why use traceroute to test load-balancing?

[Oak Ridge]

the default CEF is destination-based load balancing, I can also verify with "show ip cef " command. so any ICMP traffic with same destination will be going through the same path.

I found traceroute DID go through different path in multiple equal pathes. Why?

I use PING  but not able to go through different path.

Why?

[cadet alain]

Hi,

default CEF load-sharing( not load-balancing) is per src-dst pair not destination only. I also think that some platforms even take the ports into account. Can you explain what you did exactly and on which topology.

Regards.

Alain

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[Oak Ridge]

the topology is pretty simple.

R1 ============== R2

two equal cost path between R1 and R2. Loopback 0 is configured in R1 and R2 respectively.

we use extended traceroute source , and it shows going through different path.

so if IP CEF default load-sharing is source-destination, how come the same ICMP flow (with same source and destination IP address) goes through different path?

[Giuseppe Larosa]

Hello Oak,

traceroute makes three attempts for each given TTL, so it can discover up to 3 paths in parallel between two router hops with default settings.

This number may be equal or less of the effective number of links between the two routers.

To answer your question: traceroute traffic is ICMP only on the return path and what is really important is that this traffic is process switched  and not processed by CEF-

The traceroute probe is an UDP packet with a very high destination port (> 30000) to trigger an ICMP unreachable on receiving device.

For being process switched traceroute probes have the potential to discover multiple equal cost paths.

Hope to help

Giuseppe

[cadet alain · Accepted solution]

Hi ,

traffic originated by router or destined to router is not CEF switched but process switched and process switching does per-packet load-sharing.

Regards.

Alain

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