Elastic Networks BitStorm L3S-T, BitStorm L3S-X User Manual

BitStorm L3S-T
08-01082-01 Rev 1.5
Last Updated 9/15/00
User's Manual
includes the BitStorm L3S-X Stack Slave
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Stacking BitStorm L3S Switches
As new networks are deployed and existing ones continue to grow aggressively, managers need cost-effective products that can adapt.
Low cost, high-performance “stacked” switches that can be managed as a single entity
are the most desirable solutions, especially for small to medium-sized enterprises.
What is a “stackable” switch?
A stackable solution ensures that a “master” switch can be connected to one or more “slave” switches and that all can function or be managed as a single logical device.
Built in a predominately standalone fixed-port configuration, this type of switch is typically a single-board system that is self-contained in an enclosure with its own power supply.
Port density is increased by connecting one switch to another, unlike a chassis-based system in which ports are added using expansion boards. A stackable switch is connected in a peer-to-peer or in a master-slave relationship to switches of equal or similar size.
When a “stack” is not a stack
Many manufacturers today say their switches are “stackable” simply because they can be connected using a single Gigabit uplink on each switch.
This not only “burns” switch resources by stealing a Gigabit link, it is an ineffective design that creates severe blocking and packet loss between switches. Others use a “virtual chassis” where a separate switch is used as a “traffic cop” to interconnect switches, again, using gigabit ports for this connection and creating both non wire-speed transfers and blocking.
These switches are more accurately described as “linked” not “stacked”. They cannot truly be called stackable switches because:
these external Gigabit links introduce a significant degree of blocking
these connections consume switch ports in many cases, they are not necessarily managed as a single unit and therefore cannot truly be called
stackable switches.
Linking
Some manufacturers “stack” their switches by using a Gigabit uplink on each switch. This uses valuable resources and creates severe blocking and packet loss.
Virtual Chassis
Other manufacturers use a “virtual chassis” concept that burns Gigabit ports while creating non wire-speed transfers and blocking constraints.
Stacking
The BitStorm L3S system uses dedicated 8 Gbps stacking interfaces to guarantee wire-speed, non-blocking performance.
When a stack is a stack
The only true stacking interface is a design that uses an external, high-speed bus to interconnect separate stackable switches.
High-speed stacking bus
This interface connects the ports on the stackable “slave” switches directly to the switching fabric on the “master” switch. This is the only switch interface technology that can deliver the bandwidth necessary for wire-speed packet forwarding and eliminate blocking between connected switches.
What is blocking?
Basically, blocking is the inability of a switch to forward traffic due to bandwidth limitations. Technically, packet transfers are blocked when sufficient bandwidth is not available for all packets to be forwarded at the highest speed possible on the link.
Packets can be blocked externally as they are forwarded between switches as well as internally within the switch. Internal, or head-of-line blocking, is eliminated through complex buffering and queuing, while blocking between
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switches is a simple matter of providing sufficient bandwidth for wire-speed packet transfers.
How BitStorm L3S stacking eliminates blocking
BitStorm L3S's high performance stacking architecture guarantees that packets are forwarded at wire speed to all ports on all switches in the stack without blocking any transmissions. BitStorm L3S does this using a dedicated high-speed interface connecting all ports directly to the central switching fabric.
Blocking between switches
To avoid blocking between switches, a stackable switch must be able to forward the full traffic load from any of its switch ports to any switch port on any switch in its stack.
Using the example of a single Gigabit uplink that is used to connect two switches with 24 Fast Ethernet ports, that single Gigabit uplink is less than half the bandwidth needed to prevent blocking between two switches.
At full duplex, that single uplink delivers only 2Gbps of bandwidth instead the 4.8Gbps needed to forward packets at wire-speed over all 24 Fast Ethernet ports also operating at full duplex.
At full duplex, 24 Fast Ethernet ports talking to 24 Fast Ethernet ports need 200 Mbps x 24,
or 4.8 Gbps of bandwidth for non-blocking performance.
At full duplex, a single Gigabit link between switches only provides 2 Gbps of bandwidth,
less than half of what's required.
BitStorm L3S stacking dedicates 8Gbps of bandwidth between 24-port switches,
almost double the 4.8 Gbps required for non-blocking transfers.
Single entity management
In a BitStorm L3S stack, the management software running in the master extends its power over the ports on the slave switches. In effect, the slaves rely upon the greater power of the master. As such, slaves are very cost effective. The master CPU runs a single management system that sees all ports in the stack as its own, making the slave switches transparent to the network.
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