The VTP server switch propagates the VTP database to VTP client switches. New technologies such as 802.1x and VLAN assignment and Cisco Network Admission Control with quarantined VLAN, must be used with transparent mode. Figure 28 Mismatched Transmit/Receive Pairs. This results in fast, deterministic convergence in the event of a link or node failure. Another technique used multiple HSRP groups on a single interface and used DHCP to alternate between the multiple default gateways. To reduce whatever risk this attack may pose, set the native VLAN to an obscure ID that is not used for any real traffic in the network. Figure 14 Summaries Stop Queries at the Core. •Use Rapid PVST+ to protect against user-side loops. In general practice, the most deterministic and best-performing networks in terms of convergence, reliability, and manageability are free from L2 loops and do not require STP to resolve convergence events under normal conditions. The second document, High Availability Campus Recovery Analysis, provides extensive test results showing the convergence times for the different topologies described in this document, and is available at the following website: http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/HA_recovery_DG/campusRecovery.ht… EIGRP stub nodes are not able to act as transit nodes and as such, they do not participate in EIGRP query processing. •Tune EtherChannel and CEF load balancing to ensure optimum utilization of redundant, equal-cost links. Two types of trunks are currently available: 802.1Q is the Institute of Electrical and Electronics Engineers (IEEE) standard implementation. it uses Cat-5 twisted pair cables. For example, an Internet worm infection, such as Slammer, can cause congestion on many links in the network, and QoS can minimize the effect of this event. ), Figure 23 Virtual Trunk Protocol Operation. Enterprise edge: The enterprise edge aggregates the connectivity from the various elements at the edge of an enterprise network. They might also upgrade wiring to meet the requirements of emerging applications. Networking for a medium campus is designed for high availability, performance, and manageability. We specialize in installing network data cables, structured cabling, and design of physical data network topology that support CAT5, CAT6, & CAT7 technologies. On links between a CatOS device and a Cisco IOS software device, you should disable PAgP negotiation if EtherChannel tunnels are not required. The best practice using Cisco IOS software is shown in the following configuration snippet: Gateway Load Balancing Protocol (GLBP) protects data traffic from a failed router or circuit, like HSRP and VRRP, while allowing packet load sharing between a group of redundant routers. Because of this small amount of memory, the potential for dropped traffic because of Tx-queue starvation is relatively high. Suppose that each "node" represents 100 computer workstations. STP/RSTP convergence is required for several convergence events. The network backbone consists of a L3 switch. In a topology with three switches, the answer depends on many factors. GLBP provides HSRP-like redundancy and failure protection. EdrawMax is an advanced all-in-one diagramming tool for creating professional flowcharts, org charts, mind maps, network diagrams, UML diagrams, floor plans, electrical diagrams, science illustrations, and more. The bottom tier, the access layer, comprises switches that support end-user devices. This happens as equal-cost load sharing begins to occur and return path traffic starts to flow through the node, regardless of HSRP state because this is for return path traffic. The "Campus" is where USERS (employees) connect to the network, along with all of the devices those employees use (e.g. Even though bandwidth capacity has increased to 1 Gbps, multiples of 1 Gbps, and even 10 Gbps, it is still impractical to provide enough bandwidth to run an entire access layer switch full of ports at line rate at the same time. The throttles that OSPF places on LSA generation and SPF calculation can cause significant outages as OSPF converges around a node or link failure in the hierarchical network model. A campus network is generally the portion of the network infrastructure that provides access to network communication services and resources to end users and devices that spread over a single geographic location. There should be no need to redesign the whole network each time a module is added or removed. If your applications require spanning VLANs across access layer switches and using STP as an integral part of your convergence plan, take the following steps to make the best of this suboptimal situation: •Use Rapid PVST+ as the version of STP. However, this is no longer a concern in campus topologies with current hardware. Unless you vary the decision input for the CEF hashing algorithm at the core and distribution layers, CEF polarization can result in under-utilization of redundant paths. Most campus networks feature a high-performance, switched backbone, called the campus backbone, that connects buildings and different parts of the campus. The necessary equipments and appropriate topology required for the campus network design along with the IP address schema, IP address management, secure wireless access, internet sharing, features and services should be worked out. Typical traffic flows in the campus cross multiple redundant paths as traffic flows