Information Technology Reference
In-Depth Information
These are the basic rules that a switch will use in carrying out the frame forwarding responsibility:
If the destination MAC address is found in the CAM table, then the switch will send the frame out the
port that is associated with that destination MAC address in the CAM table. This is called
forwarding
.
If the associated port to send the frame out is the same port on which the frame originally came in, then
there is no need to send the frame back out that same port, and the frame is ignored. This is called
filtering
.
If the destination MAC address is not in the CAM table (the address is unknown), then the switch will
send the frame out
all
other ports that are in the
same
VLAN as the received frame. This is called
flooding
. It will not flood the frame out the same port on which the frame was received.
If the destination MAC address of the received frame is the broadcast address (FFFF.FFFF.FFFF), then
the frame is sent out all ports that are in the same VLAN as the received frame. This is also called
flooding
. The frame will not be sent out the same port on which the frame it was received.
Spanning-Tree Protocol
As we have seen, the transparent bridging algorithm floods unknown and broadcast frames out all the
ports that are in the same VLAN as the received frame. This causes a potential problem. If the network
devices running this algorithm are connected in a physical loop, then flooded frames (such as broadcasts)
will be passed from switch to switch, around and around the loop forever. Depending on the physical
connections involved, the frames may actually multiply exponentially as a result of the flooding
algorithm, which can cause serious network problems.
There is a benefit to having a physical loop in your network: It can provide redundancy. If one link fails,
there is still another way for the traffic to reach its destination. To allow the benefits derived from
redundancy, without breaking the network because of flooding, a protocol called the Spanning-Tree
Protocol was created. It was standardized in the IEEE 802.1d specification.
The purpose of the Spanning-Tree Protocol is to identify and temporarily block the loops in a network
segment or VLAN. The switches run the Spanning-Tree Protocol, which involves electing a root bridge
or switch. The other switches measure their distance from the root switch. If there is more than one way
to get to the root switch, then there is a loop. The switches follow the algorithm to determine which ports
should be blocked to break the loop. STP is dynamic; if a link in the segment fails, then ports that were
originally blocking may possibly be changed to forwarding mode.
Trunking
Trunking
is a mechanism that is most often used to allow multiple VLANs to function independently
across multiple switches. Routers and servers may use trunking as well, which allows them to live
simultaneously on multiple VLANs. If your network has only one VLAN in it, then you may never need
trunking; if your network has more than one VLAN, however, you will probably want to take advantage
of the benefits of trunking.
A port on a switch normally belongs to only one VLAN; any traffic received or sent on this port is
assumed to belong to the configured VLAN. A trunk port, on the other hand, is a port that can be
configured to send and receive traffic for many VLANs. It accomplishes this by attaching VLAN
information to each frame, a process called “tagging” the frame. Also, trunking must be active on both
sides of the link; the other side must be expecting frames that include VLAN information for proper
communication to occur.
Different methods of trunking exist, depending on the media being used. Trunking methods for Fast
Ethernet or Gigabit Ethernet are Inter-Switch Link (ISL) or 802.1q. Trunking over ATM uses LANE.
Trunking over FDDI uses 802.10.
