Introduction
There are certain configurations that are known to have major flaws by design and should be avoided by all means possible. Misconfigured Layer2 can sometimes cause hard to detect network errors, random performance drops, certain segments of a network to be unreachable, certain networking services to be malfunctioning or a complete network failure. This page will contain some common and not so very common configurations that will cause issues in your network.
Bridges on a single switch chip
Consider the following scenario, you have a device with a built-in switch chip and you need to isolate certain ports from each other, for this reason, you have created multiple bridges and enabled hardware offloading on them. Since each bridge is located on a different Layer2 domain, then Layer2 frames will not be forwarded between these bridges, as a result, ports in each bridge are isolated from other ports in a different bridge.
Configuration
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/interface bridge add name=bridge1 add name=bridge2 /interface bridge port add bridge=bridge1 interface=ether1 add bridge=bridge1 interface=ether2 add bridge=bridge2 interface=ether3 add bridge=bridge2 interface=ether4 |
ProblemAfter a simple performance test, you might notice that one bridge is capable of forwarding traffic at wire-speed while the second, third, etc. bridge is not able to forward as much data as the first bridge. Another symptom might be that there exists a huge latency for packets that need to be routed. After a quick inspection, you might notice that the CPU is always at full load, this is because hardware offloading is not available on all bridges, but is available only on one bridge. By checking the hardware offloading status you will notice that only one bridge has it active:
| Code Block | ||
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[admin@MikroTik] > /interface bridge port print Flags: X - disabled, I - inactive, D - dynamic, H - hw-offload # INTERFACE BRIDGE HW 0 H ether1 bridge1 yes 1 H ether2 bridge1 yes 2 ether3 bridge2 yes 3 ether4 bridge2 yes |
The reason why only one bridge has the hardware offloading flag available is because the device does not support port isolation. If port isolation is not supported, then only one bridge will be able to offload the traffic to the switch chip.
Symptoms
Below is a list of possible symptoms that might be as a result of this kind of a misconfiguration:
- Missing "H" flag to bridge ports
- Low throughput
- High CPU usage
Solution
Not all device devices support port isolation, currently only CRS1xx/CRS2xx series devices support it and only 7 isolated and hardware offloaded bridges are supported at the same time, other devices will have to use the CPU to forward the packets on other bridges. This is usually a hardware limitation and a different device might be required. Bridge split horizon parameter is a software feature that disables hardware offloading and when using bridge filter rules you need to enable forward all packets to the CPU, which requires the hardware offloading to be disabled. You can control which bridge will be hardware offloaded with the hw=yes flag and by setting hw=no to other bridges, for example:
| Code Block | ||
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/interface bridge port set [find where bridge=bridge1] hw=no /interface bridge port set [find where bridge=bridge2] hw=yes |
Sometimes it is possible to restructure a network topology to use VLANs, which is the proper way to isolate Layer2 networks.
Packet flow with hardware offloading and MAC learning
Consider the following scenario, you set up 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.
Configuration
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/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 |
Problem
When running Sniffer or Torch tools 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:
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 or Torch 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·
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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 a packet analyser analyzer or for Firewalla 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:
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When you add an interface to a bridge, the bridge becomes the master interface and all bridge ports become slave ports, this means that all traffic that is received on a bridge port is captured by the bridge interface and all traffic is forwarded to the CPU using the bridge interface instead of the physical interface. As a result VLAN interface that is created on a slave interface will never capture any traffic at all since it is immediately forwarded to the master interface before any packet processing is being done. Usual The usual side effect is that some DHCP clients receive IP addresses and some don't.
