Ethernet – How does a Hub work when sending broadcast packages

ethernet

I know that hubs send broadcast packets, so every packets is received by each node in the broadcast domain. I read that frames that are received but that aren't addressed to a NIC'S MAC address are dropped. But I mean if it is a broadcast packet, it shouldn't be accepted by each node?

Best Answer

Hubs are layer 1 devices. As they receive electrical impulses constituting a frame, they forward them out all ports other than the receiving port.

This is regardless of whether the l2 destination is unicast, multicast, or broadcast (ff:ff:ff:ff:ff:ff).

This may result in an Ethernet collision if multiple hosts transmit traffic to the hub simultaneously.

A host’s Ethernet adapter will pass certain received frames up the protocol stack for further processing ( generating a cpu interrupt)

Unicast frames which match its Ethernet unicast address
Multicast frames destined to any l2 multicast address which the host is “listening” to
All broadcast l2 frames

If the l2 frame includes a l3 packet, the host cpu will de-encapsulate and process it.

If the interface is placed in “promiscuous mode” ( by tcpdump for example) it is possible for all received Ethernet frames to be sent up the protocol stack and generate interrupts.

Regarding Path MTU Discovery

Ideally i would be relying on Path MTU discovery. But since the ethernet packets being generated are too large for any other machine to receive, there is no opportunity for IP Packet too big fragmentation messages to be returned

Based on your diagram, I agree that PMTUD cannot function between two different PCs in the same LAN segment; PCs do not generate ICMP Error messages required by PMTUD.

Jumbo frames

Some vendors (such as Cisco) have switch models which support ethernet payloads larger than 1500 bytes. Officially IEEE does not endorse this configuration, but the industry has valid needs to judiciously deviate from the original 1500 byte MTU. I have storage LAN / backup networks which leverage jumbo frame for good reason; however, I made sure that all MTUs matched inside the same vlan when I deployed jumbo frames.

Mismatched MTUs within a broadcast domain

The bottom line is that you should never have mismatched ethernet MTUs inside the same ethernet broadcast domain; if you do, it's a bug or configuration error. Regardless of bug or error, you have to solve these problems, sometimes manually.

All that discussion leads to the next question...

Why is there a spec that intentionally creates invalid ethernet frames?

I'm not sure that I agree... I don't see how the IEEE 802.3 series, or RFC 894 create invalid frames. Host implementations or host misconfigurations create invalid frames. To understand whether your implementation is following the spec, we need a lot more evidence...

This diagram is at least prima facie evidence that your MTUs are mismatched inside a broadcast domain...

+------------------+      +----------------+     +------------------+
| Realtek PCIe GBe |      | NetGear 10/100 |     | Realtek 10/100   |
|       (on-board) |      |     Switch     |     |     (on-board)   |
|                  |      +----------------+     |                  |
| Windows 7        |           ^    ^            |                  |
|                  |           |    |            |                  |
| 192.168.1.98/24  |-----------+    +------------| 192.168.1.10/24  |
| MTU = 1504 bytes |                             | MTU = 1500 bytes |
+------------------+                             +------------------+

How should an 802.3-compliant implementation respond to MTU mismatches?

What was it they [the writers of 'the spec'] expected people to do with devices that generate these too large packets?

MTU 1504 and MTU 1500 within the same broadcast domain is simply a misconfiguration; it should never be expected to work any more than mismatched IP netmasks, or mismatched IP subnets can be expected to work. Your company will have to knuckle-down and fix the root-cause of the MTU mismatches... at this time it's hard to say whether the root cause is user error, an implementation bug, or some combination of the above.

If the affected Windows machines are successfully logging into to an Active Directory Domain, one could write Windows login scripts to automatically fix MTU issues based on some well-constructed tests inside the domain login scripts (assuming the Domain Controller isn't part of the MTU issues).

If the machines are not logging into a domain, manual labor is another option.

Other possibilities to contain the damage

Use a layer3 switch^{Note 1} to build a custom vlan for anything that has broken MTUs and set the layer3 switch's ethernet MTU to match the broken machines; this relies on PMTUD to resolve MTU issues at the IP layer. Layer3 switches generate the ICMP errors required by PMTUD.

This option works best if you can re-address the broken machines with DHCP; and you can identify the broken machines by mac-address.

... why did they bump it up to 1504 bytes, and create invalid packets, in the first place?

Hard to say at this point

802.1ad vs 802.1q

How is IEEE 802.1ad (aka VLAN Tagging, QinQ) valid, when the packets are too large?

I haven't seen evidence so far that you're using QinQ; from the limited evidence I have seen so far, you're using simple 802.1q encapsulation, which should work correctly in Windows, assuming the NIC driver supports 802.1q encap.

End Notes:

^{Note 1}_{Any layer 3 switch should do... Cisco, Juniper, and Brocades all could perform this kind of function.}

How a Switch Routes Packets with Another Switch in Path

A Layer 2 switch learns most of its information about the location of other endpoints via "listening" to ingressing frames, and when it is not aware of the location, it uses floodingand will learn from the answer. Lets say the topology is:

(Host A) <--> (Switch A) <--> (Switch B) <-->(Host B).

Also important to note, a L2 Switch forwards, it does not route. By that I mean, when it receives an encapsulated frame, it forwards it out interfaces.

Host A wants to get to Host B. Host A knows Host B's MAC address. So Host A sends a frame out to Switch A.
Switch A does not have Host B in its MAC address Table. Switch A then flood out the frame on all of its ports, except for the one Host A is located. Among all those ports there is the ports that Switch B is plugged into, so Switch B receives the frame.
Switch B then flood that frame out all of its ports (considering Switch B does not have Host B in its MAC address Table).
Host B will respond to the frame, Switch B will then record Host B MAC address and send the response to Switch A. Switch A then records the MAC address of Host B in its table and send the response to Host A

Here's a related question: How does a switch learn a switch table?