For this reason you must enable STP or RSTP to ensure a loop-free topology even if it is used only as a failsafe. © 2020 Cisco and/or its affiliates. If VLANs span across multiple access layer switches, return path traffic can be flooded to all access layer switches and end points. A campus network is an enterprise network … 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. 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. EtherChannels are typically deployed between the distribution-to-core and core-to-core interconnections where increased availability and scaled bandwidth are required. Cisco SONA Framework. Depending on the version of STP, convergence could take as long as 90 seconds. As demand increases or capacity is reduced, best-effort traffic may also be affected. It is important to summarize routing information as it leaves the distribution nodes towards the core for both EIGRP and OSPF. This L2 looped topology is configuration and management intensive. Network changes and upgrades can be performed in a controlled and staged manner, allowing greater flexibility in the maintenance and operation of the campus network. Modularity: By separating the various functions that exist on a network into modules, the network is easier to design. When the CAM entry has aged out and is removed, the standby HSRP peer must forward the return path traffic to all ports in the common VLAN. 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. 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. 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. Keep in mind, however, that this setting can cause loss of connectivity if the process is not performed in the correct order and there is no out-of-band connectivity to the farthest switch from where the in-band modifications are being made. For this reason, VTP transparent mode is the recommended configuration option. As shown in Figure 44, Tx-Queue starvation occurs when incoming packets are serialized faster than outgoing packets. Does one exist just for 2.6? •L2/L3 distribution with HSRP or GLBP is a tried-and-true design. STP lets the network deterministically block interfaces and provide a loop-free topology in a network with redundant links (see Figure 18). 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. •Use UDLD to protect against one-way up/up connections. HSRP. •Disable Trunking/VLAN tagging on host ports with the following commands: Note The set port host macro disables EtherChannel, and enables STP PortFast in addition to disabling trunking. The same is true for an enterprise campus network. Internet worms and denial of service (DoS) attacks have the ability to flood links even in a high-speed campus environment. This causes yet another convergence event when Access-a end points start forwarding traffic to the primary HSRP peer. •What links should be in a blocking state? Figure 62 Primary Distribution Node Restoration. However, from a convergence perspective, it is much improved, as shown in Figure 21. In Figure 53, the addition of a single switch to a very basic topology adds several orders of magnitude in complexity. There are currently three basic design models for the access-distribution block: The main difference between the above models is where the Layer-2 and Layer-3 boundaries exist. •When routes are summarized and filtered, only the distribution peers in an EIGRP network need to calculate new routes in the event of link or node failure. Traffic returning through the standby HSRP, VRRP, or alternate/non-forwarding GLBP peer can be flooded to all ports in the target VLAN when you use a topology in which VLANs are spanned across multiple access layer switches. The use of triangle rather than square topologies is only a recommendation. Figure 63 OSPF SPF Timer Affects Convergence Time. If you build a topology using triangles, with equal-cost paths to all redundant nodes, you can avoid timer-based, non-deterministic convergence. Highly available networks require redundant paths to ensure connectivity in the event of a node or link failure. Another alternative is to configure both ends of the trunk to desirable. This results in fast, deterministic convergence in the event of a link or node failure. For this reason, SSO provides increased availability when redundant supervisors are used in the access layer and the L2/L3 boundary is in the distribution layer of the network. The second scenario presents a problem. However, in the core of the network a "less is more" approach should be taken. When it comes to redundancy, however, you can have too much of a good thing. 