Which source address will take while pinging by default

Answered Question
Aug 22nd, 2009
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Hi All,


Sorry to put this basic quiestion.i am confused with this.


From PE Router if i am pinging any customer ip address what would be default source address its taking if source address/interface is not mentioned.


XX-XX-XX#p vrf TEST-VRF

Target IP address: 172.16.XX.XX

Repeat count [5]: 10

Datagram size [100]:

Timeout in seconds [2]:

Extended commands [n]: y

Source address or interface: ==================> ???????????????

Type of service [0]:

Set DF bit in IP header? [no]:

Validate reply data? [no]:

Data pattern [0xABCD]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Sweep range of sizes [n]:

Type escape sequence to abort.

Sending 10, 100-byte ICMP Echos to 172.16.XX.XX, timeout is 2 seconds:

!!!!!!!!!!

Success rate is 100 percent (10/10), round-trip min/avg/max = 4/6/8 ms



Thanks in advance


Lijsh.N.C

Correct Answer by Peter Paluch about 7 years 11 months ago

Hello Lijesh,


Each time your router originates a packet and sends it to a particular destination, it looks into the corresponding routing table to find out how to reach that destination. The result of the routing table lookup is always the outgoing interface and optionally information which next hop should receive the packet. The address of the outgoing interface will then be used as the source IP address of the packet.


So, to answer which outgoing interface will it be - it depends on the destination IP address and on the routing table that determines how to reach that destination. That in turn also determines the source address of the packet.


Best regards,

Peter


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paolo bevilacqua Sat, 08/22/2009 - 02:43
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It takes the address of the outgoing interface.

Lijesh NC Sat, 08/22/2009 - 04:03
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Hi Paolo,


Thanks for the reply.Would like to know which Outgoing interface will it take for eg: if ip address 192.168.10.10 belongs to serial0..by befault packet will be sourced from this interface ???




Lijesh.N.C

Correct Answer
Peter Paluch Sat, 08/22/2009 - 04:20
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  • Cisco Employee,

Hello Lijesh,


Each time your router originates a packet and sends it to a particular destination, it looks into the corresponding routing table to find out how to reach that destination. The result of the routing table lookup is always the outgoing interface and optionally information which next hop should receive the packet. The address of the outgoing interface will then be used as the source IP address of the packet.


So, to answer which outgoing interface will it be - it depends on the destination IP address and on the routing table that determines how to reach that destination. That in turn also determines the source address of the packet.


Best regards,

Peter


Lijesh NC Sat, 08/22/2009 - 04:24
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Hi Peter,


Thanks for the reply.Its clear now.


cheers


Lijesh.N.C

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