Electrical computers and digital processing systems: multicomput – Computer-to-computer data routing
Reexamination Certificate
2000-09-25
2004-10-05
Jaroenchonwanit, Bunjob (Department: 2143)
Electrical computers and digital processing systems: multicomput
Computer-to-computer data routing
C370S389000
Reexamination Certificate
active
06801950
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to data communication systems for conveying information in the form of addressed data packets, such as Ethernet packets, and more particularly relates to the construction and manner of operation of a stackable unit which may be connected to other similar units in a stack. The main object of the invention is to increase the versatility of a stackable unit to enable a greater range of connections of a trunk to a stack of units.
BACKGROUND TO THE INVENTION
(a) Stackable Units.
It is well known to provide multi-port network units, such as switches and routers, which can be ‘stacked’, that is to say connected so that a multiplicity of such units form effectively a single larger unit comprising modules each constituted by one of the stackable units. The term ‘stack’ arises because frequently, though not necessarily, the units are physically designed so that they may be stacked one above the other. The facility of ‘stacking’ is very useful in the organisation of networks because it allows for expansion and scaling, though various difficulties are present. Among these is the general inability of a unit in a stack to hold information enabling a determination within that unit of the state or configuration of another unit in the stack. A specific example of such a difficulty will be further explained later. Furthermore, certain forwarding rules need to be obeyed to avoid excessive or unnecessary forwarding of packets or duplicates of packets throughout the stack.
(b) Cascades.
In order to enable the forwarding of packets from one unit in a stack to the other units in a stack, it is common to provide a connection known as a ‘cascade’. This comprises a connection between certain ports, at least one for each unit, so as to provide the physical means by which packets which may be received at one unit in a stack can be forwarded prior to ultimate dispatch from the stack by another unit. Cascades may be in comparatively simple form, wherein for example cable connections between specified ports, known as ‘cascade ports’ connect the units in a stack. Alternatively they may be more sophisticated, including arbitrated access, such as for example described in GB patent publication number 2338155-A. A suitable commercially available cascade is the ‘Trillian’ cascade made by 3Com Corporation. It is known in stackable switches to provide a ‘logical’ port which is treated as a destination port by that switch for packets which have to be transmitted via the cascade to another unit. The concept of a logical port is useful when a unit has more than one port connected to the cascade, in such a case a packet which is directed to the ‘logical’ cascade port is subject to some further logic process in order to determine the physical port from which it should be forwarded onto the cascade.
In general, a cascade is, in respect of packets, merely a transmission medium, i.e. it does not determine which particular unit should receive a packet. A packet which has been placed on the cascade will be conveyed by the cascade to all the active units in the stack. Accordingly, the units themselves need information, which may be conveyed with the packet or be stored in a unit when the stack is made and configured, that enables the units to obey predetermined operating rules that indicate whether a particular unit is required to forward a packet that it receives by way of the cascade or to discard that packet. One such rule is a ‘Next in Line’ rule which is described hereinafter. However, the operation of such a rule needs qualification when, as envisaged herein, a unit which would be designated for forwarding a packet cannot or should not do so.
(c) Trunks.
Another feature in modern network practice is a ‘trunk’ or ‘trunk connection’. Such a connection is useful where the expected traffic from one network unit to another is substantially greater than can be accommodated by a shingle link A trunk is in essence a set of parallel paths from a remote unit (which may itself be constituted by a stack of units) to a multiplicity of ports. In its simplest form, a trunk connection is made to a multiplicity of ports on a single unit, so that the total bandwidth available for the trunk is generally the aggregate of the bandwidths available to each of the ports which are ‘members’ of the trunk connection.
However, with the development of stackable units it is also desirable (and known practice) to make ports of different units within a stack members of the trunk. This presents some difficulties of organisation. These need not be severe in respect of packets which enter the stack from the trunk because a switch which receives a packet intended for a destination to which that unit is not coupled by way of one of its local ports broadly need only forward such a packet on the cascade so that that packet eventually reaches a unit having a port connected to the required destination. However, the difficulties can be severe in respect of packets which may be received by any of the units (by way of a non-trunk port) and are intended for forwarding by way of the trunk. Normal forwarding rules which govern the forwarding engine of a stacked unit are difficult to reconcile with the operation of a trunk connected to different units of a stack, or place undue restriction on the manner in which the trunk may be connected to the stack.
More particularly, it is possible to make the software control process, or the corresponding hardware, which directs packets within the unit, subject to a ‘local forwarding’ rule. According to such a rule, all traffic (i.e. packets and frames) intended for dispatch on a stack-wide trunk must be sent to the trunk from the unit by which the traffic has been received. In other words, any traffic intended for a stack-wide trunk operated in local forwarding mode may not be sent to the trunk by way of the cascade. If the unit which receives a packet has a port which is a ‘member’ of the trunk, then that packet must be sent by way of that port (or one of the trunk ports for that unit if there is more than one such trunk port). A ‘local forwarding’ rule is generally useful and convenient, but has hitherto imposed some restriction on the connections that may be made to the units in a stack. In particular, one restriction has been that all units which are capable of sending traffic onto the cascade, normally all the units in the stack, must have at least one port which is a member of the trunk.
An additional difficulty arises in relation to packets which are not only forwarded from a local port but are sent to other units, as in the cases of for example broadcast packets, multicast packets, unicast packets with addresses unknown in an originating unit or packets destined for more than one trunk neither of which is in an originating unit. It is necessary to determine when such a packet is received by way of the cascade from the originating unit whether it has been forwarded to a specific trunk so as to prevent a second forwarding of the packet to the same trunk.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a stackable network unit, such as a switch, which can in conjunction with other compatible units be connected selectively to a trunk and operated according to a ‘local forwarding’ rule modified so that the unit need not have any port which is a member of the trunk. For this purpose the stackable network unit preferably includes means, typically a register, namely some accessible storage device having a multiplicity of storage locations, which identifies those units which have ports connected to the trunk. Such a register may be accessed at an appropriate time by the forwarding engine to enable a unit which receives a packet to modify a ‘local forwarding’ rule and to enable a mode of working in which units connected in a stack need not all have a port connected to the trunk. The register facilitates a determination whether a received packet was placed on the cascade by a unit which has a port which is a member of the trunk. That may be determined b
Hughes Mark A
Loughran Kevin
Moran Paul J
O'Keeffe Daniel M
3Com Corporation
Jaroenchonwanit Bunjob
Nixon & Vanderhye P.C.
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