How router Serial port works

[mrityunjay singh]

Recently I face one issue when serial port/line protocol up and I was not able to ping my own my.

I regenerated the case as below.

R1--s1/1---------s1/1---R2

R1: s1/1: 10.10.10.1/30
R2: s1/1: 10.10.10.2/30

R1#sh ip int brief | s 1/1
Serial1/1              10.10.10.1      YES manual up                    up

R2#sh ip int br | s 1/1
Serial1/1              10.10.10.2      YES manual up                    up
R2#

Wireshark has been captured on S1/1 R2.

Scenario1:
Ping 10.10.10.1 from R1

R1#ping 10.10.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/52/100 ms

R1#ping 10.10.10.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/25/68 ms
R1#

Scenario 2:

Now I changed R2 s1/1 ip address to 1.1.1.1/30 whereas R1 s1/1 IP is same.

R2#sh ip int bri | s 1/1
Serial1/1              1.1.1.1         YES manual up                    up

Now I pinged 1.1.1.1 from R1 having source 10.10.10.1 , which won't ping( it is obvious)

R1#ping 1.1.1.1 source 10.10.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
Packet sent with a source address of 10.10.10.1
.....
Success rate is 0 percent (0/5)

Then I tried to ping my own IP 10.10.10.1 , ping failed !!,why I am not able to ping my own IP, whereas in ethernet it works fine.
Assuming serial don't used L2 address and its fail during L2 lookup, then how router serial port communicate.

I know Ethernet communication mechanism but bit confused on serial link.

Awaiting for enlightenment on this query.

[Julio E. Moisa]

Hi

Is it a packet tracer or real devices? R2 knows about this IP  10.10.10.1 on its routing table?

[Jon Marshall]

When you ping the IP on the local serial interface the ping actually goes across the link to the other router and back again which is probably why it is failing. 

See this thread for more details - 

https://supportforums.cisco.com/discussion/10962481/serial-interface-no-ping

Jon

[mrityunjay singh]

Hi Jon,

Thanks for your valuable input.

From your input I understand, in serial if we ping IP then its first redirect towards peer end and then peer redirect it towards the originator.

Here when I ping R1 ip then req goes to R2 and R2 check if the intended req ip belong to its own subnet if it then it will send its to R1.

Now lets say If i configured R1: 10.10.10.1/24 R2 10.10.10.2/24( though in real scenario we use /30 on serial) and try to ping 10.10.10.4 from R1 which do not exist.

What will happen? It should create loop.

R1#sh ip cef | s 10.10.10.
10.10.10.0/24        attached             Serial1/1
10.10.10.0/32        receive              Serial1/1
10.10.10.1/32        receive              Serial1/1
10.10.10.255/32      receive              Serial1/1

R2#sh ip cef | s 10.10.10.
10.10.10.0/24        attached             Serial1/1
10.10.10.0/32        receive              Serial1/1
10.10.10.2/32        receive              Serial1/1
10.10.10.255/32      receive              Serial1/1

R1#traceroute 10.10.10.4
Type escape sequence to abort.
Tracing the route to 10.10.10.4
VRF info: (vrf in name/id, vrf out name/id)
  1 10.10.10.2 80 msec 12 msec 12 msec
  2 10.10.10.1 20 msec 16 msec 24 msec
  3 10.10.10.2 20 msec 16 msec 20 msec
  4 10.10.10.1 32 msec 32 msec 32 msec
  5 10.10.10.2 32 msec 36 msec 36 msec
  6 10.10.10.1 48 msec 48 msec 36 msec
  7 10.10.10.2 44 msec 80 msec 80 msec
  8 10.10.10.1 68 msec 60 msec 52 msec
  9 10.10.10.2 56 msec 72 msec 60 msec
 10 10.10.10.1 80 msec 72 msec 68 msec
 11 10.10.10.2 60 msec 68 msec 60 msec
 12 10.10.10.1 72 msec 72 msec 68 msec
 13 10.10.10.2 72 msec 68 msec 72 msec
 14 10.10.10.1 76 msec 80 msec 80 msec
 15 10.10.10.2 80 msec 80 msec 80 msec
 16 10.10.10.1 88 msec 92 msec 88 msec
 17 10.10.10.2 92 msec 88 msec 92 msec
 18 10.10.10.1 100 msec 144 msec 128 msec
 19 10.10.10.2 160 msec 140 msec 128 msec
 20 10.10.10.1 116 msec 136 msec 160 msec
 21 10.10.10.2 144 msec 120 msec 160 msec
 22 10.10.10.1 240 msec 240 msec 188 msec
 23 10.10.10.2 140 msec 128 msec 120 msec
 24 10.10.10.1 132 msec 140 msec 140 msec
 25 10.10.10.2 144 msec 124 msec 132 msec
 26 10.10.10.1 184 msec 172 msec 148 msec
 27 10.10.10.2 152 msec 180 msec 160 msec
 28 10.10.10.1 172 msec 148 msec 152 msec
 29 10.10.10.2 152 msec 148 msec 152 msec
 30 10.10.10.1 160 msec 168 msec 160 msec
R1#

My last query, what serial link use for resolution at layer2 , if no layer2 info used then packet should have drop.

Thanks,

[Richard Burts]

I am not clear whether you are asking a question about the traceroute output. You say it should create a loop, and the output does demonstrate looping behavior. Basically this is what is happening"

first traceroute probe has TTL = 1. It goes 1 to 2 where it times out so 2 generates the response that shows up in the output.

next traceroute probe has TTL = 2. It goes 1 to 2 and then 2 to 1 where it times out so 1 generates the response that shows up in the output.

next traceroute probe has TTL = 3. It goes 1 to 2 and then 2 to 1 then 1 to 2 where it times out so 2 generates the response that shows up in the output.

next traceroute probe has TTL = 4. It goes 1 to 2 and then 2 to 1 then 1 to 2 then 2 to 1 where it times out so 1 generates the response that shows up in the output.

next traceroute probe has TTL = 5. It goes 1 to 2 and then 2 to 1 then 1 to 2 then 2 to 1 then 1 to 2 where it times out so 2 generates the response that shows up in the output.

the pattern continues like that.

The answer to your question about layer 2 resolution is that some serial protocols do have layer 2 resolution, such as Frame Relay that uses the DLCI for layer 2 resolution or SDLC which had a station polling address for layer 2 resolution. But the serial protocols that we use most often, PPP and HDLC do not have layer 2 resolution. With PPP or HDLC when the router prepares to send a frame out the interface it just sends it without trying to specify a layer 2 address. If you think about it this makes good sense. Layer 2 resolution is important if the connection is multi-point and you may have more than one peer on the serial connection.  So Frame Relay and SDLC which are multi-point do have layer 2 addresses to specify which peer you are sending to. But PPP and HDLC operate as point to point. With these protocols there is no ambiguity about which peer they are sending to and so there is no layer 2 resolution for these protocols.

HTH

Rick

[Ankt Tiwari]

thank you sir your explanation help me :)

[Richard Burts]

I am glad that my explanation was helpful to you. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick