5CC514 Network Routing and Switching
5CC514 Network Routing and Switching
Abstract
This report examines solutions for looping issues in layer 2 switched and layer 3 routed networks with redundant paths. The benefits of parallel connectivity for reliability and availability are weighed against inherent broadcast storm and switching loop risks. Rapid Spanning Tree Protocol (RSTP) remedies Ethernet looping topology through strategic link blocking and fast re-convergence. Enhancements like Multiple Spanning Tree Protocol (MSTP) improve complex environments and access port experience. At layer 3, equal cost multipathing (ECMP) balances traffic but can still route loop from errors. The Time to Live (TTL) field bounds endless packet cycles while Interior Gateway Protocols (IGPs) like Open Shortest Path First (OSPF) utilize sophisticated loop prevention mechanisms for stability. With well-designed protocols and configurations, networks can utilize redundant infrastructure resiliently. Further research could pursue optimizing layer 2 port roles for blocking selectivity while accelerating routing convergence through loop mitigation techniques. Careful interworking between enhanced spanning tree and integrated routing provides high availability parallel topologies with low looping risk.
Keywords:
Reliability, availability, broadcast storm, switching/routing loop, redundancy, Spanning Tree (RSTP/MSTP), convergence, re-convergence, access port, Time-to-Live (TTL), Interior Gateway Protocol (IGP), Open Shortest Path First (OSPF)
Introduction
Network redundancy through parallel paths is desirable to improve reliability and availability. By using the parallel redundency in networks and the implementation of the availability and validity helps to provide the automatic failover at the time of link failures. Without the implementation of the specific designing and the protocols , the broadcast storm may create in the multiple layers of 2 and 3 in the following paths. Inadvertently the routing loops and the switching loops also may create failure in links. Thus here in this report the analysis on the causes of the issues generated in the loops and the testing of the solutions provides the in-depth knowledge about the application. The multiple layer 2 and layer 3 paths can also create issues such as broadcast storms, switching loops and routing loops if not carefully configured. This report examines the causes and solutions for loops in layer 2 switching and layer 3 routing. This report explains the causes of looping issues and examines solutions in depth using enhanced layer 2 Spanning Tree protocols and tuned layer 3 routing.
Layer 2 Switching Loops
In layer 2 switched ethernet networks, a switching loop (or bridge loop) can occur when there are multiple paths between switches, forming a looped topology. This can happen by accident through cabling mistakes or intentionally by redundant links.
When the layer 2 network contains these looping paths, switches can continuously forward broadcasts in circles between one another [1]. The broadcasts multiply exponentially as they traverse the loop, flooding links with repetitive traffic. This exponentially growing broadcast storm overwhelms the available bandwidth, congesting connectivity for legitimate user traffic. Multicasting exhibit similar behavior in the presence of switching loops. Ultimately, the broadcast overload can exhaust switch resources leading to failed packet forwarding and crashing vital network devices. A switching loop allows broadcast frames to be continuously forwarded in a circle between switches, multiplying broadcast traffic with each iteration. This exponentially growing broadcast storm consumes available bandwidth, prevents legitimate traffic from flowing and can crash connected devices.
To mitigate layer 2 loops, Spanning Tree Protocol (STP) automatically blocks redundant paths to prevent the physical loops in the network [2]. STP views the layer 2 topology logically as a tree, with the root bridge at the base of a hierarchy and switches branching out similar to a tiered distribution layer. By exchanging bridge protocol data units (BPDUs), STP determines port roles and selectively blocks ports to prune the physical topology into a loop-free logical spanning tree. This tree structure forwards traffic along the fastest singular path while retaining redundancy. If links in the tree fail, STP recalculates and re-converges port roles to restructure the tree automatically.
Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP) evolve STP with faster convergence and features like root guard, to improve reliability and availability. Legacy STP can be slow to converge, so enhancements like Rapid Spanning Tree Protocol speed up topology changes using explicit handshaking instead of timers [3]. RSTP provides sub-second convergence in most environments. Further improvements like Multiple Spanning Tree Protocol (MSTP) allow tuning parameters for faster failover while preventing loops across multiple VLANs.
However, a downside of STP is that access ports directly connected to clients can be blocked to prevent loops. This disrupts connectivity when clients roam between wireless access points or dual-homed wall ports. Portfast can be enabled on access ports to skip the blocking delay when links activate. Root guard can prevent access ports from becoming the root bridge. Access ports directly facing end users can be configured with Portfast to skip the traditional listening and learning phases when first activated [4]. This prevents the 30-60 second delay to reach forwarding. Portfast disabled ports still listen first to prevent potential loops. Root guard can further configure access ports to prevent them from ever becoming the root bridge. This enforcement narrows the potential STP failure domain while allowing fast access port activation for improved end-user experience.
Layer 3 Routing Loops
In routed layer 3 networks, parallel paths enable load balancing and redundancy. However, routing loops can still occur when route advertisements propagate incorrectly. In layer 3 routed environments, parallel paths and equal cost multipathing (ECMP) allows routers to load balance traffic across multiple links between networks. This increases bandwidth, speed and redundancy. However, routing loops can still form at layer 3 despite the benefits of multipathing.
A routing loop forms when routers forward packets to the same destination back and forth endlessly. This congests links while preventing delivery. Routing loops most often arise from configuration errors with static routes or route redistribution. This can happen when route advertisements get corrupted propagating the layer 3 topology [5]. Routing protocol malfunctions or configuration mistakes often trigger sustained looping. When routing loops form, links carrying the looping traffic become severely congested. This disrupts connectivity and prevents the delivery of users’ legitimate application traffic.
For example, if router R1 has a static default route to router R3 configured via router R2 as the next hop, while R2 also has a static default back to R3 configured via R1, a forwarding loop is introduced [6]. Packets destined to the default route loop endlessly between R1 and R2 without reaching R3. This loop can overwhelm R1 and R2’s connectivity. Static route misconfigurations easily cause sustained looping problems compared to dynamic interior gateway protocols (IGPs).
IP routing implements the time to live (TTL) field to prevent packets looping endlessly in case of routing errors. Each router decrements the IP header TTL by one before forwarding. If a packet’s TTL reaches zero, it is discarded. The maximum TTL, typically 64 or 255, enables packets to traverse a reasonable number of hops before expiration. So while routing loops may form, TTL bounds the lifespan of packets trapped recycling in loops.
Furthermore, modern dynamic interior routing protocols utilize enhanced algorithms and loop mitigation to achieve rapid convergence and stability in layer 3 environments. For example, Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS) and Enhanced Interior Gateway Routing Protocol (EIGRP) all leverage Dijkstra’s algorithm to determine the shortest path with least cost routing [7]. They also deploy split horizon, route poisoning, and hold down timers to reduce instability from route withdrawals. Poison reverse even advertises loss of connectivity back out failed links. This means converged routing protocols can achieve robust multipathing, fast re-convergence and failure recovery with very rare or brief routing loops.
Conclusion
In conclusion, redundant layer 2 and layer 3 paths improve network resilience but can result in broadcast storms, switching loops and routing loops if poorly configured. STP mitigates layer 2 loops, and TTL bounds layer 3 looping. Well-designed protocols like RSTP and integrated routing handle parallel paths reliably. Modern spanning tree evolutions like RSTP and MSTP mitigate layer 2 issues. While TTL bounds layer 3 looping, integrated routing protocols converge quickly using enhanced algorithms.So network designers must weigh the benefits of redundancy against the risks of loops. The solution is not avoiding parallel paths entirely but managing them intelligently by topology and protocol.This report examined causes and solutions for layer 2 and layer 3 looping issues. Further research could explore optimising redundancy designs and tuning protocols for fastest loop recovery. With careful configuration, parallel paths can enhance network availability despite inherent looping risks.
Reference List
Journals
[3] Sangeetha, Y. and Narayanan, K., 2023. Physical layer link quality metrics-based stable routing for QoS enhancement in adhoc network. Applied Nanoscience, 13(3), pp.2393-2403.
[4] Sharma, A.K., Wadhawan, S., Datta, R. and Mittal, S.K., 2022. Inter-networking: An Elegant Approach for Configuring Layer-2/Layer-3 Devices for Attaining Impulsive Outcomes. In Sustainable Communication Networks and Application: Proceedings of ICSCN 2021 (pp. 1-27). Singapore: Springer Nature Singapore.
[5] Al-Heety, O.S., Zakaria, Z., Ismail, M., Shakir, M.M., Alani, S. and Alsariera, H., 2020. A comprehensive survey: Benefits, services, recent works, challenges, security, and use cases for sdn-vanet. IEEE Access, 8, pp.91028-91047.
[6] Kumar, M. and Dhiman, P., 2023. Network Packet Analyzer.
[7] Lamba, K., Rawat, A., Sharma, S. and Jain, P., An Analysis based on Comparative Study of Routing Protocols in MANET. International Journal of Engineering Research & Technology (IJERT) ISSN, pp.2278-0181.
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