from the access layer across the distribution and core and into the data center. In this topology, SSO provides for protection against supervisor hardware or software failure with 1-3 seconds of packet loss and no network convergence. This requirement is discussed in detail in the next section. VPTv3 contains many enhancements for security and reliability. Additionally, you can use QoS to reduce the priority of unwanted traffic. The campus backbone provides redundant and fast-converging connectivity. Although individual purchase decisions might seem harmless, network designers must not forget that this separate equipment still works together to form a network. Cisco switches let you tune the hashing algorithm used to select the specific EtherChannel link on which a packet is transmitted. As a result, no additional end stations are affected by the flooded traffic (see Figure 52). Hot Standby Routing Protocol (HSRP) provides fast recovery of link failures. •Routed Access—This option is interesting from a convergence performance perspective, but is not yet widely deployed. For the same events, where 60-200 milliseconds of packet loss occurred without redundant supervisors when dual supervisor nodes were used in the core or distribution, 1.8 seconds of loss was measured. Whether you need to retrofit an existing office space or lay cable in a new construction project we can help make the process simple. Routing integrates these switched networks, and provides the security, stability, and control needed to build functional and scalable networks. IGMP snooping helps control multicast packet flooding for multicast applications. At the time of this writing, test results show that EIGRP is better suited to a campus environment than OSPF. Figure 13 Convergence Around a Failed Node. L3 recovery using NSF happens after the SSO convergence event, minimizing L3 disruption and convergence. If an L3 link between the distribution nodes is not present, return traffic (from the core to the access layer) could be dropped if an access layer link fails and the distribution nodes are not interconnected with an L3 link, as shown in Figure 14. Consequently, it was rare to make a hardware design mistake. ATA-FL serving Florida Health Plans & Therapists since 2006. Tree topology. This area contains all the network elements for independent operation within one campus location. Rapid PVST+ provides the rapid convergence of 802.1w while avoiding the complexity of 802.1s. These techniques worked but were not optimal from a configuration, maintenance, or management perspective. When you configure switch-to-switch interconnections to carry multiple VLANs, set DTP to on/on with no negotiate to avoid DTP protocol negotiation. When a distribution is re-introduced to the environment, there is no disruption of service as compared to the four-second outage measured in the 40-node test bed for the L2/L3 distribution layer boundary topology. However, there are many opportunities to increase your availability and optimize convergence with alternative designs. •L2/L3 distribution with HSRP or GLBP is a tried-and-true design. Results vary depending on the size of the L2 domain supported by the distribution pair. Figure 32 Port Aggregation Protocol Operation. •Layer 2 Loop-Free—This is the time-tested solution. Star Topology. PAgP or LACP enable the automatic formation of EtherChannel tunnels between interconnected switches (see Figure 32). While PVST+, Rapid PVST+, and EIGRP all converged in less than one second (EIGRP in sub 200 ms), OSPF required at least 1.65 seconds to converge around this specific failure. This behavior caused a considerable amount of traffic being dropped; more than 40 seconds in the tested topology. Summarization is required to facilitate optimum EIGRP or OSPF convergence. The following configuration snippets demonstrate how EIGRP was configured to achieve sub-200ms convergence for link and node failure scenarios. Ensure that the distribution node has connectivity to the core before it preempts its HSRP/GLBP standby peer so that traffic is not dropped while connectivity to the core is established. The distribution layer provides default gateway redundancy using the Gateway Load Balancing Protocol (GLBP), Hot Standby Router Protocol (HSRP), or Virtual Router Redundancy Protocol (VRRP). Building distribution submodule: The job of this submodule is to aggregate wiring closets within a building and provide connectivity to the campus backbone via routers (or switches with routing modules). This submodule provides routing, QoS, and access control methods for meeting security and performance requirements. The distribution switches become ABRs with their core-facing interfaces in area 0 and the access layer interfaces in unique totally stubby areas for each access layer switch. Its concepts, however, can be applied to any network, including Data Centers. (See Figure 23. •When something goes wrong, how do you find the source of the problem? Cisco has incorporated a number of these features into the following versions of STP: •Per-VLAN Spanning Tree Plus (PVST+)—Provides a separate 802.1D spanning tree instance for each VLAN configured in the network. •Client—Receives updates but cannot make changes. For example, higher-speed technologies-such as Fast Ethernet, Gigabit Ethernet, and ATM as a backbone architecture-and Layer 2 switching provide dedicated bandwidth to the desktop. You can use BPDU Guard to stop all bridge devices, such as switches, from being added to your network. A routing protocol like EIGRP, when properly tuned, can achieve better convergence results than designs that rely on STP to resolve convergence events. From a connectivity perspective, some network designers recommend dual distribution nodes that are individually connected to a single core node member. However, adding redundant supervisors to redundant core and distribution layers of the network can increase the convergence time in the event of a supervisor failure. A campus backbone must provide access to management devices that support monitoring, logging, troubleshooting, security, and other common management functions. This L2 looped topology is configuration and management intensive. In the 3750 family of stackable switches, you can create a cross-stack channel where members of the EtherChannel exist on different members of the stack, yielding very high availability. In the campus, as we transition from 10 Gbps or 1 Gbps to 10/100 Gbps to the desktop, packets must be queued as they wait to serialize out the 10 or 100 Mbps link. The recommendation is 4:1 for the distribution-to-core links. Figure 55 Convergence Events with an Uplink Failure. When redundant paths are present, failover depends primarily on hardware link failure detection instead of timer-based software failure detection. You can minimize this by using RFC1918 private address space and Variable Length Subnet Masking (VLSM). You might think that completely removing loops in a topology that requires the spanning of multiple VLANs across access layer switches might be a good thing. 3. However, emerging applications like these are built upon the campus foundation. Increasingly, these two networking functions are being integrated into common platforms. You don’t design a network topology based on the type of institution it will be used in, but rather based on the internal structure of the building. Routing protocols are utilized in a hierarchical network design to reroute around a failed link or node. Additionally, it should be noted that in soft failure conditions where keepalives (BPDU or routing protocol hellos) are lost, L2 environments fail open, forwarding traffic with unknown destinations on all ports and causing potential broadcast storms; while L3 environments fail closed, dropping routing neighbor relationships, breaking connectivity, and isolating the soft failed devices. During a transition from LAN to WAN, a router has to make the rate transition from 10/100 Ethernet to WAN speeds. Use StackWise technology in the Cisco Catalyst 3750 family or modular chassis implementations to avoid these complications. With standard STP, this can take as long as 50 seconds. A specific situation can cause considerable periods of packet loss during channel negotiation when mixing CatOS in the access layer and Cisco IOS software in the distribution layer. This document is the first in a series of two documents describing the best way to design campus networks using the hierarchical model. In the topology tested, the recovering distribution node had not fully established connectivity to the core, yet it was distributing a default route to the access layer switch. In most cases, network redundancy is not the top priority, but cost effectiveness is. When connecting a Cisco IOS software device to a CatOS device, make sure that PAgP settings are the same on both sides. The ability of EIGRP to provide route filtering and summarization maps easily to the tiered hierarchical model, while the more rigid requirements of OSPF do not easily integrate to existing implementations and require more complex solutions. Additionally, this option is not as widely deployed in the field as the L2/L3 distribution layer boundary model. The second scenario presents a problem. This means that the core does not know that it cannot send traffic to the distribution member where the link has failed. This removes any possibility that a double 802.1Q-tagged packet can hop VLANs. These can easily happen by accident because of misconfigured hosts. The CAM timer expires because no traffic is sent upstream towards the standby HSRP peer after the end point initially ARPs for its default gateway. In this topology, no VLANs span access layer switches and the distribution layer interconnection is an L3 point-to-point link. In this case, L2 loops are common (see Figure 20). As discussed earlier in this document, you should summarize at the distribution layer towards the core layer to stop EIGRP queries from propagating beyond the core of the network. At the time of this writing, there is no workaround for this situation except using normal areas instead of totally stubby areas for the access layer switches. When a link or node has failed, an OSPF peer cannot take action until this timer has expired. Additionally, the media types common in the access layer are not susceptible to the same half up or rapid transitions from up to down to up (bouncing) as are those commonly found in the WAN. With OSPF in the same topology, the default route is propagated to the totally stubby peer (the access layer switch in this case) when the neighbor relationship is established, regardless of the ability of the distribution node to forward traffic to the core. As stated earlier, this problem only occurs in a topology where VLANs span multiple access layer switches in a large L2 domain. The end result is that for return path traffic, the distribution node that is coming back online can not resolve all the IP to MAC addresses for the L2 domain that it supports for a considerable period of time. The difference between a WAN router and a campus switch is the number of interfaces and the amount of memory associated with each. An enterprise can have more than one campus. STP is also required to protect against inadvertent loops introduced on the user side or end point-facing access layer ports. If this happens, traffic can be dropped until full connectivity is established. You must consider this limitation before selecting OSPF as a routing protocol in campus environments. Additional requirements of these designs typically include: The Cisco solution provides high-performance switched infrastructure for a building-sized intranet with hundreds of networked devices. You should enable UDLD in global mode so you do not have to enable it on every individual fiber optic interface. The hardware and software attributes of the access layer that support high availability include the following: •System-level redundancy using redundant supervisor engines and redundant power supplies. –By default, one of the possible adjacencies is selected by a hardware hash where the packet source and destination IP address are used. Additionally, in a less than optimal design where VLANs span multiple access layer switches, the distribution nodes must be linked by an L2 connection. Use the following command to disable PAgP negotiation: Additionally, port aggregation should be disabled on interfaces facing end users. It is … Additionally, the distribution-to-distribution link is an L3 routed link. The Catalyst 6500 also provides L3 Non-Stop Forwarding (NSF), which allows the redundant supervisor to assume L3 forwarding responsibilities without resetting or re-establishing neighbor relationships with the surrounding L3 peers in the event of the failure of the primary supervisor. As with Trunking/DTP, the long-standing practice for EtherChannel/PAgP has been to set one side of the interconnection (typically the access switch) to auto and the other side (typically the distribution switch) to desirable. View with Adobe Reader on a variety of devices, High Availability Campus Recovery Analysis, "Using HSRP, VRRP, or GLBP for Default Gateway Redundancy" section, "Gateway Load Balancing Protocol" section, http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html. The corresponding access layer switches also do not have a CAM entry for the target MAC, and they also broadcast the return traffic on all ports in the common VLAN. In the reference hierarchical design, L2 links are deployed between the access and distribution nodes. Use Loop Guard to protect the network from a soft failure where physical connectivity and packet forwarding are intact but STP (BPDU generation, forwarding, and evaluation) fails. Figure 41 GLBP with STP Blocking Distribution-to-Distribution Link. It has a root node and all other nodes are connected to it forming a hierarchy. A common practice is to set one side of the interconnection (typically the access) to auto and the other end (typically the distribution) to desirable. VTP runs only on trunks and provides the following four modes: •Server—Updates clients and servers. Highly available networks require redundant paths to ensure connectivity in the event of a node or link failure. Before the development of GLBP, methods used to utilize uplinks more efficiently were difficult to implement and manage. Figure 8 Triangle and Square Network Topologies. Congestion on a Cisco Catalyst switch interface is not typically caused by oversubscription or an anomaly such as an Internet worm. Adding an L3 link between the distribution switches allows the distribution node that loses connectivity to a given VLAN or subnet to reroute traffic across the distribution-to-distribution link. Video Network [PDF] Diagram of the topology of the video network for … Figure 26 Double 802.1Q-Encapsulated Packets. During periods of congestion, scavenger-class traffic is the first to experience Tx-queue starvation and packet loss because the bandwidth is reserved for higher priority traffic. On the interface facing the primary root switch, the following Cisco IOS command was entered in interface configuration mode to accomplish the desired effect. Some form of redundancy is required because this environment can be large and a considerable outage could occur if the device acting as default gateway failed. When an end point ARPs for its default gateway, the virtual MACs are checked out on a round-robin basis. Use the default L3 information for the core nodes and use L3 with L4 information for the distribution nodes. The first challenges this network design will face is economic and budget issue. The topology of the network from the distribution layer to the access layer is logically a hub-and-spoke topology, which reduces complexity of design and troubleshooting. Because of these two differences, you can safely tune the OSPF timers (hello, dead-interval, and SPF) to their minimum allowable values of 1, 3, and 1 second, respectively. Finally, VTP transparent mode should be used because the need for a shared common VLAN database is reduced. The following configuration example shows how to change the EtherChannel input algorithm on a Cisco Catalyst 6000 Series switch using CatOS. Additionally, because both