Why is it using the first IP in the interface for tracerouts?


Hello, I have an issue when performing traceroutes which can make it difficult to troubleshoot issues. We are using VyOS as one of our core routers and we typically assign a /31 on VyOS and on the customer CPE.

Here is a snippet of my config on VyOS:

ethernet eth4 {
duplex auto
hw-id XX:XX:ba:1d:XX:XX
smp_affinity auto
speed auto
vif 500 {
address 204.555.111.8/31
address 204.555.111.10/31
address 204.555.111.42/31
description “VLAN 500 FIBER NETWORK”

On Customer CPE:

interfaces {
ethernet eth0 {
address 204.555.111.43/31


system {

gateway-address 204.555.111.42


When doing a traceroute from the internet:

Tracing route to 204.555.111.43 over a maximum of 30 hops

1 <1 ms <1 ms <1 ms
2 <1 ms <1 ms <1 ms 204.555.111.8 <--------why is it using this ip??? should be 204.555.111.42 , right??
3 1 ms 1 ms <1 ms 204.555.111.43

Why does VyOS respond on the first IP of the interface instead of using the 204.555.111.42 IP since that is the IP that vyos is sending the packets from when sending to 204.555.111.43 ?


Traceroute normally gets answered from source IP=1st IP address configured on the interface where it sends out the response. You complicated matters by having multiple subnets on same VLAN
Reply2:Note this packet came in with a TTL , that was unable to reach 204.555.111.42
Reply3: This packet came in with higher TTL, now packet makes it all the way to 204.555.111.43


Does this mean what Caesar is trying can’t be done or is there a way to configure it better and make it to work?


There is no “better” or “make it work.”
It works as designed, same behavior is seen on other router brands.

closed #5

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