicate the symptoms you are seeing. I just got back home and I will be off for a couple of days. I can tell you what I find later on.

2lazybutsmart

We could get a better glimpse of the config in your environment if you could post the sh ip route, sh ip int brief, and the sh run results here.

I'll bet you have some shady config hiding somewhere

2lbs.

Mr.Noe

Can you do a debug ip packet detail then ping your local interface and post it.

I know that you can't ping your local interface when you configured Frame relay using static mapping and disabling inverseARP, because sometime there's no need to map your local interface in frame relay. Thats the only thing I can think of why you couldn't ping a local interface.

EdTheLad

As requested here are the outputs of one of my 2500's


RouterA#show ip int brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 unassigned YES NVRAM administratively down down
Serial0 11.0.0.2 YES manual up up
Serial1 unassigned YES manual administratively down down
RouterA#ping 11.0.0.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.0.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 60/60/60 ms
RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#int s0
RouterA(config-if)#shut
RouterA(config-if)#shutdown
RouterA(config-if)#ip add
RouterA(config-if)#ip address
1w2d: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down
RouterA(config-if)#ip address 12.0.0.1 255.255.255.252
RouterA(config-if)#no shut
RouterA(config-if)#no shutdown
RouterA(config-if)#
1w2d: %LINK-3-UPDOWN: Interface Serial0, changed state to up
RouterA(config-if)#
1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to up
RouterA(config-if)#end
RouterA#ping 12.0.0.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.0.0.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
RouterA#show ip int bri
RouterA#show ip int brief
Interface IP-Address OK? Method Status Protocol
Ethernet0 unassigned YES NVRAM administratively down down
Serial0 12.0.0.1 YES manual up up
Serial1 unassigned YES manual administratively down down
RouterA#sh
RouterA#show ip rou
RouterA#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is not set

12.0.0.0/30 is subnetted, 1 subnets
C 12.0.0.0 is directly connected, Serial0
RouterA#sh
RouterA#show runn
RouterA#show running-config
Building configuration...

Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname RouterA
!
enable secret 5 $1$H3.R$aA6xw7FdbcPSr7HR3Kfel/
!
!
!
!
!
ip subnet-zero
no ip domain-lookup
ip host RouterB 11.0.0.1
!
!
!
!
interface Ethernet0
no ip address
no ip directed-broadcast
shutdown
!
interface Serial0
ip address 12.0.0.1 255.255.255.252
no ip directed-broadcast
no ip mroute-cache
clockrate 64000
!
interface Serial1
no ip address
no ip directed-broadcast
shutdown
!
ip classless
no ip http server
!
!
line con 0
exec-timeout 0 0
logging synchronous
transport input none
line aux 0
line vty 0 4
access-class 10 in
exec-timeout 0 0
logging synchronous
no login
!
end

RouterA#debug ip packet detail
IP packet debugging is on (detailed)
RouterA#ping 12.0.0.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 12.0.0.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
RouterA#
1w2d: IP: s=12.0.0.1 (local), d=12.0.0.1 (Serial0), len 100, sending
1w2d: ICMP type=8, code=0
1w2d: IP: s=12.0.0.1 (local), d=12.0.0.1 (Serial0), len 100, sending
1w2d: ICMP type=8, code=0
1w2d: IP: s=12.0.0.1 (local), d=12.0.0.1 (Serial0), len 100, sending
1w2d: ICMP type=8, code=0
1w2d: IP: s=12.0.0.1 (local), d=12.0.0.1 (Serial0), len 100, sending
1w2d: ICMP type=8, code=0
1w2d: IP: s=12.0.0.1 (local), d=12.0.0.1 (Serial0), len 100, sending
1w2d: ICMP type=8, code=0
RouterA#undebug all
All possible debugging has been turned off
RouterA#

RouterA#sh ip int s0
Serial0 is up, line protocol is up
Internet address is 12.0.0.1/30
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP fast switching on the same interface is enabled
IP Flow switching is disabled
IP Fast switching turbo vector
IP multicast fast switching is disabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Probe proxy name replies are disabled
Policy routing is disabled
Network address translation is disabled
WCCP Redirect outbound is disabled
WCCP Redirect exclude is disabled
BGP Policy Mapping is disabled


RouterA#sh int s0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 12.0.0.1/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
Last input 00:00:05, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0 (size/max/drops); Total output drops: 0
Queueing strategy: weighted fair
Output queue: 0/1000/64/0 (size/max total/threshold/drops)
Conversations 0/2/256 (active/max active/max total)
Reserved Conversations 0/0 (allocated/max allocated)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
5135 packets input, 314115 bytes, 0 no buffer
Received 2587 broadcasts, 0 runts, 0 giants, 0 throttles
3 input errors, 3 CRC, 0 frame, 0 overrun, 0 ignored, 1 abort
5405 packets output, 315552 bytes, 0 underruns
0 output errors, 0 collisions, 7786 interface resets
0 output buffer failures, 0 output buffers swapped out
37 carrier transitions
DCD=up DSR=up DTR=up RTS=up CTS=up
RouterA#

rossonieri#1

hi,

can you draw a picture of your network here?
i think there should be no problem changing one intf. ip and change it back again.
you cant ping a remote machine only if you dont have a route to destination - and vice versa from destination going back again. the router only check if the packet has a destination in its routing table to forward - and if not - check default route - and if not, drop it.

