A Ring Network with VLAN Tag

HIROSHI SHIMIZUNEC Corp, Japan

SUMMARY

The proposed Ring Network with VLAN Tag offersthe features of wrapping/steering control functions and 1+1path protection function, maintaining compatibility withthe Ethernet media access control scheme. The key technol-ogy for “Path concept” is VLAN tag swapping operation.A set of primary and backup paths is defined between ringnodes, which are distinguished by a flag bit in VLAN tagfield. On failure detection, the path is switched within thepath set by the tag swapping. Tag swapping at the failuredetection node, while tag swapping at the source nodeachieves steering operation, achieves Wrapping operation.The restoration behavior is almost the same as that ofResilient Packet Ring (RPR). Since the tag swapping con-trol is based on hardware processing, high-speed operationis also expected. Furthermore, because the paths are inde-pendently designed from the physical topology, the schemecan be applied to networks other than physical ring net-works. The proposed scheme will fit to the path control fornext-generation Ethernet over WDM system. © 2008 WileyPeriodicals, Inc. Electr Eng Jpn, 166(1): 60–66, 2009;Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20422

Key words: VLAN tag; tag swapping; ring net-work; RPR.

1. Introduction

A variety of new network technologies have beendeveloped aiming at both larger capacity and lower cost. Inparticular, the application of 10-Gbit Ethernet and otherEthernet technologies to MAN (Metropolitan Area Net-work) and WAN (Wide Area Network) is reported to beprogressing steadily [1]. Although there are also VPN serv-ices implemented on IP networks using the MPLS mecha-nism, Ethernet services are spreading in this country due to

their economic performance [3]. Specifically, the develop-ment of Ethernet QoS (Quality of Service) guarantee tech-niques makes possible better economic performance, thusdriving increased demand. In the future, we can expect theimplementation of high-standard wide-area Ethernet serv-ices offering not only large capacity and low cost, but alsoreliability and scalability, thus competing with conven-tional digital leased-line services. Actually, corporate VoIPservices, IP-TV conference and surveillance systems, andother IP services requiring guaranteed bandwidth are rap-idly spreading. In this context, higher standards of wide-area Ethernet services are necessary in order to expandbusiness based on video services.

In this study, we propose an Ethernet ring networkintended for metropolitan area applications such as ac-cess/backbone networks or community, campus, and pri-vate networks.

2. Metro L2 (Layer 2) Network Technology

First we give a brief explanation of the Metro L2technology.

2.1 Wide-area Ethernet

These techniques aim at the application of Ethernetto wide-area networks, while placing the emphasis on lowcost. In particular, the following functions were developedfor VLAN tag (IEEE 802.IQ) [4, 5].

1. Stacking [6–8] An extended VLAN tag managedby the operator is added outside the VLAN tag managed bythe user. This allows independent VLAN management bythe operator.

2. QoS control Bandwidth is secured for VLAN tagsto be traffic units. The development of LAN switchesprovided with this function has made possible advancedband services.

© 2008 Wiley Periodicals, Inc.

Electrical Engineering in Japan, Vol. 166, No. 1, 2009Translated from Denki Gakkai Ronbunshi, Vol. 125-C, No. 10, October 2005, pp. 1602–1607

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3. Swapping This function offering tag ID exchangewas developed for interconnection between wide-area Eth-ernet networks with different operators.

However, a spanning tree is usually employed forrestorative control in the case of network failure, but it isnot necessarily appropriate for metropolitan-area networksin terms of speed. Vendor-specific routing techniques weredeveloped to solve this problem, but they are inferior to the1+1 protection of about 50 ms offered by SDH.

2.2 Resilient packet ring (RPR)

RPR [9] is a ring network technology featuring fastrestorative control of about 50 ms. RPR uses MAC technol-ogy rather than Ethernet, and offers efficient utilization oftransmission bandwidth. In particular, as distinct fromSONET ring with dual backup configuration, MAC canactivate both systems. Furthermore, MAC supports wrap-ping for on-site fault recovery, and steering for recovery bytracing back to the transmission source. However, thesefunctions cannot be implemented across ring networks, andthus, the spanning tree must be employed for faults at relaynodes between rings, which considerably degrades per-formance (recovery times in seconds).

3. VLAN Tag Networks

In the aforementioned Ethernet-based wide-area net-works, QoS control, stacking and swapping are imple-mented as VLAN tag functions. However, there is noestablished method of integrating such functions, and thus,while aiming at low-cost large-capacity communicationservices, one still must rely on other systems, namely, onSDH for protection, and on RPR for high reliability control.

