Torching a specific port on a MikroTik switch or router running RouterOS
Posted by jpluimers on 2023/04/11
On most recent [Wayback/Archive] RouterOS configurations of MikroTik Routers and Switches, running [Wayback/Archive] Torch a port will show zero traffic when they are part of a bridge configuration. The same holds for the Packet Sniffer.
The reason is that these bridges have hardware acceleration turned on, which makes all traffic go through the switch chip instead of the device CPU. Torch works on the CPU level, so won’t show hardly any traffic except for some configuration stuff (depending on the combination of switch chip and CPU type).
This is not documented in the Torch documentation, but it is documented in the Packet Sniffer documentation.
Further reading:
- [Wayback/Archive] Manual:Troubleshooting tools – Torch – MikroTik Wiki
- [Wayback/Archive] Manual:Tools/Packet Sniffer – MikroTik Wiki
Note: Unicast traffic between Wireless clients with client-to-client forwarding enabled will not be visible to sniffer tool. Packets that are processed with hardware offloading enabled bridge will also not be visible (unknown unicast, broadcast and some multicast traffic will be visible to sniffer tool).
- [Wayback/Archive] mikrotik nonhardware bridge – Google Search (yes that was a typo, but Google still got good results)
- [Wayback/Archive] Can not see trafic in TORCH – MikroTik
As the ethernet ports are marked as S(laves) in the tables, I would assume that they are member ports of bridges and “hardware acceleration” is enabled (the value of hw in the respective rows of /interface bridge port is set to yes). So any frames which pass through these ports to other ports of the same switch chip are counted by the switch chip counters, but as they never get to the CPU, the torch cannot see them.
- [Wayback/Archive] mikrotik torch ip ports of bridge – Google Search
- [Wayback/Archive] Manual:Layer2 misconfiguration; Packet flow with hardware offloading and MAC learning – MikroTik Wiki
Consider the following scenario, you setup a bridge and have enabled hardware offloading in order to maximize the throughput for your device, as a result your device is working as a switch, but you want to use Sniffer or Torch tools for debugging purposes, or maybe you want to implement packet logging.
- [Wayback/Archive] Manual:Layer2 misconfiguration; Packet flow with hardware offloading and MAC learning; Configuration – MikroTik Wiki
/interface bridge add name=bridge1 /interface bridge port add bridge=bridge1 hw=yes interface=ether1 learn=yes add bridge=bridge1 hw=yes interface=ether2 learn=yes - [Wayback/Archive] Manual:Layer2 misconfiguration; Packet flow with hardware offloading and MAC learning; Problem – MikroTik Wiki
When running Sniffer or Torch tool to capture packets you might notice that barely any packets are visible, only some unicast packets, but mostly broadcast/multicast packets are captured, while the interfaces report that much larger traffic is flowing through certain interfaces than the traffic that was captured. Since RouterOS v6.41 if you add two or more Ethernet interfaces to a bridge and enable Hardware Offloading, then the switch chip will be used to forward packets between ports. To understand why only some packets are captured, we must first examine how the switch chip is interconnected with the CPU, in this example we can use a block diagram from a generic 5-Port Ethernet router:
Generic Ethernet router
For this device each Ethernet port is connected to the switch chip and the switch chip is connected to the CPU using the CPU port (sometimes called the switch-cpu port). For packets to be visible in Sniffer tools, the packet must be sent from an Ethernet port to the CPU port, this means that the packet must be destined to the CPU port (destination MAC address of the packet matches the bridge’s MAC address) or the packet’s MAC address has not be learnt (packet is flooded to all ports), this behavior is because of MAC learning·The switch chip keeps a list of MAC addresses and ports called the Hosts table· Whenever a packet needs to be forwarded, the switch chip checks the packet’s destination MAC address against the hosts table to find which port should it use to forward the packet. If the switch chip cannot find the destination MAC address, then the packet is flooded to all ports (including the CPU port). In situations where packet is supposed to be forwarded from, for example, ether1 to ether2 and the MAC address for the device behind ether2 is in the hosts table, then the packet is never sent to the CPU and therefore will not be visible to Sniffer or Torch tool.. - [Wayback/Archive] Manual:Layer2 misconfiguration; Packet flow with hardware offloading and MAC learning – MikroTik Wiki
Packets with a destination MAC address that has been learned will not be sent to the CPU since the packets are not not being flooded to all ports. If you do need to send certain packets to the CPU for packet analyser or for Firewall, then it is possible to copy or redirect the packet to the CPU by using ACL rules. Below is an example how to send a copy of packets that are meant for
4C:5E:0C:4D:12:4B:/interface ethernet switch ruleadd copy-to-cpu=yes dst-mac-address=4C:5E:0C:4D:12:4B/FF:FF:FF:FF:FF:FF ports=ether1 switch=switch1Note: If the packet is sent to the CPU, then the packet must be processed by the CPU, this increases the CPU load.
- [Wayback/Archive] mikrotik torch mac address – Google Search
- [Wayback/Archive] How can I see the mac address connected directly to switch port – MikroTik
The following script is used to associate IP address of directly connected stations to physical port of the switch.
Warning: Running this script will make the CPU go to 100% for about 30-40 seconds, so please run the script when needed or by using scheduler.
- [Wayback/Archive] How can I see the mac address connected directly to switch port – MikroTik
- [Wayback/Archive] Manual:Layer2 misconfiguration; Packet flow with hardware offloading and MAC learning – MikroTik Wiki
–jeroen
The Wiert Corner - irregular stream of stuff 
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