Web / SNMP Intelligent Products
WGSW-801SX
8-Port Gigabit Ethernet Backbone Switch
Advance Topics
The Spanning Tree Algorithm (that is, the STA configuration algorithm as outlined in IEEE 802.1D) can be used to detect and disable network loops, and to provide link backup. This allows the switch to interact with other bridging devices (including STA compliant switches, bridges or routers) in your network to ensure that only one route exists between any two stations on the network. If redundant paths or loops are detected, one or more ports are put into a blocking state (stopped from forwarding packets) to eliminate the extra paths. Moreover, if one or more of the paths in a stable spanning tree topology fail, this algorithm will automatically change ports from blocking state to forwarding state to re-establish contact with all network stations.
The STA uses a distributed algorithm to select a bridging device (STA compliant switch, bridge or router) that serves as the root of the spanning tree network. It selects a root port on each bridging device (except for the root device) which incurs the lowest path cost when forwarding a packet from that device to the root device. Then it selects a designated bridging device from each LAN which incurs the lowest path cost when forwarding a packet from that LAN to the root device. All ports connected to designated bridging devices are assigned as designated ports. After determining the lowest cost spanning tree, it enables all root ports and designated ports, and disables all other ports. Network packets are therefore only forwarded between root ports and designated ports, eliminating any possible network loops.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge Protocol Data Units) transmitted from the Root Bridge. If a bridge does not get a Hello BPDU after a predefined interval (Maximum Age), the bridge assumes that the link to the Root Bridge is down. This bridge will then initiate negotiations with other bridges to reconfigure the network to re-establish a valid network topology.
The following figure gives an illustration of how
the Spanning Tree Algorithm assigns bridging device ports.
Switches do not inherently support broadcast domains, which can lead to broadcast storms in large networks that handle a lot of IPX or NetBeui traffic. In conventional networks with routers, broadcast traffic is split up into separate domains to confine broadcast traffic to the originating group and provide a much cleaner network environment. By supporting VLANs, this switch allows you to create segregated broadcast domains. However, note that if you need to support intra-VLAN communications, you must use a router or Layer 3 switch.
An IEEE 802.1Q VLAN is a group of ports that can be located anywhere in the network, but communicate as though they belong to the same physical segment. VLANs help to simplify network management by allowing you to move devices to a new VLAN without having to change any physical connections. VLANs can be easily organized to reflect departmental groups (such as Marketing or R&D), usage groups (such as e-mail), or multicast groups (used for multimedia applications such as video conferencing).
VLANs provide greater network efficiency by reducing broadcast traffic, but also allow you to make network changes without having to update IP addresses or IP subnets. VLANs inherently provide a high level of network security, since traffic must pass through a Layer 3 switch or a router to reach a different VLAN. This switch supports the following VLAN features:
9.2.1 Assigning Ports to VLANs
Before enabling VLANs for the switch, you must first assign each port to the VLAN group(s) it will participate in. (By default all ports are assigned to VLAN 1 as untagged ports.) Add a port as a tagged port (that is, a port attached to a VLAN-aware device) if you want it to carry traffic for one or more VLANs and the device at the other end of the link also supports VLANs. Then assign the port at the other end of the link to the same VLAN(s). However, if you want a port on this switch to participate in one or more VLANs, but the device at the other end of the link does not support VLANs, then you must add this port as an untagged port (that is, a port attached to a VLAN-unaware device).
Port-based VLANs are tied to specific ports. The switch's forwarding decision is based on the destination MAC address and its associated port. Therefore, to make valid forwarding and flooding decisions, the switch learns the relationship of the MAC address to its related port--and thus to the VLAN--at run-time.
When the switch receives a frame, it classifies the frame in one of two ways:
Port overlapping can be used to allow access to commonly shared network resources among different VLAN groups, such as file servers or printers. Note that if you implement VLANs which do not overlap, but still need to communicate, you must connect them using a router or Layer 3 switch.
9.2.4 Forwarding Tagged/Untagged Frames
Ports can be assigned to one untagged VLAN and multiple tagged VLANs. Each port on the switch is therefore capable of passing tagged or untagged frames. To forward a frame from a VLAN-aware device to a VLAN-unaware device, the switch first decides where to forward the frame, and then strips off the VLAN tag. However, to forward a frame from a VLAN-unaware device to a VLAN-aware device, the switch first decides where to forward the frame, and then inserts a VLAN tag reflecting this port's default VID. The default PVID is VLAN 1, but this can be changed (refer to VLAN Port Configuration).