In this study, we propose a method of implementingvarious protection functions by Ethernet alone, by func-tional integration using the concept of VLAN tag paths.Actually, Ethernet technologies of network control play agreat role in systems using only Ethernet and WDM, suchas the Ethernet Over WDM systems considered as next-generation networks.

3.1 VLAN tag path topology and features

Regarding path topology, one can consider three op-tions, namely, point-to-point type with paths assigned toevery node pair, branch type with paths branching fromsource node to multiple target nodes, as shown in Fig. 1,and merge-type with paths gathering toward target nodes,as shown in Fig. 2. An important issue in path-based trafficassignment is the possibility of path management usinglabel information, in particular, route control. From this

standpoint, branch-type topology with the target unspeci-fied by the label information is not appropriate for routecontrol.

Another important point is that the number of pathsshould be made as small as possible in the case of 12-bitVLAN tags, which are short compared to the 20-bit labelsin MPLS. In this context, the point-to-point type requiresN(N – 1)/2 label paths for N nodes, while only N paths areneeded in the branch-type with VLAN tags assigned tosource nodes, and in merge-type with VLAN tags assignedto target nodes.

A comparison of different path topologies is shownin Table 1. It is seen that the merge-type topology offers thebest combination of required paths (VLAN count) androute control. In the merge-type path topology, as shown inFig. 2, the same path label is given to all packages addressedto node B, and the paths originating from nodes A, X, Y aremerged. Methods of setting VLANs for target nodes arebeing developed in the framework of GoE (Global onEthernet) technology [10, 11]. Extending this approach topath architecture, we here consider the merge-path topol-ogy, which supports route control. The proposed merge-type VLAN tag paths have the following features.

(1) As in MPLS, paths are based on statistical multi-plexing, and priority control is supported. In addition, as inSDH and ATM paths, bandwidth guarantee is possible.

Fig. 1. VLAN tag path topology.

Fig. 2. Ring network with merge-type path.

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(2) Path label values (VLAN tags) pertain to merge-type paths that specify target nodes.

(3) Route control independent of MAC address canbe implemented.

(4) Transmission paths and reception paths can be setup separately (asymmetrically).

(5) Path labels (VLAN tags) can be swapped tochange paths in the course of routing.

(6) Due to label swapping control, mutual traffic canbe established among disjoint networks without overlapbetween System 0 and System 1.

3.2 Configuration of VLAN tag network

Here we consider the case in which ring networksfunction as backbones of broadband access systems asshown in Fig. 2. Corporate users connected to the terminalsof broadband access systems are placed at the end nodes ofring networks, and the end nodes are connected via VLANtag network. At the end nodes, an extended VLAN field isset outside the user’s VLAN field (see the Ethernet MACframe in Fig. 7).

Two paths are set among the end nodes, clockwiseand counterclockwise, so as to avoid overlap. For example,path 1, shown by a solid line, and path 2, shown by a dottedline, are established between nodes A and B. Since pathsetting reduces to VLAN design, even networks that are notphysical rings can be treated logically as ring networksprovided that two nonoverlapping paths are set.

The path label information (VLAN ID) correspondsto the target node ID in unicast communications, and to thesource node ID in multicast communications. Therefore,packets on a ring network are removed from the ring at thetarget nodes in unicast communications, and at the sourcenodes in multicast communications. Every node is config-ured as shown in Fig. 3, being connected to the ring trans-mission path, namely, clockwise System 0 andanticlockwise System 1. Path receivers R0, R1, and theADD unit feeding user’s packets into LAN switch areprovided with the VLAN ID swapping function (path labelswitching).

3.3 Wrapping (loopback) control

This control is implemented by loopback functionsat the nodes neighboring a failed portion, providing fastrestorative control at the RPR level by wrapping at theneighbor nodes.

Usually, LAN switching devices are designed to pre-vent forward routes from looping, assuming that the datado not return to the same port. Thus, wrapped packets arenot forwarded unaltered. Therefore, VLAN ID values arerewritten as wrapping control takes place. Let us considerthis processing in more detail. Every node is assigned twonode IDs corresponding to the logical rings of System 0 andSystem 1, and the target node ID is related to the VLAN ID.For simplicity of explanation, suppose that these two IDsassigned to every node differ only in the least significant bit(LSB). Therefore, switching between System 0 and System1 is implemented by swapping of the LSB of VLAN ID.That is, the VLAN ID path on System 0 is transferred to theVLAN-ID path on System 1 by inversion of the LSB of theVLAN ID. When a link fault is detected downstream,receiver R0 of System 0 on the opposite side inverts the LSBto 1 for all received VLAN IDs. In addition, the LSB isinverted to 1 at the ADD as well. As a result of such control,packets that would usually be switched to transmitter X0 ofSystem 0 are all forwarded to transmitter X1 of System 1,and wrapped. The wrapped packets are routed on System 1to the target node via the source node.