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Change the interface on which the VLAN interface will be listening for traffic, change it to the master interface:
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/interface vlan set VLAN99 interface= |
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bridge1 |
VLAN on a bridge in a bridge
Consider the following scenario, you have a set of interfaces (don't have to be physical interfaces) and you want all of them to be in the same Layer2 segment, the solution is to add them to a single bridge, but you require that traffic from one port tags all traffic into a certain VLAN. This can be done by creating a VLAN interface on top of the bridge interface and by creating a separate bridge that contains this newly created VLAN interface and an interface, which is supposed to add a VLAN tag to all received traffic. Network A network diagram can be found below:
VLAN on bridge in bridge topology
Configuration
| Code Block | ||
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/interface bridge
add name=bridge1
add name=bridge2
/interface vlan
add interface=bridge1 name=VLAN vlan-id=99
/interface bridge port
add bridge=bridge1 interface=ether1
add bridge=bridge1 interface=ether2
add bridge=bridge2 interface=VLAN
add bridge=bridge2 interface=ether3 |
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Problem
To better understand the underlying problems, lets first look on at bridge host table.
| Code Block |
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[admin@switch] /interface bridge host print where !local
Flags: X - disabled, I - invalid, D - dynamic, L - local, E - external
# MAC-ADDRESS VID ON-INTERFACE BRIDGE AGE
0 D CC:2D:E0:E4:B3:A1 ether1 bridge1 42s
1 D CC:2D:E0:E4:B3:A2 ether2 bridge1 42s
2 D CC:2D:E0:E4:B3:A1 VLAN bridge2 42s
3 D CC:2D:E0:E4:B3:A2 VLAN bridge2 42s
4 D CC:2D:E0:E4:B3:A3 ether3 bridge2 42s |
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|
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Devices on ether1 and ether2 need to send tagged packets with VLAN-ID 99 in order to reach the host on ether3 (other packets do not get passed towards VLAN interface and further bridged with ether3). We can see in the host table that bridge2 have has learned these hosts. Packets coming from ether3 to ether1 will be correctly sent out tagged and traffic will not be flooded in bridge1. But since MAC learning is only possible between bridge ports and not on interfaces that are created on top of the bridge interface, packets sent from ether2 to ether3 will be flooded in bridge1.
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Use bridge VLAN filtering. The proper way to tag traffic is to assign a VLAN ID whenever traffic enters a bridge, this behaviour behavior can easily be achieved by specifying PVID value for a bridge port and specifying which ports are tagged (trunk) ports and which are untagged (access) ports. Below is an example how such setup should have been configured:
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/interface bridge
add name=bridge1 vlan-filtering=yes
/interface bridge port
add bridge=bridge1 interface=ether1
add bridge=bridge1 interface=ether2
add bridge=bridge1 interface=ether3 pvid=99
/interface bridge vlan
add bridge=bridge1 tagged=ether1,ether2 untagged=ether3 vlan-ids=99 |
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| Warning |
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By enabling |
VLAN in a bridge with a physical interface
Very similar case to VLAN on a bridge in a bridge, there are multiple possible scenarios where this could could have been used, most popular use case is when you want to send out tagged traffic through a physical interface, in such a setup you want traffic from one interface to receive only certain tagged traffic and send out this tagged traffic as tagged through a physical interface (simplified trunk/access port setup) by just using VLAN interfaces and a bridge.
Configuration
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/interface vlan
add interface=ether1 name=VLAN99 vlan-id=99
/interface bridge
add name=bridge1
/interface bridge port
add interface=ether2 bridge=bridge1
add interface=VLAN99 bridge=bridge1 |
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Problem
This setup and configuration will work on in most cases, but it violates the IEEE 802.1W standard when (R)STP is used. If this is the only device in your Layer2 domain, then this should not cause problems, but problems can arise when there are other vendor switches. The reason for this is that (R)STP on a bridge interface is enabled by default and BPDUs coming from ether1 will be sent out tagged since everything sent into ether1 will be sent out through ether2 as tagged traffic, not all switches can understand tagged BPDUs. Precautions should be made with this configuration in a more complex network where there are multiple network topologies for certain (group of) VLANs, this is relevant to MSTP and PVSTP(+) with mixed vendor devices. In a ring-like topology with multiple network topologies for certain VLANs, one port from the switch will be blocked, but in MSTP and PVSTP(+) a path can be opened for a certain VLAN, in such a situation it is possible that devices that don't support PVSTP(+) will untag the BPDUs and forward the BPDU, as a result, the switch will receive its own packet, trigger a loop detection and block a port, this can happen to other protocols as well, but (R)STP is the most common case. If a switch is using a BPDU guard function, then this type of configuration can trigger it and cause a port to be blocked by STP. It has been reported that this type of configuration can prevent traffic from being forwarded over certain bridge ports over time when using 6.41 or later. This type of configuration does not only break (R/M)STP, but it can cause loop warnings, this can be caused by MNDP packets or any other packets that are directly sent out from an interface.
