Layer 2 Traceroute
Posted by Aragoen Celtdra on 1st May 2009
I found a cool simple command to trace the Layer 2 hop of a packet when going from one source device to another.
According to Cisco docs:
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The Layer 2 traceroute feature allows the switch to identify the physical path that a packet takes from a source device to a destination device.
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Layer 2 traceroute supports only unicast source and destination MAC addresses.
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It finds the path by using the MAC address tables of the switches in the path. When the switch detects a device in the path that does not support Layer 2 traceroute, the switch continues to send Layer 2 trace queries and lets them time out.
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The switch can only identify the path from the source device to the destination device.
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It cannot identify the path that a packet takes from source host to the source device or from the destination device to the destination host.
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CDP must be enabled on all devices.
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The maximum number of hops is 10.
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Must be on the same subnet.
ont-sw01#traceroute mac ip 10.100.194.116 10.100.194.4
Translating IP to mac …..
10.100.194.116 => 1234.1fe6.1116
10.100.194.4 => 5678.4640.1114
Source 000f.1fe6.d8e9 found on ont-sw01
1 ont-sw01 (10.100.194.10) : Fa0/7 => Fa0/12
2 switch01 (10.100.194.41) : Fa0/15 => Fa0/6
Destination 5678.4640.1114 found on switch01
Layer 2 trace completed
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ont-sw01#traceroute mac ip 10.100.194.116 10.100.194.1
Translating IP to mac …..
10.100.194.116 => 1234.1fe6.1116
10.100.194.1 => 4321.43da.1111
Source 000f.1fe6.d8e9 found on ont-sw01
1 ont-sw01 (10.100.194.10) : Fa0/7 => Fa0/2
Destination 4321.43da.1111 found on ont-sw01
Layer2 trace completed.
ont-sw01#
Reference:
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