9.2.5 Forwarding Traffic with Unknown VLAN Tags
Up to 2048 VLANs are supported by the IEEE 802.1Q protocol, but this switch only supports 16 VLANs. Therefore, if this switch is attached to any device that forwards frames with unknown VLAN tags, or to endstations which issue VLAN registration requests for unknown VLANs, this traffic will be dropped.
9.3 Class-of-Service (CoS) Support
The WGSW-801SX provides two transmit queues on each port, with a weighted round-robin scheme. This function can be used to provide independent priorities for various types of data such as real-time video or voice, and best-effort data.
Priority assignment to a packet in the Web Switch is accomplished through explicit assignment by end stations which have applications that require a higher priority than best-effort. This switch utilizes the IEEE 802.1p and 802.1Q tag structure to decide priority assignments for the received packets.
9.4 IGMP Snooping and IP Multicast Filtering
The Internet Group Management Protocol (IGMP) runs between hosts and their immediately neighboring multicast router/switch. The protocol's mechanisms allow a host to inform its local router/switch that it wants to receive transmissions addressed to a specific multicast group.
A router, or multicast-enabled switch, can periodically ask their hosts is they want to receive multicast traffic. If there is more than one router/switch on the LAN performing IP multicasting, one of these devices is elected "querier" and assumes the responsibility of querying the LAN for group members.
Based on the group membership information learned from IGMP, a router/switch can determine which (if any) multicast traffic needs to be forwarded to each of its ports. At Layer-3, multicast routers use this information, along with a multicast routing protocol, to support IP multicasting across the Internet.
IGMP provides the final step in an IP multicast packet delivery service since it is only concerned with forwarding multicast traffic from the local router/switch to group members on directly attached subnetwork or LAN segment. This switch supports IP Multicast Filtering by:
The purpose of IP multicast filtering is to optimize a switched network's performance, so multicast packets will only be forwarded to those ports containing multicast group hosts or multicast routers/switches instead of flooding to all ports in the subnet (VLAN).
The WGSW-801SX, with IP multicast filtering capability, not only passively monitors IGMP Query and Report messages; it can also actively send IGMP Query messages to learn locations of multicast routers/switches and member hosts in multicast groups within each VLAN.
However, note that IGMP neither alters nor routes any IP multicast packets. Since IGMP is not concerned with the delivery of IP multicast packets across subnetworks, an external IP multicast router is needed if IP multicast packets have to be routed across different subnetworks.
SNMP (Simple Network Management Protocol) is a communication protocol designed specifically for managing devices or other elements on a network. Network equipment commonly managed with SNMP includes hubs, switches, bridges, routers and host computers. SNMP is typically used to configure these devices for proper operation in a network environment, as well as monitor them to evaluate performance and detect potential problems.
Remote Monitoring (RMON) provides a cost-effective way to monitor large networks by placing embedded or external probes on distributed network equipment (hubs, switches or routers). RMON has already become a valuable tool for network managers faced with a quickly changing network landscape that contains dozens or hundreds of separate segments. RMON is the only way to retain control of the network and analyze applications running at multi-megabit speeds. It provides the tools you need to implement either reactive or pro-active policies that can keep your network running based on real-time access to key statistical information.
This switch provides support for mini-RMON which contains the four key groups required for basic remote monitoring. These groups include:
Statistics: Includes all the tools needed to monitor your network for common errors and overall traffic rates. Information is provided on bandwidth utilization, peak utilization, packet types, errors and collisions, as well as the distribution of packet sizes.
History: Can be used to create a record of network utilization, packet types, errors and collisions. You need a historical record of activity to be able to track down intermittent problems. Historical data can also be used to establish normal baseline activity, which may reveal problems associated with high traffic levels, broadcast storms, or other unusual events. Historical information can also be used to predict network growth and plan for expansion before your network becomes too overloaded.
Alarms: Can be set to test data over any specified time interval, and can monitor absolute or changing values (such as a statistical counter reaching a specific value, or a statistic changing by a certain amount over the set interval). Alarms can be set to respond to either rising or falling thresholds.
Events: Defines the action to take when an alarm is triggered. The response to an alarm can include recording the alarm in the Log Table or sending a message to a trap manager. Note that the Alarm and Event Groups are used together to record important events or immediately respond to critical network problems.
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