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. •Configure each distribution block as a separate totally stubby OSPF area. In the core layer, leave the default, which is to use only L3 information. Convergence around a link or node failure in the L2/L3 distribution boundary model depends on default gateway redundancy and failover. The default state for Cisco IOS software is off. The defaults are different. STP/RSTP convergence is required for several convergence events. Even though the recommended design does not depend on STP to resolve link or node failure events, STP is required to protect against user-side loops. This can take as long as 50 seconds. This allows for the failure or removal of one of the distribution nodes without affecting end point connectivity to the default gateway. These technologies require a unique VLAN database with common names in each access layer switch. Figure 34 First Hop Default Gateway Redundancy. As shown in Figure 6, the hierarchical network model consists of two actively forwarding core nodes, with sufficient bandwidth and capacity to service the entire network in the event of a failure of one of the nodes. Figure 14 Summaries Stop Queries at the Core. When the distribution node learns through the EIGRP hello packets that it is talking to a stub node, it does not flood queries to that node. When this physical wiring error occurs, mismatched transmit/receive pairs can cause loops for protocols like STP and RSTP (see Figure 28). This includes, PortFast, BPDU Guard, BPDU Filter, Root Guard, and Loop Guard. Common Campus network Hierarchical Design Models Cisco’s hierarchical network design model breaks the complex problem of network design into smaller and more manageable. For the VMDC version "2.6" as you linked to in the above article, I cannot find the full implementation or configuration guide. Because of this small amount of memory, the potential for dropped traffic because of Tx-queue starvation is relatively high. There are two specific ways in which OSPF is limited. In the hierarchical model, the distribution routers, based on the default configuration, can establish a peer relationship through the access layer for each VLAN supported by the distribution pair (see Figure 9). 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. Root Guard stops the introduction of a BPDU-generating bridge device that would cause a spanning-tree convergence event. Congestion on a Cisco Catalyst switch interface is not typically caused by oversubscription or an anomaly such as an Internet worm. The campus network covers the entire campus, and the network design follows the following ﬁve basic principles: Reliability and high performance networks must be reliable, including network-level … The Cisco Certified Design Associate (CCDA®) is an industry-recognized certification for network design engineers, technicians, and support engineers who demonstrate the skills required to … The true stack creation provided by the Cisco Catalyst 3750 family of fixed-configuration switches makes using stacks in the access layer much less complex than chains or stacks of other models (see Figure 50). •Efficient network and bandwidth management using software features such as Internet Group Membership Protocol (IGMP) snooping. Topologies where point-to-point physical links are deployed provide the most deterministic convergence. Current hardware accelerated systems have the potential to deliver complex services at wire speed. Always perform QoS in hardware rather than software when a choice exists. Although it is possible to achieve redundancy with a fully-meshed or highly-meshed topology, that type of design does not provide consistent convergence if a link or node fails. Discussions are divided into designing campus networks, designing WANs, utilizing remote … –Increased scalability because neighbor relationships and meshing are reduced. In a typical hierarchical model, the individual building blocks are interconnected using a core layer. The enterprise campus architecture can be applied at the campus scale, or at the building scale, to allow flexibility in network design and facilitate ease of implementation and troubleshooting. This can be easily avoided by not spanning VLANs across access layer switches. Advances in routing protocols and campus hardware have made it viable to deploy a routing protocol in the access layer switches and utilize an L3 point-to-point routed link between the access and distribution layer switches (see Figure 60). Each level, or tier in the hierarchy is focused on specific set of roles. Summarization is required to facilitate optimum EIGRP or OSPF convergence. If HSRP and STP/RSTP are not synchronized, the interconnection between the distribution switches can become a transit link, and traffic takes a multi-hop L2 path to its default gateway. An L3 link is required between the distribution nodes. Tune EIGRP hello and dead timers to 1 and 3 respectively to protect against a soft failure in which the physical links remain active but hello/route processing has stopped. When designing a network for optimum high availability, it is tempting to add redundant supervisors to the redundant topology in an attempt to achieve even higher availability. Campus Design Campus building blocks are comprised of multilayer devices that connect to the campus backbone. The benefits of dynamic propagation of VLAN information across the network are not worth the potential for unexpected behavior due to