vlan + sniffing = problem

RouterOS General
1

This is ROS 4.17. I have several Vlans defined over an interface.

When i use packet sniffer over the base interface i have these problems:

  • It sniffs all packets coming from all VLANS (this is not really a problem, but there should be an option like in Torch).
  • All packets have “VLAN Id” field empty, despite they come from different vlans.
  • In the packet dump i can see they have vlan header stripped.

I think this is also causing dhcp-server alert giving false positives.

DHCP Server on VLAN Issue
2

i think that you are sniffing on VLAN interface - at that time, VLAN tag is already removed, you should try to sniff on etnernet where VLAN is assigned to see the tags.

3

Afaik It’s an ethernet interface.
There are VLAN interfaces on top of that one, but that’s another story.

4

I have also seen that kind of problem.

Built like this on a RB450G 4.5 (yeah, old):

Bridge (Ports: ether2 + ether3)
No VLAN Interfaces configured, so full VLAN trunk is passing between ether2 and ether3

Torching on the bridge shows no VLAN ID on tagged packets arriving from a Cisco Router on VLAN 20 at ether2 (ether3 is not connected).
A DHCP Alert on the bridge interface shows false-positives from a DHCP Server which is situated in VLAN 20, and sends tagged packets.
Creating a DHCP Client on bridge interface does not result in the Mikrotik receiving an IP address. Creating bridge.20 VLAN 20 subinterface
and running dhcp client there gives an IP address (as expected).

5

I can confirm packet sniffer on 4.17 and 5.7 does not capture properly the frames that come out of the MT router. VLAN info is missing where as they should be tagged.
Frames coming from outside clearly show that they are tagged.

I can also confirm that DHCP does not work properly with v5.7 on a VLAN interface.


I think MikroTik should invest in QA, testing, debugging or sell the business to some company that can release updates without so much bugs and incompatibilities.

6

huh, I was almost ready to upgrade… and what’s improperly in DHCP on VLAN?..

7

Says it’s Invalid.

Furthermore troubleshooting is a pain because the sniffer does not capture outbound frames with the tags.

8

wait till 5.8

on routerboard VLAN tag should be visible while sniffing.

9

and on non-routerboards?..

and what about x86 routerboards? :slight_smile:

10

ok i rephrase it:

for sniffer to show vlan tag on all RouterOS flavors wait till 5.8, 5.7 should display outgoing traffic VLAN tag.

11

Hey guys the DHCP server on VLAN that I am trying to bring up does not work when its set to Static-only. When set to some IP Pool - it works again.

But the situation requires static.

12

I am seeing something similar in RoS6.0 on a CCR1016

I have a VLAN 10 configured on ether12 and with a pppoe-client configured to use VLAN 10.

Torch shows the tagged outgoing pppoe-discovery(8863) packets (vlan10) and shows the replies have no vlan tag. (This is the actual issue I am trying to diagnose)
torch.jpg
When I do a packet sniffer on ether12 and look at the packets in RoS6 it shows vlan10 yet when I view the file in Wireshark the outgoing tags are missing in the trace.
wireshark.jpg