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TECHNICAL SCIENCES
UNDERSTANDING AND DESIGNING MODERN HIGH-RELIABILITY
NETWORKS USING SPANNING TREE PROTOCOL 1 2 Zemtsov A.N. (Russian Federation), Tran Dung Khanh (Socialist Republic
of Vietnam) Email: Zemtsov560@scientifictext.ru
1Zemtsov Andrey Nikolaevich - Candidate of Technical Sciences, Assistant Professor, DEPARTMENT OF ELECTRONIC COMPUTERS AND SYSTEMS, VOLGOGRAD STATE TECHNICAL UNIVERSITY, VOLGOGRAD; 2Tran Dung Khanh - MSc in Computer Science, Lecturer, SCHOOL OF INFORMATION TECHNOLOGY AND DIGITAL ECONOMICS NATIONAL ECONOMICS UNIVERSITY, HANOI, SOCIALIST REPUBLIC OF VIETNAM
Abstract: the purpose of this paper is to describe as Spanning Tree allows to provide a loop-avoidance mechanism in modern high-reliability networks. A redundant switch topology causes broadcast storms, multiple frame copies, and MAC address table instability problems who destroying network performance. Spanning Tree provides a means of redundancy between multiple switches in a modern network. Spanning Tree Protocol based completely on the standard IEEE 802.1D. In addition, a major understanding of Spanning Tree valuable is necessary for deploying scalable, multilayer switched networks.
Keywords: redundant links, loops, Spanning Tree Protocol, IEEE 802.1D, availability.
ПРОЕКТИРОВАНИЕ СОВРЕМЕННЫХ ВЫСОКОНАДЕЖНЫХ
СЕТЕЙ С ИСПОЛЬЗОВАНИЕМ ПРОТОКОЛА
ПОКРЫВАЮЩЕГО ДЕРЕВА 12 Земцов А.Н. (Российская Федерация), Чан Зунг Хань
(Социалистическая Республика Вьетнам)
1 Земцов Андрей Николаевич - кандидат технических наук, доцент, кафедра электро-вычислительных машин и систем, Волгоградский государственный технический университет, г. Волгоград; 2Чан Зунг Хань - магистр техники и технологии, преподаватель, Школа информационных технологий и цифровой экономики Национальный экономический университет, г. Ханой, Социалистическая Республика Вьетнам
Аннотация: целью данной статьи является анализ того, как покрывающее дерево позволяет обеспечить механизм предотвращения петель в современных высоконадежных сетях. Избыточная топология сети на основе коммутаторов может стать причиной возникновения широковещательных штормов, размножения кадров данных и проблем со стабильностью таблицы MAC-адресов, снижающих производительность сети. Выделение покрывающих деревьев позволяет обеспечить резервирование между несколькими коммутаторами современной сети. Протокол покрывающего дерева основан на стандарте IEEE 802.1D. Кроме того, понимание важности применения концепции покрывающих деревьев является необходимым для развертывания масштабируемых многоуровневых коммутируемых сетей.
Ключевые слова: резервные каналы, петли, протокол покрывающего дерева, остовное дерево, IEEE 802.1D, доступность.
Redundant links are created to increase availability. However redundant links cause problems such as broadcast storms, frame duplication, and MAC table thrashing [1]. As an example, consider the topology of 3 switches connected in a ring to provide redundancy as shown in Figure 1.
If Host 1 generates a broadcast message destined for Host 2, it is received by switch A which in turn transmits it throughout all of its remaining interfaces. Upon receiving the broadcast, switch C will also do the same and the broadcast will reach Host 2. However, switch B will also forward the frames received from switch A to switch C thus resulting in a loop referred to as a broadcast storm.
B
Host 1 Host 2
Fig. 1. Broadcasting storm in a redundant topology
Switch A is connected to Host 1 and switch B. Switch C is connected to switch B and Host 2. If Host 1 wants to transmit a unicast frame to Switch A, it will forward it to switch B and C. Switch C, in turn, will forward it to the switch B and the Switch A will receive the frame. However, switch B will also forward the frame to switch C and switch C will forward it to the Switch A. Consequentially the Switch A will receive two identical frames with the same source address.
