Ping the Fast Ethernet or Gigabit Ethernet Interface
Purpose
Use the ping command to verify the loopback connection.
Action
To send ping packets from the Ethernet interface, use the following Junos OS CLI operational mode command:
Sample Output
user@router> ping 10.108.120.2 bypass-routing
interface ge-7/2/1 count 100 rapid PING 10.108.120.2 (10.108.120.2): 56 data bytes 36 bytes from 10.108.120.1: Time to live exceeded Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 0054 e871 0 0000 01 01 cc5c 10.108.120.1 10.108.120.2 .36 bytes from 10.108.120.1: Time to live exceeded Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 0054 e874 0 0000 01 01 cc59 10.108.120.1 10.108.120.2 .36 bytes from 10.108.120.1: Time to live exceeded Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 0054 e878 0 0000 01 01 cc55 10.108.120.1 10.108.120.2 .36 bytes from 10.108.120.1: Time to live exceeded Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 0054 e87c 0 0000 01 01 cc51 10.108.120.1 10.108.120.2 .36 bytes from 10.108.120.1: Time to live exceeded Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 0054 e880 0 0000 01 01 cc4d 10.108.120.1 10.108.120.2 .36 bytes from 10.108.120.1: Time to live exceeded Vr HL TOS Len ID Flg off TTL Pro cks Src Dst 4 5 00 0054 e884 0 0000 01 01 cc49 10.108.120.1 10.108.120.2 .36 bytes from 10.108.120.1: Time to live exceeded
Meaning
The sample output shows that the time to live (TTL) expired, indicating that the link is receiving the frames from the ping test. The MAC address used is the same as the physical address of the port being tested because this allows the port to accept the frames from the ping test. As the packet is looped over the link, you expect to receive a TLL exceeded message for each ping sent. These messages are generated because the ping packets are repeatedly looped between the router and the physical loopback. When the packet is sent to the other end of the link, which does not exist, the loopback returns the packet back to the same interface, where it is again subjected to the Packet Forwarding Engine fabric for routing. After the route lookup, the TTL is decremented, and the packet is again sent out of the looped interface. This process repeats until the packed is either lost, or the TLL expires with subsequent TTL expired message displayed. Should any errors occur, the packet is discarded and a time-out error is displayed, rather than the expected TTL expired message. Note that the default TTL for ICMP echo packets in Junos OS is 64. This means a given test packet must be successfully sent and received 63 times before a TTL expired message can be generated. You can alter the TTL value to adjust the tolerance for loss, for example, a value of 255 is the most demanding test because now the packet must be sent and received error free 254 times.