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To avoid compatibility issues you should use bridge VLAN filtering. Below you can find an example how the same traffic tagging effect can be achieved with a bridge VLAN filtering configuration:
| Code Block | ||
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/interface bridge
add name=bridge1 vlan-filtering=yes
/interface bridge port
add bridge=bridge1 interface=ether1 pvid=99
add bridge=bridge1 interface=ether2
/interface bridge vlan
add bridge=bridge1 tagged=ether2 untagged=ether1 vlan-ids=99 |
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| Warning |
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By |
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enabling |
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you will be filtering out traffic destined to the CPU, before enabling VLAN filtering you should make sure that you set up |
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Bridged VLAN on physical interfaces
Very A very similar case to VLAN on a bridge in a bridge, consider the following scenario, you have a couple of switches in your network and you are using VLANs to isolate certain Layer2 domains and connect these switches are connected to a router that assigns addresses and routes the traffic to the world. For redundancy, you connect switches all switches directly to the router and have enabled RSTP, but to be able to setup DHCP Server you decide that you can create a VLAN interface for each VLAN on each physical interface that is connected to a switch and add these VLAN interfaces in a bridge. Network diagram can be found bellow:
Configuration
Only the router part is relevant to this case, switch configuration doesn't really matter as long as ports are switched. Router configuration can be found bellow:below:
| Code Block | ||
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/interface bridge
add name=bridge10
add name=bridge20
/interface vlan
add interface=ether1 name=ether1_v10 vlan-id=10
add interface=ether1 name=ether1_v20 vlan-id=20
add interface=ether2 name=ether2_v10 vlan-id=10
add interface=ether2 name=ether2_v20 vlan-id=20
/interface bridge port
add bridge=bridge10 interface=ether1_v10
add bridge=bridge10 interface=ether2_v10
add bridge=bridge20 interface=ether1_v20
add bridge=bridge20 interface=ether2_v20 |
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Problem
You might notice that the network is having some weird delays or even the network is unresponsive, you might notice that there is a loop detected (packet received with own MAC address) and some traffic is being generated out of nowhere. The problem occurs because a broadcast packet that is coming from either one of the VLAN interface created on the Router will be sent out the physical interface, packet will be forwarded through the physical interface, through a switch and will be received back on a different physical interface, in this case, broadcast packets sent out ether1_v10 will be received on ether2, packet will be captured by ether2_v10, which is bridged with ether1_v10 and will get forwarded again the same path (loop). (R)STP might not always detect this loop since (R)STP is not aware of any VLANs, a loop does not exist with untagged traffic, but exists with tagged traffic. In this scenario, it is quite obvious to spot the loop, but in more complex setups it is not always easy to detect the network design flaw. Sometimes this network design flaw might get unnoticed for a very long time if your network does not use broadcast traffic, usually Nieghbor Neighbor Discovery Protocol is broadcasting packets from the VLAN interface and will usually trigger a loop detection in such a setup. Sometimes it is useful to capture the packet that triggered a loop detection, this can by using sniffer and analysing analyzing the packet capture file:
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/tool sniffer
set filter-mac-address=4C:5E:0C:4D:12:44/FF:FF:FF:FF:FF:FF \
filter-interface=ether1 filter-direction=rx file-name=loop_packet.pcap |
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Or a more convenient way using logging:
| Code Block | ||
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/interface bridge filter
add action=log chain=forward src-mac-address=4C:5E:0C:4D:12:44/FF:FF:FF:FF:FF:FF
add action=log chain=input src-mac-address=4C:5E:0C:4D:12:44/FF:FF:FF:FF:FF:FF |
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Symptoms
Below is a list of possible symptoms that might be as a result of this kind of a misconfiguration:
- Port blocked by (R)STP
- Loops in network
- Low throughput
- Port flapping
- Network inaccessible
Solution
Partial A solution is to use Multiple Spanning Tree Protocol across the whole network, but it is required to use bridge VLAN filtering in order to make all bridges compatible with IEEE 802.1W and IEEE 802.1Q.