EdTheLad

Hi Rossonieri,
The picture is 2 2500's connected back to back via serial ports.I'm not trying to ping a remote machine, im trying to ping my local interface.As you can see from the outputs above i can ping the remote machine.Then i change the local IP address on my serial port and for some reason i cannot ping this local interface anymore even though its UP UP.I do understand how routing works and this is not a routing issue.

rossonieri#1

ed_the_lad wrote:

Hi Rossonieri,
The picture is 2 2500's connected back to back via serial ports.I'm not trying to ping a remote machine, im trying to ping my local interface.As you can see from the outputs above i can ping the remote machine.Then i change the local IP address on my serial port and for some reason i cannot ping this local interface anymore even though its UP UP.I do understand how routing works and this is not a routing issue.

hello ed,
a picture i mean simply rtrA:ip's

---

rtrB:ip's. ; )
if you are really sure it is not a routing issue - what then?
loopback?

EdTheLad

You want a picure ill give u a picture

RouterA

---

RouterB
11.0.0.2/30 11.0.0.1/30

RouterA#ping 11.0.0.2 100% perfect
RouterA#ping 11.0.0.1 100% perfect

Now i change the IP address of RouterA

RouterA

---

RouterB
12.0.0.1/30 11.0.0.1/30

RouterA#ping 12.0.0.1 0% Why?????
RouterA#ping 11.0.0.1 0% Expected

So as you can see my question is why cant i ping 12.0.0.1 since its configured on a local interface with Line Up, Protocal Up.

Worktruck

Actually it's real simple both interfaces have to be on the same network or subnetwork in order to see each other and communicate. That is the reason why you can see it in your first example and not in the second.

Deadmaster200

Worktruck wrote:

Actually it's real simple both interfaces have to be on the same network or subnetwork in order to see each other and communicate. That is the reason why you can see it in your first example and not in the second.

Apparently, alot of people don't seem to be able to read the English language too well. Here let's take a look at the quite by the original poster that sums up the whole problem:

"RouterA#ping 12.0.0.1 0% Why?????
RouterA#ping 11.0.0.1 0% Expected "

Obviously, the original poster knows why he can't ping RouterB. He IS NOT asking about RouterB.

2lazybutsmart

ed_the_lad: here's my 2 lb's worth of insight in this little tricky thing.

I've checked, double checked, and triple checked that scenario on my lab and the only conclusions I can arrive at are:

A- Serial interfaces need to have the following properties before they can be pinged or be available for communication from other serial interfaces:

1- Layer 2 communication must be up and running (physical link, clock rates, dce-dte, etc.). Once everything's up up,
2- If you're running IP, then both serial interfaces must have valid logical addressing at Layer 3. By this, I mean addresses have to be on the same network. This is probably due, IMO, to the fact that serial interfaces don't use ARP or MAC addresses or anything of that sort to physically track interfaces on the same network as there is only one physical network segment possible between a point-to-point serial link. I've also tried getting the serial interfaces to use the ip's of the other ethernet interfaces that use them for communication. What can be or can not be done with serial interfaces is pretty interesting.

B- Ethernet interfaces must be active at Layer 2, but don't need to have valid logical addressing at Layer 3 for the interface to respond to local pings. I think this is due, again IMO, to the fact that Ethernet interfaces use ARP to resolve the MAC address and the logical address is just used when communication is being made to other interfaces.



HTH
2lbs

darkmagic

ed_the_lad wrote:

You want a picure ill give u a picture

RouterA

---

RouterB
11.0.0.2/30 11.0.0.1/30

RouterA#ping 11.0.0.2 100% perfect
RouterA#ping 11.0.0.1 100% perfect

Now i change the IP address of RouterA

RouterA

---

RouterB
12.0.0.1/30 11.0.0.1/30

RouterA#ping 12.0.0.1 0% Why?????
RouterA#ping 11.0.0.1 0% Expected

So as you can see my question is why cant i ping 12.0.0.1 since its configured on a local interface with Line Up, Protocal Up.

Now i change the IP address of RouterA

RouterA

---

RouterB
12.0.0.1/30 11.0.0.1/30

The IP address has been changed on Router A, but the IP Address on Router B is still the same, so obviously all of us know that he would not be able to ping RouterB from RouterA, which is what "Ed" said by

RouterA#ping 11.0.0.1 0% Expected

The question is why can't he ping the local interface of Router A from RouterA

RouterA#ping 12.0.0.1 0% Why?????