This control is initiated only at the nodes adjoiningfailed portions in order to quickly switch tag paths (imple-mented as loopback). Packets are removed from the ring atthe target node, and hence the same packets are not receivedredundantly even though a loopback state occurs.

Now let us consider multicasting. In this case, thepackets make the rounds along rings, and are removed from

Fig. 3. Wrapping control by tag swapping.

Table 1. Path topology comparison

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the rings at the source nodes. Therefore, in multicast com-munications, two source node IDs are assigned as theVLAN IDs for System 0 and System 1. Since VLAN IDallocation is different in unicast and multicast communica-tions, a discrimination flag is set in the VLAN ID. Inmulticasting, the wrapped packets pass through the samenode twice, and thus are received twice. This is avoided bydefining a reverse flag. At a source node, the VLAN ID isset to multicast mode, and the reverse flag is set to 0. Thepackets are forwarded as usual as for a node adjoining thefailed portion. Simultaneously with wrapping (inversion ofthe tag’s LSB), the reverse flag is set to 1, and the normalmode is switched to the reverse mode. In the reverse mode,the intermediate relay nodes only pass packets through anddo not receive them. The packets are forwarded in reversemode to a node adjoining the fault on the opposite side. Atthat node, the LSB of the path label is inverted, the datareturn to the corresponding path of System 0, and thereverse flag is reset to 0. As a result, the normal mode isestablished between the wrapping node and source node.Due to the reverse mode, packets forwarded in the oppositedirection resume normal mode from the node adjacent tothe fault on the opposite side.

3.4 Steering control

Loopback at a fault neighbor node is implemented bywrapping control. However, superfluous traffic occurs inthe loopback interval from the wrapping node to the sourcenode. In steering control, System 0 is switched over toSystem 1 after the loopback state in order to control thesuperfluous traffic. In multicast communications, targetnodes may exist even in the loopback interval, and thereforesteering control is applied only to unicasting. Looped-backreverse packets are forwarded in reverse mode to the faultneighbor node on the opposite side, and there, in the course

of loopback control, the reverse flag is reset to 0 to restorethe normal mode. In the normal mode thus resumed, recep-tion is performed at every node. The looped-back reversepackets are monitored, and the VLAN-ID (that is, targetnode ID) is recorded. At the same time, at the ADD unit ofsource node, the LSB of the VLAN-ID (that is, the targetnode ID) is inverted, and system switching is performed.As a result, the wrapped traffic disappears, thus improvingthe usability of forward routes. When a fault is recovered,the neighbor node issues a notification in multicast mode.As a result, all source modes cancel the steering state andreturn to normal operation.

The control scheme described above can be summa-rized as follows.

(1) Wrapping control: At a fault neighbor node, thereverse flag is inverted and VLAN ID swapping control isperformed.

(2) Steering control: For unicast paths with a reverseflag of 1, the reverse flag is set to 1 at the source nodes, andVLAN ID swapping control is performed.

(3) Reception control: At every node, reception con-trol and removal from the ring is performed only for packetswith a reverse flag of 0, and packets with a reverse flag of1 are simply relayed.

3.5 1+1 protection switching control

In link aggregation control and wrapping control,bandwidth is not necessarily guaranteed in the case offaults. In addition, the management strategy may requireSDH-type protection functions. In this section, we explain1+1 protection control using tag swapping, referring to Fig.6.

All nodes are linked via System 1 (primary) andSystem 0 (backup), where signals propagate in the sameFig. 4. Wrapping control for multicast communication.

Fig. 5. Steering control for unicast communication.

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direction. When a link fault is detected in System 1, thereceiver R on the opposite side inverts the LSB in VLANID to 0. As a result, System 1 is switched over to System 0.For packets received from System 0, the LSB in the VLANID is set to 1 at the corresponding receiver R. Thus, packetsreceived in System 0 are returned to System 1. Conse-quently, tag swapping control makes possible protectionswitching control without using the SDH framework.