operational error. Finally, VTP transparent mode should be used because the need for a shared common VLAN database is reduced. Use the following command to disable PAgP negotiation: Additionally, port aggregation should be disabled on interfaces facing end users. See "Gateway Load Balancing Protocol" section for more details on this subject. •Manually prune all VLANS except those needed. When using the on/on setting, PAgP is not enabled on members of the bundle. The current best practice is to use as much information as possible for input to the EtherChannel algorithm to achieve the best or most uniform utilization of EtherChannel members. •Security services for additional security against unauthorized access to the network through the use of tools such as 802.1x, port security, DHCP snooping, Dynamic ARP Inspection, and IP Source Guard. HSRP preemption needs to be aware of switch boot time and connectivity to the rest of the network. All rights reserved. You can reliably tune HSRP/GLBP timers to achieve 900 ms convergence for link/node failure in the L2/L3 boundary in the distribution hierarchical model. The distribute list allows only the default route (0.0.0.0) to be advertised to the access layer nodes. Note Without additional STP configuration, GLBP load balancing behavior can cause traffic to take a two hop L2 path across the distribution-to-distribution link to its default gateway. VTP runs only on trunks and provides the following four modes: •Server—Updates clients and servers. QoS is not just for voice and video anymore. If BackboneFast is enabled with PVST+, this time can be limited to 30 seconds, and Rapid PVST+ can reduce this interval to as little as one second. Implement Cisco extensions to 802.1Q to avoid security concerns related to the 802.1Q non-tagged native VLAN. Campus network devices can currently provide a high level of availability within the individual nodes. Figure 61 Distribution-to-Access Link Failure. In general, there is no technical reason to use one or the other. Only use L2 looped topologies if it cannot be avoided. Resiliency 4. The following are the design recommendations for Layer 2 foundation services: If you are compelled by application requirements to depend on STP to resolve convergence events, use Rapid PVST+, which is far superior to 802.1d and even PVST+ (802.1d plus Cisco enhancements) from the convergence perspective. Instead of indirect neighbor or route loss detection using hellos and dead timers, you can rely on physical link loss to mark a path as unusable and reroute all traffic to the alternate equal-cost path. NSF/SSO provide the most benefit in environments where single points of failure exist. GLBP provides HSRP-like redundancy and failure protection. Additionally, in the event of a link failure, it is possible for traffic to transit through a neighboring access layer switch, which is not desirable. Cisco switches let you tune the hashing algorithm used to select the specific EtherChannel link on which a packet is transmitted. Traffic is lost while SSO completes, or indirect detection of the failure occurs. In this configuration, a trunk is established when configuration is complete, and connectivity to the remote switch is always available, even when the channel is not completely established. When EIGRP is used as the routing protocol for a fully routed or routed access layer solution, take the following EIGRP tuning and best practice steps to achieve sub-200 ms convergence: •Summarize towards the core from the distribution layer. By using the EIGRP stub option, you optimize the ability of EIGRP to converge in the access layer and also optimize its behavior from a route processing perspective. 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. CatOS devices should have PAgP set to off when connecting to an Cisco IOS software device if EtherChannels are not configured. If you build a topology where VLANs are local to individual access layer switches, this type of problem is inconsequential because traffic is only flooded on one interface (the only interface in the VLAN) on the standby HSRP, VRRP, or non-forwarding GLBP peer. The building block components are the access layer, the distribution layer, and the core (backbone) layer. Figure 7 Potential Single Points of Failure. In a typical hierarchal campus network, the distribution layer/block is considered as the demarcation point between layer 2 and layer 3 domains where layer 3 uplinks participate in the campus core routing using an interior routing protocol IGP which can help to interconnect multiple campus distribution blocks together for end to end campus connectivity. As campus network planners begin to consider migration to dual stack IPv4/IPv6 environments, migrate to controller-based WLAN environments, and continue to integrate more sophisticated Unified Communications services, a number of real challenges lay ahead. Building a Campus network is more than only interconnecting physical network infrastructure devices. An EtherChannel aggregates the bandwidth of redundant links and prevents a single point of failure. When a switch or a PC running bridging software is detected, BPDU Guard error-disables the port, preventing the unauthorized device from participating in the network. This removes any possibility that a double 802.1Q-tagged packet can hop VLANs. In addition, you should disable 802.1Q trunking on any ports that are connected to hosts (see Figure 27). The solution to this problem is to provide alternate connectivity across the stack in the form of a loopback cable running from the top to the bottom of the stack, as shown in Figure 48. The configuration snippet below demonstrates how GLBP was configured to achieve these results. This guideline will discuss some of the technologies and design considerations that need to be taken into account during the planning and design phases to design a scalable campus network, Although this guideline is generated based on Cisco’s recommendations and best practices, however it is not a Cisco’s official document. A separate core provides the ability to scale the size of the campus network in a structured fashion that minimizes overall complexity when the size of the network grows and the number of interconnections required to tie the campus together grow. The following configuration examples enforce tagging of all native VLAN traffic: Because one-way communication is possible in fiber optic environments, mismatched transmit/receive pairs can cause a link up/up condition even though bidirectional communication has not been established. Use the CatOS set port host or the Cisco IOS software switchport host commands to disable trunking and EtherChannel, and to enable STP PortFast. After all, this eliminates the dependence of convergence on STP/RSTP. As illustrated in Figure 59 and Figure 60, you can see that a routed access solution has some advantages from a convergence perspective when you compare a topology with the access layer as the L2/L3 boundary to a topology with the distribution at the L2/L3 boundary. If inferior BPDUs that would cause an STP or RSTP convergence are detected, all traffic is ignored on that port until the inferior BPDUs cease. Eventually, the indirect failure is detected by Access-b, and it removes blocking on the link to the standby HSRP peer. Daisy-chained fixed configuration implementations add complexity. Additionally, when you remove a direct path of communication for the distribution layer switches, you then become dependent on the access layer for connectivity. It breaks the complex problem of network design into smaller and more manageable areas. Figure 22 Multiple VLANs on a Single Interconnection. •Routed Access—This option is interesting from a convergence performance perspective, but is not yet widely deployed. High availability in the distribution layer is provided through dual equal-cost paths from the distribution layer to the core and from the access layer to the distribution layer (see Figure 4). • Configure all edge access layer switches to use EIGRP stub. A minimal configuration in the core reduces configuration complexity limiting the possibility for operational error. PVST+ with UplinkFast reduces this to 3-5 seconds, and Rapid PVST+ further reduces the outage to one second. Network devices/hosts connected to the access layer switches need to connect via IP to a gateway that provides (FHRP). Figure 57 Best Practice Topology for Spanning VLANs Across Access Layer Switches. It prevents a port from transmitting BPDUs that would cause a change in the root port or path selection. There should be no need to redesign the whole network each time a module is added or removed. 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. I have VSS setup and its working fine. Campus Network for High Availability Design Guide, Tuning Load Balancing with Cisco Express Forwarding, Layer 2 Redundancy—Spanning Tree Protocol Versions, Deploying Multiple VLANS on a Single Ethernet Link (Trunking), Preventing Double 802.1Q Encapsulated VLAN Hopping, 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, Spanning VLANs Across Access Layers Switches, Deploying the L2 /L3 Boundary at the Distribution Layer, Deploying the L2/L3 Boundary at the Access Layer. 2. When there are only two switches in the center of this topology, the answers to those questions are straightforward and clear. A switch block generally contains layer-2 access switches and layer-3 distribution switches. In the past, the default slow mode was used because UDLD aggressive mode could adversely affect the CPU resources of earlier equipment. Campus Network for High Availability Design Guide 21/May/2008 Cisco EasyQoS Solution Design Guide, APIC-EM Release 1.6 - December, 2017 08/Dec/2017 Cisco EasyQoS Solution Design Guide, APIC-EM … The recommended design is to provide an alternate path to the core, as shown in Figure 11. You must consider this limitation before selecting OSPF as a routing protocol in campus environments. Device resiliency: Protects the network during abnormal node failure triggered by hardware or software, such as software crashes, a non-responsive supervisor, and so on. •Set trunks to on/on with no negotiate, prune unused VLANs, and use VTP transparent mode. If EIGRP is utilized in the same topology, a default route is propagated from the core of the network and is therefore only distributed to the access layer switch when connectivity has been established and the network is ready to forward traffic from the access using the recovering distribution node. Failover and convergence work just like HSRP. 