| Code Block | ||
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/interface bridge add name=bridge vlan-filtering=yes /interface bridge port add bridge=bridge interface=ether1 add bridge=bridge interface=ether2 /interface bridge vlan add bridge=bridge tagged=ether1,ether2,bridge vlan-ids=10 add bridge=bridge tagged=ether1,ether2,bridge vlan-ids=20 /interface vlan add name=vlan10 interface=bridge vlan-id=10 add name=vlan20 interface=bridge vlan-id=20 |
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| Warning |
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By enabling |
Even though rewriting your configuration to use bridge VLAN filtering will fix loop occurrence because of broadcast traffic that is coming from a VLAN interface, there still might exist loops with tagged unknown unicast or broadcast traffic. To make sure that loops don't exist with tagged and untagged traffic you should consider implementing MSTP in your network instead of (R)STP.
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will be filtering out traffic destined to the CPU, before enabling VLAN filtering you should make sure that you set up |
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Bridged VLAN
A more simplified scenario of Bridged VLAN on physical interfaces, but in this case, you simply want to bridge two or more VLANs together that are created on different physical interfaces. This is a very common type of setup that deserves a separate article since misconfiguring this type of setup has caused multiple network failures. This type of setup is also used for VLAN translation.
Configuration
| Code Block | ||
|---|---|---|
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/interface vlan
add interface=ether1 name=ether1_v10 vlan-id=10
add interface=ether2 name=ether2_v10 vlan-id=10
/interface bridge
add name=bridge1
/interface bridge port
add bridge=bridge1 interface=ether1_v10
add bridge=bridge1 interface=ether2_v10 |
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Problem
You may notice that certain parts of the network is are not accessible and/or certain links keep flapping. This is due to (R)STP, this type of configuration forces the device to send out tagged BPDUs, that might not be supported by other devices, including RouterOS. Since a device receives a malformed packet (tagged BPDUs should not exist in your network when running (R)STP, this violates IEEE 802.1W and IEEE 802.1Q), the device will not interpret the packet correctly and can have unexpected behaviour.
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/interface bridge add name=bridge1 vlan-filtering=yes /interface bridge port add bridge=bridge1 interface=ether1 add bridge=bridge1 interface=ether2 pvid=20 add bridge=bridge1 interface=ether3 pvid=30 add bridge=bridge1 interface=ether4 pvid=40 /interface bridge vlan add bridge=bridge1 tagged=ether1 untagged=ether2 vlan-ids=20 add bridge=bridge1 tagged=ether1 untagged=ether3 vlan-ids=30 add bridge=bridge1 tagged=ether1 untagged=ether4 vlan-ids=40
Problem
For example, you use this configuration on a CRS1xx/CRS2xx series device and you started to notice that the CPU usage is very high and when running a performance test to check the network's throughput you notice that the total throughput is only a fraction of the wire-speed performance that it should easily reach. The cause of the problem is that not all devices support bridge VLAN filtering on a hardware level. All devices are able to be configured with bridge VLAN filtering, but only few of them will be able to offload the traffic to the switch chip. If improper configuration method is used on a device with a built-in switch chip, then the CPU will be used to forward the traffic.