EIGRP and OSPF load share over equal-cost paths, this provides a benefit similar to GLBP. • Configure all edge access layer switches to use EIGRP stub. It includes the following topics: •Layer 2 Redundancy—Spanning Tree Protocol Versions, •Protecting Against One-Way Communication with UniDirectional Link Detection, •Link Aggregation—EtherChannel Protocol and 802.3ad, •Using HSRP, VRRP, or GLBP for Default Gateway Redundancy, •Ensuring Connectivity in Case of Failure, •Tuning Load Balancing with Cisco Express Forwarding. •What links should be in a blocking state? Ethernet This is wired LAN technology. This ensures that the HSRP primary distribution node has established full connectivity to all parts of the network before HSRP preemption is allowed to occur (see Figure 35). It has a low start-up cost, and it is easier to add more devices to the network. The recommended best practice is to measure the system boot time, and set the HSRP preempt delay statement to 50 percent greater than this value. •Rapid PVST+—Provides an instance of RSTP (802.1w) per VLAN. You can edit this Network Diagram using Creately diagramming tool and include in your report/presentation/website. This tuning can save seconds of outage when restoring a failed link or node. •Do not extend area 0 to the edge switch. As a side effect, a convergence event on the uplink or on the primary distribution node affects only half as many hosts, giving a convergence event an average of 50 percent less impact (see Figure 39). Figure 39 GLBP, HSRP, and VRRP Test Results. As such, you can safely configure each access layer switch into its own unique totally stubby area. You can use QoS policies to protect mission-critical applications while giving a lower class of service to suspect traffic. The Enterprise Composite Network Model is a blueprint that network designers can use to simplify the complexity of a large internetwork. Additionally, you must consider the tradeoffs between totally stubby areas and regular areas for the access layer. Figure 60 Fully Routed Solution with Point-to-Point L3 Links. When spanning-tree convergence is required, Rapid PVST+ is superior to PVST+ or plain 802.1d. However, a routed access layer topology is not a panacea. Although geographical sites can appear on the map, the purpose of the map is to show the geometry of the network, After all, this eliminates the dependence of convergence on STP/RSTP. The networking equipments (switches, routers) and transmission media (optical fiber, copper plant, Cat5 cabling etc.) Therefore, when tuning for optimum performance, disable PAgP and set the channel members to on/on. Network redundancy are provided to clients and servers. Campus network is the category of network that lies between a local area network that services a single location and a wide area network services locations across a large geographical area. The building blocks of modular networks are easy to replicate, redesign, and expand. This is a benefit, however it makes this design less flexible than other configurations. You can create channels containing up to eight parallel links between switches. Convergence based on these functions, which are implemented in hardware, is the most deterministic. The following are general design considerations: •Use HSRP or GLBP for default gateway redundancy (sub-second timers). To avoid this situation the Spanning Tree environment must be tuned so that the L2 link between the distribution switches is the blocking link while the uplinks from the access layer switches are in a forwarding state. Therefore, make sure you maintain connectivity when applying this configuration. While this is not optimum, it is also not detrimental from the perspective of outbound traffic. It is possible for HSRP neighbor relationships to form and preemption to occur before the primary switch has L3 connectivity to the core. This section describes why QoS is needed and discusses specific cases where QoS is most beneficial. Link up/down topology changes can be propagated almost immediately to the underlying protocols. This section includes the following topics: You can use the hierarchical model to design a modular topology using scalable "building blocks" that allow the network to meet evolving business needs. Using these oversubscription ratios, congestion on the uplinks occurs by design (see Figure 42). A minimal configuration in the core reduces configuration complexity limiting the possibility for operational error. CatOS devices should have PAgP set to off when connecting to a Cisco IOS software device if EtherChannels are not configured. Boggs in the year 1970. The design principles and implementation best practices described in this document are tried-and-true lessons learned over time. PAgP has four modes related to the automatic formation of bundled, redundant switch-to-switch interconnections: •On—Always be an EtherChannel tunnel member, •Desirable—Request that the other side become a member, •Auto—Become a member at the request of the other side. A topology is a map of an internetwork that indicates network segments, interconnection points, and user communities. Support for applications based on Novell IPX, DECnet, AppleTalk, and SNA. From an STP perspective, both access layer uplinks are forwarding, so the only convergence dependencies are the default gateway and return path route selection across the distribution-to-distribution link (see Figure 58). Topologies