CS402: Computer Communications and Networks

Ethernet Switches

Increasing the physical layer bandwidth as in Fast Ethernet was only one of the solutions to improve the perfor- mance of Ethernet LANs. A second solution was to replace the hubs with more intelligent devices. As Ethernet hubs operate in the physical layer, they can only regenerate the electrical signal to extend the geographical reach of the network. From a performance perspective, it would be more interesting to have devices that operate in the datalink layer and can analyse the destination address of each frame and forward the frames selectively on the link that leads to the destination. Such devices are usually called Ethernet switches 10. An Ethernet switch is a relay that operates in the datalink layer as is illustrated in the figure below.

Figure 6.24: Ethernet switches and the reference model

An Ethernet switch understands the format of the Ethernet frames and can selectively forward frames over each interface. For this, each Ethernet switch maintains a MAC address table. This table contains, for each MAC address known by the switch, the identifier of the switch’s port over which a frame sent towards this address must be forwarded to reach its destination. This is illustrated below with the MAC address table of the bottom switch. When the switch receives a frame destined to address B, it forwards the frame on its South port. If it receives a frame destined to address D, it forwards it only on its North port.

Figure 6.25: Operation of Ethernet switches

One of the selling points of Ethernet networks is that, thanks to the utilisation of 48 bits MAC addresses, an Ethernet LAN is plug and play at the datalink layer. When two hosts are attached to the same Ethernet segment or hub, they can immediately exchange Ethernet frames without requiring any configuration. It is important to retain this plug and play capability for Ethernet switches as well. This implies that Ethernet switches must be able to build their MAC address table automatically without requiring any manual configuration. This automatic configuration is performed by the the MAC address learning algorithm that runs on each Ethernet switch. This algorithm extracts the source address of the received frames and remembers the port over which a frame from each source Ethernet address has been received. This information is inserted into the MAC address table that the switch uses to forward frames. This allows the switch to automatically learn the ports that it can use to reach each destination address, provided that this host has previously sent at least one frame. This is not a problem since most upper layer protocols use acknowledgements at some layer and thus even an Ethernet printer sends Ethernet frames as well.

The pseudo-code below details how an Ethernet switch forwards Ethernet frames. It first updates its MAC address table with the source address of the frame. The MAC address table used by some switches also contains a timestamp that is updated each time a frame is received from each known source address. This timestamp is used to remove from the MAC address table entries that have not been active during the last n minutes. This limits the growth of the MAC address table, but also allows hosts to move from one port to another. The switch uses its MAC address table to forward the received unicast frame. If there is an entry for the frame’s destination address in the MAC address table, the frame is forwarded selectively on the port listed in this entry. Otherwise, the switch does not know how to reach the destination address and it must forward the frame on all its ports except the port from which the frame has been received. This ensures that the frame will reach its destination, at the expense of some unnecessary transmissions. These unnecessary transmissions will only last until the destination has sent its first frame. Multicast and Broadcast frames are also forwarded in a similar way.

# Arrival of frame F on port P
# Table: MAC address table dictionary: addr->port
# Ports: list of all ports on the switch
src=F.SourceAddress
dst=F.DestinationAddress
Table[src]=P #src heard on port P
if isUnicast(dst):
  if dst in Table:
    ForwardFrame(F,Table[dst])
  else:
    for o in Ports:
      if o!= P: ForwardFrame(F,o)
else:
  # multicast or broadcast destination
  for o in Ports:
    if o!= P: ForwardFrame(F,o)

Note: Security issues with Ethernet hubs and switches

From a security perspective, Ethernet hubs have the same drawbacks as the older coaxial cable. A host attached to a hub will be able to capture all the frames exchanged between any pair of hosts attached to the same hub. Ethernet switches are much better from this perspective thanks to the selective forwarding, a host will usually only receive the frames destined to itself as well as the multicast, broadcast and unknown frames. However, this does not imply that switches are completely secure. There are, unfortunately, attacks against Ethernet switches. From a security perspective, the MAC address table is one of the fragile elements of an Ethernet switch. This table has a fixed size. Some low-end switches can store a few tens or a few hundreds of addresses while higher-end switches can store tens of thousands of addresses or more. From a security point of view, a limited resource can be the target of Denial of Service attacks. Unfortunately, such attacks are also possible on Ethernet switches. A malicious host could overflow the MAC address table of the switch by generating thousands of frames with random source addresses. Once the MAC address table is full, the switch needs to broadcast all the frames that it receives. At this point, an attacker will receive unicast frames that are not destined to its address. The ARP attack discussed in the previous chapter could also occur with Ethernet switches [Vyncke2007]. Recent switches implement several types of defences against these attacks, but they need to be carefully configured by the network administrator. See [Vyncke2007] for a detailed discussion on security issues with Ethernet switches.

The MAC address learning algorithm combined with the forwarding algorithm work well in a tree-shaped network such as the one shown above. However, to deal with link and switch failures, network administrators often add redundant links to ensure that their network remains connected even after a failure. Let us consider what happens in the Ethernet network shown in the figure below.

When all switches boot, their MAC address table is empty. Assume that host A sends a frame towards host C. Upon reception of this frame, switch1 updates its MAC address table to remember that address A is reachable via its West port. As there is no entry for address C in switch1’s MAC address table, the frame is forwarded to both switch2 and switch3. When switch2 receives the frame, its updates its MAC address table for address A and forwards the frame to host C as well as to switch3. switch3 has thus received two copies of the same frame. As switch3 does not know how to reach the destination address, it forwards the frame received from switch1 to switch2 and the frame received from switch2 to switch1... The single frame sent by host A will be continuously duplicated by the switches until their MAC address table contains an entry for address C. Quickly, all the available link bandwidth will be used to forward all the copies of this frame. As Ethernet does not contain any TTL or HopLimit, this loop will never stop.

Figure 6.26: Ethernet switches in a loop

The MAC address learning algorithm allows switches to be plug-and-play. Unfortunately, the loops that arise when the network topology is not a tree are a severe problem. Forcing the switches to only be used in tree-shaped networks as hubs would be a severe limitation. To solve this problem, the inventors of Ethernet switches have developed the Spanning Tree Protocol. This protocol allows switches to automatically disable ports on Ethernet switches to ensure that the network does not contain any cycle that could cause frames to loop forever.

Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
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