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/interface bridge add name=bridge1 /interface bridge port add bridge=bridge1 interface=ether1 add bridge=bridge1 interface=ether2 add bridge=bridge1 interface=ether3 add bridge=bridge1 interface=ether4 add bridge=bridge1 interface=ether5 add bridge=bridge1 interface=ether6 add bridge=bridge1 interface=ether7 add bridge=bridge1 interface=ether8 add bridge=bridge1 interface=ether9 add bridge=bridge1 interface=ether10 /interface vlan add interface=bridge1 name=VLAN10 vlan-id=10 /interface ethernet switch port set ether1,ether2,ether3,ether4,ether5,ether6,ether7,ether8,ether9 default-vlan-id=10 vlan-header=always-strip vlan-mode=secure set ether10 vlan-header=add-if-missing vlan-mode=secure set switch1-cpu,switch2-cpu vlan-mode=secure /interface ethernet switch vlan add ports=ether1,ether2,ether3,ether4,ether5,switch1-cpu switch=switch1 vlan-id=10 add ports=ether6,ether7,ether8,ether9,ether10,switch2-cpu switch=switch2 vlan-id=10
Problem
After running a few tests you might notice that packets from ether6-ether10 are forwarded as expected, but packets from ether1-ether5 are not always forwarded correctly (especially through the trunk port). Most noticeable issue would be that packets from ether1-ether5 through ether10 are simply dropped, this is because these ports are located on different switch chip, this means that VLAN filtering is not possible on a hardware level since the switch chip is not aware of the VLAN table's contents on a different switch chip. Packets that are being forwarded between ports that are located on different switch chips are also processed by the CPU, which means you won't be able to achieve wire-speed performance.
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/interface bridge add name=bridge1 /interface bridge port add bridge=bridge1 interface=ether1 add bridge=bridge1 interface=ether2 /interface vlan add interface=bridge1 name=VLAN99 vlan-id=99
Problem
As soon as you try to increase the MTU size on the VLAN interface, you receive an error that RouterOS Could not set MTU. This can happen when you are trying to set MTU larger than the L2MTU. In this case you need to increase the L2MTU size on all slave interfaces, which will update the L2MTU size on the bridge interface. After this has been done, you will be able to set a larger MTU on the VLAN interface. The same principle applies to bonding interfaces. You can increase the MTU on interfaces like VLAN, MPLS, VPLS, Bonding and other interfaces only when all physical slave interfaces have proper L2MTU set.
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/interface bridge add name=bridge1 /interface bridge port add interface=ether1 bridge=bridge1 add interface=ether2 bridge=bridge1
Problem
This is a very simplified problem, but in larger networks this might not be very easy to detect. For instance, ping might be working since a generic ping packet will be 70 bytes long (14 bytes for Ethernet header, 20 bytes for IPv4 header, 8 bytes for ICMP header, 28 bytes for ICMP payload), but data transfer might not work properly. The reason why some packets might not get forwarded is that MikroTik devices running RouterOS by default has MTU set to 1500 and L2MTU set to something around 1580 bytes (depends on the device), but the Ethernet interface will silently drop anything that does not fit into the L2MTU size. Note that L2MTU parameter is not relevant to x86 or CHR devices. For a device that is only supposed to forward packets, there is no need to increase the MTU size, it is only required to increase the L2MTU size, RouterOS will not allow you to increase the MTU size that is larger than the L2MTU size. If you require the packet to be received on the interface and the device needs to process this packet rather than just forwarding it, for example, in case of routing, then it is required to increase the L2MTU and the MTU size, but you can leave the MTU size on the interface to the default value if you are using only IP traffic (that supports packet fragmentation) and don't mind that packets are being fragmented. You can use the ping utility to make sure that all devices are able to forward jumbo frames:
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/interface bridge add name=bridge1 protocol-mode=rstp /interface bridge port add interface=ether1 bridge=bridge1 add interface=eoip1 bridge=bridge1
Problem
Both devices are able to communicate with each other, but some protocols do not work properly. The reason is that as soon as you use any STP variant (STP, RSTP, MSTP), you make the bridge compliant with IEEE 802.1D and IEEE 802.1Q, these standards recommend that packets that are destined to 01:80:C2:XX:XX:XX should NOT be forwarded. In cases where there are only 2 ports added to a bridge (R/M)STP should not be used since a loop cannot occur from 2 interfaces and if a loop does occur, the cause is elsewhere and should be fixed on a different bridge. Since (R/M)STP is not needed in transparent bridge setups, it can be disabled. As soon as (R/M)STP is disabled, the RouterOS bridge is not compliant with IEEE 802.1D and IEEE 802.1Q and therefore will forward packets that are destined to 01:80:C2:XX:XX:XX.