where point-to-point physical links are deployed provide the most deterministic convergence. In this figure you can see that this campus network has two buildings that are both three stories tall. When using the on/on setting, PAgP is not enabled on members of the bundle. After the STP/RSTP convergence, the Access-b uplink to the standby HSRP peer is used as a transit link for Access-a return path traffic. This three-tier architecture is the most scalable and resilient solution. The campus network is mostly owned by an enterprise, university, government, etc. The following configuration snippets illustrate the OSPF configuration: The design recommendations described in this design guide are best practices designed to achieve the best convergence possible. See "Using HSRP, VRRP, or GLBP for Default Gateway Redundancy" section for more details on default gateway redundancy. VTP is an essential component of VLAN Trunking. This model reduces peering relationships and interface count at the core. A medium campus consists of one large building or several buildings. The high performance collapsed backbone uses layer three switching. Return path traffic has a 50/50 chance of arriving on a distribution switch that does not have physical connectivity to the half of the stack where the traffic is destined. When an indirect failure is detected and STP/RSTP converges, the distribution nodes reestablish their HSRP relationships and the primary HSRP peer preempts. It was created by Bob Metcalfe and D.R. This document presents recommended designs for the campus network, and includes descriptions of various topologies, routing protocols, configuration guidelines, and other considerations relevant to the design of highly available and reliable campus networks. Campus network can be additional to the set of wireless connections, connect several buildings to the same network, but it's not the same thing. Topologies with redundant equal-cost load sharing links are the most deterministic and optimized for convergence measured in milliseconds. In the recommended topologies, the same VLAN should not appear in any two access layer switches. A CAN is also known as a corporate area network (CAN). Figure 41 illustrates the STP topology after changing STP port cost on the secondary root switches interface facing the primary root switch (the distribution to distribution link) allowing traffic to flow up both uplinks from the access layer switches to both GLBP Virtual MAC addresses. The rule-of-thumb recommendation for oversubscription is 20:1 for access ports on the access-to-distribution uplink. The benefits of dynamic propagation of VLAN information across the network system drawings STP to resolve convergence events a... Can cause unexpected and unwanted Internal gateway Protocol ( VTP ) in transparent mode to and! Edit this network design 39 GLBP, HSRP, or malicious users can create a Loop can be introduced the! 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Design mistake by reducing serviceability and determinism improves the detection of the campus backbone including voice video! 802.1Q trunking on any ports that are required not often deployed today link! •L2/L3 distribution with HSRP or GLBP for default gateway redundancy and high availability, performance, and provides following! And normal reroute processing is completed campus-area network topology IOS supports QoS, and SPF timers to 1 3!, with some manual configuration required a WAN router and a campus to... The development of GLBP, methods used to select the specific EtherChannel on! Promoting deterministic traffic patterns represents 100 computer workstations uplinks are available to actively forward traffic, multiple convergence events a! The ports in question Diagram using Creately diagramming tool and include in your report/presentation/website export, print, and network... Network topologies with hundreds of networked devices save seconds campus network topology packet loss and no network convergence topology. Root Guard are tools that can not currently be tuned below one second, HSRP, GLBP... Information from the distribution switches to avoid VLAN hopping attack by making it to. Process to make its final forwarding decision: 1 topology adds several orders of magnitude in.. Switches or workstations running a version of STP, this eliminates the dependence of on... Which optimizes EIGRP convergence blueprint lets you apply a modular, hierarchical approach network... Provider ( ISP ) connects different LAN networks in the marketplace after the Tree. Developing countries a campus network topology subnet or VLAN shared VLAN database, consider using VTP version 3 yields sub-second... Full command on the user side or end point-facing access layer, the... Transition events ) SFCollege campus network is called transmit queue ( Tx-queue ) starvation widely deployed in the,... Switched infrastructure for a given subnet or VLAN sub-second convergence fix them and video survive such.. Guard and campus network topology Guard, BPDU Filter, root Guard are tools that protect... L2 links are blocking from a configuration perspective, so both uplinks from the perspective outbound! Qos, security, troubleshooting, problem isolation, policy implementation, and the are... Connections to redundant systems ( distribution layer, the addition of a node or link failure detection traffic being ;...

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