That's what this means

Hope that's clear enough, for people having trouble understanding what it is all about

rossonieri#1

ed_the_lad wrote:

I configure an ip address on a local serial interface which is physically up with protocol up.This serial is connected back to back with another router in the same state. When the ip addresses are in the same subnet all is good as expected.Now when i change one of the ip addresses to a different subnet i am unable to ping my local interface.
This seems a little stange for me as now im wondering what is exactly happening when you ping your local interface.Is the router sending your ping to the remote router which sends it back?
Internally how does the router behave in this situation?
For me the router should check the arp cache, see the mac is a local interface and process the packet, why does the remote device effect this?

my answers based on this Q.
A serial doesnt have a MAC so the router will not check the ARP table - as a WAN Serial is considered a Non-Local interface (even its local).
changing IP doesnt have have relation to DTE/DCE - so you will have your port UP-UP as long clockrate's set.
i've check on my router : your config doing fine - changing any ips - pinging everywhere - no problemo.
maybe that back2back cable is the problem...??
or - we agree that our discussion should stop here..

2lazybutsmart

2lazybutsmart wrote:

ed_the_lad: here's my 2 lb's worth of insight in this little tricky thing.

I've checked, double checked, and triple checked that scenario on my lab and the only conclusions I can arrive at are:

A- Serial interfaces need to have the following properties before they can be pinged or be available for communication from other serial interfaces:

1- Layer 2 communication must be up and running (physical link, clock rates, dce-dte, etc.). Once everything's up up,
2- If you're running IP, then both serial interfaces must have valid logical addressing at Layer 3. By this, I mean addresses have to be on the same network. This is probably due, IMO, to the fact that serial interfaces don't use ARP or MAC addresses or anything of that sort to physically track interfaces on the same network as there is only one physical network segment possible between a point-to-point serial link. I've also tried getting the serial interfaces to use the ip's of the other ethernet interfaces that use them for communication. What can be or can not be done with serial interfaces is pretty interesting.

B- Ethernet interfaces must be active at Layer 2, but don't need to have valid logical addressing at Layer 3 for the interface to respond to local pings. I think this is due, again IMO, to the fact that Ethernet interfaces use ARP to resolve the MAC address and the logical address is just used when communication is being made to other interfaces.



HTH
2lbs

BTW, I might have been a bit equivocal there in my post, but what I'm reffering to is both the pings you do on a router to a local interface and a ping you do to remote ones as well.

I know a few people got ed_the_lad's post confused at first, but It's pretty clear now. Even though I'm starting to wonder why in the world he'd want to ping a local interface that can't communicate to the outside world ..... Nevertheless, it certainly is one of those things that I always like to clear up. It's like why is the sky blue and not black?!

FYI, I know why the sky isn't black but blue

To sum things up: the nature of serial links being a point-to-point connection probably gives them these sensitive layer 3 attributes. There's only one physical network segment there anywayz.

Do you remember that DRs and BDRs aren't elected on point-to-point networks, too? Looks like there're a lot of fishy things on point-to-points.

2lbs.

2lazybutsmart

rossonieri#1 wrote:

i've check on my router : your config doing fine - changing any ips - pinging everywhere - no problemo.
maybe that back2back cable is the problem...??
or - we agree that our discussion should stop here..

It doesn't work for me..... my two 2520s are connected with normal DB60 - V.35s

EdTheLad

I think it must be down to these db60 v.35 cables i bought on ebay.
Prob 2lbs has bought the same type.
Very annoying if you ask me!
Thanks alot guys...

darkmagic

2lazybutsmart wrote:

2- If you're running IP, then both serial interfaces must have valid logical addressing at Layer 3. By this, I mean addresses have to be on the same network. This is probably due, IMO, to the fact that serial interfaces don't use ARP or MAC addresses or anything of that sort to physically track interfaces on the same network as there is only one physical network segment possible between a point-to-point serial link. I've also tried getting the serial interfaces to use the ip's of the other ethernet interfaces that use them for communication. What can be or can not be done with serial interfaces is pretty interesting.

What does "IMO" means here ?

2lazybutsmart wrote:

To sum things up: the nature of serial links being a point-to-point connection probably gives them these sensitive layer 3 attributes. There's only one physical network segment there anywayz.

Can you elaborate that a little bit, i am sorry to ask for it, but it isn't as clear to me, thanks...

mayh3m

Darkmagic:
IMO = In My Opinion

(Correct me if I am wrong) The way I understand it is that if you do not have correct IP addresses, the serial (point to point) link will not be ping-able due to the fact that it does not check an ARP cache, as there is no need for a cache, 'cause there are only ever 2 addresses on the one network segment.

ed_the_lad:
I'd say there is nothing wrong with your hardware personally.
I tried the problem in two 2610 routers running 12.3 and as soon as I changed the local interfaces IP addess, I could no longer ping it. Same goes if I change the remote interface addess, and try pinging the local interface.

And just an interesting side note, kind of different but....
The same thing happens if you kill the keepalive (no keepalive), it's still up up, but cannot telnet or ping.