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Where X corresponds to and IP address for each device.
Problem
While traffic is being forwarded properly between R1 and R2, load balancing, link fail-over is working properly as well, but devices between R1 and R2 are not not always accessible or some of them are completely inaccessible (in most cases AP2 and ST2 is inaccessible). After examining the problem you might notice that packets do not always get forwarded over the required bonding slave and as a result never is received by the device you are trying to access. This is a network design and bonding protocol limitation. As soon as a packet needs to be sent out through a bonding interface (in this case you might be trying to send ICMP packets to AP2 or ST2), the bonding interface will create a hash based on the selected bonding mode and transmit-hash-policy and will select an interface, through which to send the packet out, regardless if the destination is only reachable only through a certain interface. Some devices will be accessible because the generated hash matches the interface, on which the device is located on, but it might not choose the needed interface as well, which will result in inaccessible device. Only broadcast bonding mode does not have this kind of protocol limitation, but this bonding mode has a very limited use case.
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Bad way to test bandwidth or throughput
Problem
As soon as you start Bandwidth test or Traffic generator you notice that the throughput is much smaller than expected. For very powerful routers, which should be able to forward many Gigabits per second (Gbps) you notice that only a few Gigabits per second gets forwarded. The reason why this is happening is because of the testing method you are using, you should never test throughput on a router while using the same router for generating traffic, this is especially true when using Bandwidth test since it is only able to generate traffic on a single CPU core and also applies when using Traffic-generator, though it can run on multiple cores, but you are still adding a load on the CPU that reduces the total throughput.
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/interface bridge add name=bridge1 /interface bridge port add bridge=bridge1 horizon=1 hw=no interface=ether1 add bridge=bridge1 horizon=2 hw=no interface=ether2 add bridge=bridge1 horizon=3 hw=no interface=ether3 add bridge=bridge1 horizon=4 hw=no interface=ether4
Problem
After setting the bridge split-horizon on each port, you start to notice that each port is still able to send data between each other. The reason for this is misuse of bridge split-horizon. A bridge port is only not able to communicate with ports that are in the same horizon, for example, horizon=1 is not able to communicate with horizon=1, but is able to communicate with horizon=2, horizon=3 and so on.
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Consider the following scenario, you have decided to use optical fibre cables to connect your devices together by using SFP or SFP+ optical modules, but for convenience reasons you have decided to use SFP optical modules that were available.
Problem
As soon as you configure your devices to have connectivity on the ports that are using these SFP optical modules, you might notice that either the link is working properly or are experiencing random connectivity issues. There are many vendors that manufacture SFP optical modules, but not all vendors strictly follow SFP MSA, SFF and IEEE 802.3 standards, which can lead to unpredictable compatibility issues, which is a very common issue when using not well known or unsupported SFP optical modules in MikroTik devices.
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