Dec
02

You might have a request in the Core Knowledge, Troubleshooting, or the Configuration section to have a Cisco router be able to PING its own Frame-Relay IP address. In this blog post, we will make sure we can accomplish this. Here is the topology that we will use:

ping thyself

The first order of business here is to ensure that R5 can PING the remote device of R3.

Rack9R5#ping 173.9.0.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 173.9.0.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/58/60 ms
Rack9R5#

Excellent! Now let us see if R5 can PING itself. I am hoping this fails or it is going to be my shortest blog post ever! :-p

Rack9R5#ping 173.9.0.5

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 173.9.0.5, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Rack9R5#

OK. As expected, it does not work. But why? Well, let us debug frame-relay packets and see if we can determine why.

Rack9R5#debug frame-relay packet
Frame Relay packet debugging is on
Rack9R5#ping 173.9.0.5          

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 173.9.0.5, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Rack9R5#
Dec  2 14:05:50.399: Serial0/0/0:Encaps failed--no map entry link 7(IP)
Dec  2 14:05:52.399: Serial0/0/0:Encaps failed--no map entry link 7(IP)
Dec  2 14:05:54.399: Serial0/0/0:Encaps failed--no map entry link 7(IP)
Dec  2 14:05:56.399: Serial0/0/0:Encaps failed--no map entry link 7(IP)
Dec  2 14:05:58.399: Serial0/0/0:Encaps failed--no map entry link 7(IP)
Rack9R5#

There is the answer. Frame-Relay does not know what DLCI to use for this destination IP address. We need to add a Frame-Relay mapping for our own IP address pointing to the DLCI of our neighbor.Let us do that now and then perform our verifications:

Rack9R5#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Rack9R5(config)#int s0/0/0
Rack9R5(config-if)#frame map ip 173.9.0.5 503
Rack9R5(config-if)#do sh run int s0/0/0
Building configuration...

Current configuration : 198 bytes
!
interface Serial0/0/0
ip address 173.9.0.5 255.255.255.0
encapsulation frame-relay
frame-relay map ip 173.9.0.5 503
frame-relay map ip 173.9.0.3 503 broadcast
no frame-relay inverse-arp
end

Rack9R5(config-if)#do ping 173.9.0.5

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 173.9.0.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 116/118/124 ms

Would I engage in this extra configuration unless I was asked to do so in the exam? No way...but at least now we fully understand this under-documented little nuance.

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