3.6 Priority control

Two kinds of service are provided, the bandwidthguarantee type and the best effort type. For users of theformer service, shaving/polishing is performed at the trans-ceiver of every node (ADD/DROP) according to the con-tracted bandwidth, and a high priority is assigned to thePriority field (see Fig. 7).

On the other hand, in best effort service, a low priorityis assigned. For bandwidth guaranteed traffic with diversioncontrol (wrapping or steering), a medium priority is as-signed simultaneously to system switching so as not toaffect nondiverted bandwidth guaranteed traffic.

3.7 Extended VLAN field and control

Figure 7 shows the frame configuration used to im-plement the aforementioned control. A 16-bit TCI fieldincluding the node ID corresponding to the proposedVLAN ID path is tagged as the extended VLAN field.Therefore, the user VLAN ID exists. The 12-bit VLAN IDspecifies the path, including discrimination between uni-cast and multicast, and System 0 and System 1, and whileflag B is used for wrapping/steering control. Flag A is a bitused to prevent packets from making infinite cycles. Since

node ID contains 8 bits, the largest number of nodes islimited to 250. In addition, extended TCI can be added toincrease the network scale. Since 20-bit node ID1 + nodeID2 is employed, a network of up to a million nodes can besupported.

The extended VLAN field has already been set inexisting wide-area Ethernet services. Thus, the field can beused for our purpose. In addition, under the RPR specifica-tion, a 16-bit ring header is added for ring control, whichcompares favorably with other technologies in terms oftransfer performance.

System switching is implemented by hardware-basedinversion control of the LSB of the VLAN ID, and thereforewe may expect high-speed RPR level performance (50 ms).

4. Architecture Model

In case of a multiple ring configuration, the systemcan be configured as a single logical ring. However, inlarge-scale networks, management by division into subsys-tems is desirable. The configuration shown in Fig. 2 isdivided into area rings interconnected via a core ring. As

Fig. 6. 1+1 Protection control with tag swapping.

Fig. 7. Ethernet frame with extended VLAN field.

Fig. 8. Architecture model.

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shown in Fig. 8(a), ring access among end nodes is imple-mented by means of three independent rings. As shown inFig. 9, another option is to apply 1+1 protection controlbetween relay (transit) nodes. In this case, the architecturemodel becomes that shown in Fig. 8(b). In contrast to 1+1protection using SDH, here protection extends to faults atrelay nodes. Control over the load distribution among edgenodes and link aggregation control are performed sepa-rately from low-level control. Thus, the VLAN ID paths donot depend on the physical network configuration, and thesame control mechanisms can be used for flexible responseto various needs.

5. Comparison with SDH and RPR

Here we compare the proposed network with SDHand RPR schemes; see Table 2.

In SDH, redundancy can be realized as 1+1 protec-tion or an SDH ring. However, dual-system links cannot beused simultaneously as a primary system, and one route isalways on standby, thus lowering efficiency. The RPRscheme does not work across rings, and therefore, when afault occurs at a relay node in the configuration shown inFig. 2, a spanning tree or some other technique has to beused. As a result, the RPR features are not used in full. Onthe other hand, in the proposed technology, rapid switching

within 50 ms can be performed for any link/node fault bytwo nonoverlapping VLAN systems between the sendingend and receiving end, regardless of the physical networkconfiguration (including multiple ring networks as shownin Fig. 9). In addition, efficient utilization of transfer chan-nels is assured by using both paths as a primary system. Inaddition, the only tag/label information to be managed isthe VLAN tag, and therefore, seamless network control canbe implemented regardless of whether Dark Fiber, SDH,ATM, or other low-level layers are used.

6. Conclusions

In this study we show that a ring-type logical networkcan be built regardless of the physical network shape byestablishing VLAN tag paths. System switching controlimplemented by swapping of L2 labels (VLAN ID) on thislogical network makes possible wrapping control and steer-ing control at the RPR level, and 1+1 protection controloffered by SDH. Since all control and management isconcentrated in the VLAN tag field of the Ethernet frame,we may expect flexible and simple network design andreliable management. The proposed technology may con-tribute to the further spread of wide-area Ethernet services.

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Fig. 9. Multiple ring network with 1+1 protection.

Table 2. Comparison with SDH and RPR schemes

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AUTHOR

Hiroshi Shimizu (member) graduated from the University of Tokyo in 1973, completed the M.E. program in 1975, andjoined NEC. His research interests are wide-area LAN, IVDLAN, frame relays, and routers.

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