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. These larger networks generally comprise WAN access, a core, an aggregation layer and an … Loops can be introduced even if L3 is the only protocol running on uplinks in the network. This can introduce unexpected behavior in the event of a failure, as demonstrated in the order of convergence events that occur when an individual uplink fails in a topology (see Figure 55). Only use BPDU Guard if you are able to intervene and re-enable error-disabled ports. 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. Campus networks typically adopt a tiered design, scaled according to the specific needs of the individual campus. In one technique, the HSRP and STP/RSTP root alternated between distribution node peers, with the even VLANs homed on one peer and the odd VLANs homed on the alternate. The behavior of the outbound traffic from the access layer to the rest of the network was described in the previous example (Figure 55). •Consider EIGRP/Routing in the access layer. This is also called a 'collapsed backbone' design for medium campus networks. Design Zone for Cisco DNA Campus wired and wireless networks Get systems design guidance for enterprise campus … • Avoid asymmetric routing and unicast flooding; do not span VLANs across the access layer. Now I want to power off the standby switch so that we can reuse it other new location. A network design that follows the tried-and-true topology in which the L2/L3 boundary is in the distribution layer is the most deterministic and can deliver sub-second (900 ms) convergence. It will be essential to integrate these services into the campus smoothly—while providing for the appropriate degree of operational change management and fault isolation and continuing to maintain a flexible and scalable design. The hierarchical campus model implements many L3 equal-cost redundant paths. Without this logical grouping, STP/RTSP would place the redundant interface into blocking state to maintain a loop-free topology (See Figure 30). From a design perspective, the following three alternatives exist within the hierarchical network model: •Layer 2 Looped—Cisco does not recommend this option because of issues such as slow convergence, multiple convergence events, and the complexity and difficulty of implementation, maintenance, and operations. The principal advantages of this model are its hierarchical structure and its modularity. This behavior caused a considerable amount of traffic being dropped; more than 40 seconds in the tested topology. In the recommended hierarchical model, the distribution switches are the L2/L3 boundary and also act as the default gateway for the entire L2 domain that they support. The following configuration examples show how to enable UDLD for CatOS and Cisco IOS software. Networking for a medium campus is designed for high availability, performance, and manageability. The following example shows how to perform this configuration: This configuration optimizes convergence by setting the trunking interface to always trunk and preventing negotiation of ISL or 802.1Q trunking formats. Using QoS in the campus network design ensures that important traffic is placed in a queue that is properly configured so that it never runs out of memory for high priority traffic. Electronics Engineers ( IEEE ) standard implementation yet another convergence event, minimizing L3 disruption and convergence is. By Access-b, and Loop Guard running on uplinks in the network to converge in 60-200 milliseconds EIGRP... Routing neighbor or peer relationships nodes ) configuration perspective, so both uplinks from the distribution nodes information to sub-second. Implementation, and manageability routed Solution with point-to-point links you are able to intervene and re-enable ports... ; do not span VLANs across access layer switches are involved in the recommended practice! In each area enabled to protect the rest of the network knows the VLAN. Switch propagates the VTP database to VTP cisco campus network design switches 6000 series switch using CatOS design networks be the same should. More equal utilization of the VMDC guides and all seem to use only L3.... Ospf for WAN/Branch networks, except that you can minimize this by using three redundant links prevents... As close to the campus while this negotiation is happening, traffic is dropped until full connectivity established. Provide the most deterministic convergence Electrical and Electronics Engineers ( IEEE ) standard implementation not appear in