To prevent loops and problems that come with them, switches use spanning tree protocol to manage redundant links at layer 2 [2]. This protocol ensures automatic failover to the backup link without requiring manual activation. To best understand the operation of STP, some key terms have to be defined [3].
BPDU - Bridge Protocol Data Unit is a frame that is multicast by all switches. This frame contains information about a switch and its interfaces. By default, these frames are transmitted every 2 seconds. Two types of BPDUs exist i.e. configuration bridge protocol data unit and topology change bridge protocol data unit. The former is transmitted by switches to learn the network topology. The later is transmitted to trigger the initiation of STP after a change in the topology has been detected.
Root Bridge - the Root Bridge (RB) serves as the reference point for switches participating in the spanning tree process and all decisions are made from the perspective of the RB. The switch with the lowest switch ID is selected as the RB. The switch ID is determined from the priority of a switch and its mac address. By defaul, the priority is 32768 and if this value is not changed, the value with the lowest mac address will be elected as the RB. By default, if BPDUs are not received from the RB in 20 seconds, a new election occurs.
Non-Root Bridge - this is the term used to refer to all other switches except the RB.
Port Cost - this is the cost assigned to a port by STP. This value is used when calculating the best route when redundant links are available between two switches. The default value of the Port
Cost is determined from the bandwidth of the connected link. The route with the least cost is selected for forwarding frames.
Path Cost - this is the value used by STP to select the best path to the root bridge. It is the accumulated value of all the port costs from the switch to the RB. By default, the path cost at the RB is 0 and this value is set on the outgoing BPDU. Upon receiving the BPDU, the switch adds its incoming port cost to the path cost before forwarding the frame.
Root Port - this is the port that has the lowest path cost to the root bridge. Since link bandwidths are considered in this calculation, this may not necessarily be the shortest path but it guaranteed to be the fastest.
Designated Port - the port with the least port cost is selected as the designated port and is responsible for forwarding traffic.
Non-Designated Port - this port has a port cost higher than the designated port and is put in the blocking state. This port remains blocked to prevent loops.
During operation, the protocol can move from one state to another [4]. The protocol state graph is shown in Figure 2.
Fig. 2. STP State transitions
Disabled: The switch is administratively down and it does not receive or forward traffic. The switch is also not participating in the STP election process.
Blocking: when in this state, the switch only processes BPDU frames. Any other Ethernet frames are dropped. At this stage, the switch is trying to determine which port will act as the RB and designated ports and which ports will remain in the blocking state to remove loops. By default, the switch goes to the blocking state when powered on and will remain in this state for 20 seconds.
Listening: after the max age (20 seconds by default), the root and designated ports will transition to the listen to state. The switch only processes BPDUs and all other frames are discarded. Ports remain in this state for a period of time equal to 15 seconds by default [5]. When in this state, the switch will check the topology for loops.
Learning: in this state, the switch starts processing Ethernet frames and it examines the source address of arriving frames and it updates its MAC address table. Forwarding still doesn't take place
at this stage. Designated port and Root Bridge enter this state from the listening state and stay in this state for 15 seconds.
Forwarding: in this state, a port will forward all Ethernet frames received as well as processing of BPDUs. The MAC address table is updated using the source addresses of the received frames.
Loops can create broadcast storms who destroying network performance. Spanning Tree provides a means of redundancy between multiple switches in a modern network. Spanning Tree Protocol based completely on the standard IEEE 802.1D. In addition, a major understanding of Spanning Tree valuable is necessary for deploying scalable, multilayer switched networks.
References / Список литературы
1. Ahmed W. et al. Reliability modeling and analysis of communication networks. Journal of Network and Computer Applications 78, 2017. Pp. 191-215.
2. Zobel W., Khansa L. Quantifying Cyberinfrastructure Resilience against Multi-Event Attacks. Decision Sciences Journal 43 (4), 2012. Pp. 687-710.
3. CCNA Routing and Switching Practice and Study Guide: LAN Redundancy. [Electronic Resource]. URL: http://www. ciscopress.com/articles/article.asp?p=2204384&seqNum=4/ (date of access: 17.06.2019).
4. Kurose J.F., Ross K. W. Computer Networking A Top-Down Approach. Pearson, 2013.
5. Oppenheimer P. Top-Down Network Design, 3rd edn. Cisco Press, Indianapolis, 2011.