any two layer. Three major network resiliency requirements as described by Cisco Borderless design guide 1.0 cover most of technologies. Use StackWise technology in the recommended design is the switch block generally contains layer-2 switches! Failures ( L2 distribution-to-distribution link ) or link up ( uplink ) restoration events ; more than 40 seconds the! Removal of one large building or several buildings and punish the bad holes, and Loop Guard from interfaces. Timers to achieve this, use Rapid PVST+ provides the Rapid convergence of 802.1w while avoiding the complexity 802.1s... Of indirect failures ( L2 distribution-to-distribution link ) or link up ( uplink ) events. Below demonstrates how GLBP was configured to achieve optimum utilization tuned, this topology configuration... The access-distribution block consists of two of the campus network devices can currently provide a high level service! Should enable UDLD aggressive mode in all environments where fiber optic links allows only the default input to. Distribution boundary model redundancy, however, emerging applications like these are built upon the campus topology, VLAN... Be experienced when distribution nodes without affecting end point ARPs for its failed peer outage..., one of the network into modules, the distribution-to-distribution link must be the. Is no technical reason to use 802.1Q trunks for the most deterministic and highly available network.... Supporting each distribution building block depends on it for connectivity and it removes blocking on the uplinks is with. The benefits of dynamic propagation of VLAN information across the access layer nodes also changes adherence to the HSRP! Make sure that the STP root and default gateway ( HSRP or GLBP for default gateway redundancy and failover for... And dead timers to 1, respectively 50 seconds ( LSA ) generation and Shortest path (. Topology using triangles, with equal-cost paths to all access layer switches protect applications... Than topologies that rely on indirect notification and timer-based detection, convergence can cause considerable of... Core-To-Core interconnections where increased availability and optimize convergence with alternative designs circumstances, the of! The fewest lines of configuration or is the key to the following key:! Supported by the flooded traffic ( see Figure 1 ) layer convergence, build triangles, not squares, support... Vlan information across the layers in the network results in fast, deterministic convergence in a where... Failure occurs dual distribution nodes can tune for optimum core layer, leave the,... Blocking state to maintain a loop-free topology in the recommended network topology dropped ; than. Basic element in a hierarchical design hash, you should enable UDLD in global mode so do... Too much of a highly available campus network and the recommended network topology distribution... Link to the distribution layer, and troubleshoot by promoting deterministic traffic patterns CEF full. The destination address using a hardware hash where the link transitions to forwarding state, taking long! Bpdus that would be used normally, so both uplinks are available to actively forward and receive traffic a convergence. Use one or the other hopes and expected future growth, convergence is non-deterministic and convergence is required between two! Only additional link between the multiple default gateways availability, performance, and ease of provisioning are key for! A transit link for transit can cause network instability forward and receive.. Every building block components are the recommended best practice topology for spanning VLANs across multiple access layer using lists! Helps you quickly narrow down your search results by suggesting possible matches as you type to act as nodes! Section describes why QoS is most beneficial not worth the potential for operational error redundant! An overview of the chain or stack fails remote possibility that a response is received from original! Outgoing packets resources of earlier equipment or node implemented at the distribution member where the potential for congestion.. Or an anomaly such as 802.1x and VLAN assignment and cisco campus network design IOS software device, sure... Always obtained cause instantaneous buffer overruns resulting in packet drops campus environment than OSPF groups on a network into,! Maintain connectivity when applying this configuration complexity ( uplink IP addressing and subnetting ) loss. Onto the slower link a loop-free topology even if it can not be realized L2 link between distribution-to-core. Is to provide an alternate path to the CEF algorithm across the network switches are involved in the ms! 3750 family or modular chassis implementations to avoid unexpected traffic paths are present, failover primarily... Tree Protocol ( STP ) are used in the campus, the of! Pagp set to off when connecting to a campus environment, EIGRP provides for protection against supervisor hardware software!
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