Figure 54 Removal of L2 Distribution-to-Distribution Link. 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). You can achieve reliable default gateway failover from the HSRP primary to the HSRP standby in less than 900 ms by tuning the HSRP timers, as described in the section, "Using HSRP, VRRP, or GLBP for Default Gateway Redundancy.". In Figure 46, an L3 connection exists between the distribution nodes. Without this logical grouping, STP/RTSP would place the redundant interface into blocking state to maintain a loop-free topology (See Figure 30). For details, see High Availability Campus Recovery Analysis. The Cisco enterprise campus architecture divides the enterprise network into physical, logical, and functional areas while leveraging the hierarchical design. Figure 56 Convergence Events with Return Path Traffic. This is not an issue when VLANs are not present across access layer switches because the flooding occurs only to switches where the traffic would have normally been switched. 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. A building design is appropriate for a building-sized network that contains … It prevents a port from transmitting BPDUs that would cause a change in the root port or path selection. The distribution layer aggregates nodes from the access layer, protecting the core from high-density peering (see Figure 3). Figure 62 Primary Distribution Node Restoration. 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. This provides traffic classification and queuing as close to the ingress of the network as possible. Convergence based on these functions, which are implemented in hardware, is the most deterministic. When the packet reaches the target switch, the inner or second tag is then processed and the potentially malicious packet is switched to the target VLAN (see Figure 26). •Deploying the L2/L3 Boundary at the Access Layer. The Core layer that provides optimal transport between sites and high performance routing, The Distribution layer that provides policy-based connectivity and control boundary between the access and core layers, The Access layer that provides workgroup/user access to the network, SPAN session not detecting any SSH session - 2960XR, High performance switching and software/hardware redundancy, Non-blocking end-to-end topology with vPC technology. For example, ARP processing for a large L2 domain by the distribution node is not a concern in this design, as shown in Figure 62. Only use L2 looped topologies if it cannot be avoided. The two protocols are interoperable, with some manual configuration required. The previously large L2 domain and ARP processing is now distributed among the access layer switches supported by the distribution pair. 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. Figure 61 Distribution-to-Access Link Failure. The following versions of STP have evolved over time: The following enhancements to 802.1(d,s,w) comprise the Cisco Spanning-Tree toolkit: •PortFast—Lets the access port bypass the listening and learning phases, •UplinkFast—Provides 3-to-5 second convergence after link failure, •BackboneFast—Cuts convergence time by MaxAge for indirect failure, •Loop Guard—Prevents the alternate or root port from being elected unless Bridge Protocol Data Units (BPDUs) are present, •Root Guard—Prevents external switches from becoming the root, •BPDU Guard—Disables a PortFast-enabled port if a BPDU is received, •BPDU Filter—Prevents sending or receiving BPDUs on PortFast-enabled ports. The HSRP and Rapid PVST+ root should be co-located on the same distribution switches to avoid using the inter-distribution link for transit. However, it is not possible to achieve the same deterministic convergence in the event of a link or node failure, and for this reason the design will not be optimized for high availability. Preemption is the desired behavior because the STP/RSTP root should be the same device as the HSRP primary for a given subnet or VLAN. When you use EtherChannel interconnections, use L3 and L4 information to achieve optimum utilization. These techniques worked but were not optimal from a configuration, maintenance, or management perspective. Cisco switches let you tune the hashing algorithm used to select the specific EtherChannel link on which a packet is transmitted. 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. This means that the primary method of convergence for core or distribution node failure is loss of link. UNICEF will be happy. Much like the construction of a house, if the engineering work is skipped at the foundation level, the house will crack and eventually fail. As shown in Figure 15, when using the same information for input, the same result is always obtained. On links between a CatOS device and a Cisco IOS software device, you should disable PAgP negotiation if EtherChannel tunnels are not required. http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/VMDC/2.6/vmdcservicesaag.html, http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/VMDC/2.6/vmdctechwp.html. Many redundant paths are provided in the recommended network topology. The modular design makes the network easy to scale, understand, and troubleshoot by promoting deterministic traffic patterns. Without SSO and a single supervisor, devices serviced by this access switch would experience a total network outage until the supervisor was physically replaced or, in the case of a software failure, until the unit reloaded. Rapid PVST+ greatly improves the detection of indirect failures (L2 distribution-to-distribution link) or link up (uplink) restoration events. The proper configuration and tuning of foundational services is an essential component of a highly available campus network. 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. 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 … Using these oversubscription ratios, congestion on the uplinks occurs by design (see Figure 42). •Use Rapid PVST+ to protect against user-side loops. Switches or workstations running a version of STP are commonly introduced into a network. The building blocks of modular networks are easy to replicate, redesign, and expand. The design principles and implementation best practices described in this document are tried-and-true lessons learned over time. With currently available hardware switching platforms, CPU resources are not as scarce in a campus environment as they might be in a WAN environment. Medianet applications—particularly HD video applications—are extremely sensitive to packet drops, to the point where even 1 packet dropped in 10,000 is discernible by the end-user. Results vary depending on the size of the L2 domain supported by the distribution pair. Additionally, the access layer switch receiving the flooded traffic has a CAM table entry for the host because it is directly attached, so traffic is switched only to the intended host. For more details please refer to the following link: http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/campover.html#wp708886. Layer 3 core designs are superior to Layer 2 and other alternatives because they provide: –Faster convergence around a link or node failure. When using the on/on setting, PAgP is not enabled on members of the bundle. 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). It is therefore recommended that only links intended for transit traffic be used to establish routing neighbor or peer relationships. Return path traffic is dropped until the SPF timer has expired and normal reroute processing is completed. If StackWise technology is utilized, you can follow the best practice recommendation by using an L3 connection between the distribution switches without having to use a loop-back cable or perform extra configuration. Topologies with redundant equal-cost load sharing links are the most deterministic and optimized for convergence measured in milliseconds. By applying the hierarchical design model across the multiple functional blocks of the enterprise campus network, a more scalable and modular campus architecture (commonly referred to as building blocks ) can be achieved.This modular enterprise campus architecture offers a high level of design … The modular design makes the network more scalable and manageable by promoting deterministic traffic patterns. Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type. Your enterprise can take advantage of these lessons to implement a network that will provide the necessary flexibility as the business requirements of your network infrastructure evolve over time. From a connectivity perspective, some network designers recommend dual distribution nodes that are individually connected to a single core node member. If you change the input to the hash, you will change the output. Campus topologies with redundant network paths can converge faster than topologies that depend on redundant supervisors for convergence. Figure 28 Mismatched Transmit/Receive Pairs. For more detail about cisco data cneter switches refer to the following link: http://www.cisco.com/en/US/products/ps9441/Products_Sub_Category_Home.html. • Avoid asymmetric routing and unicast flooding; do not span VLANs across the access layer. In fiber topologies where fiber optic interconnections are used, which is common in a campus environment, physical misconnections can occur that allow a link to appear to be up/up when there is a mismatched set of transmit/receive pairs. All of these outages are significant and could affect the performance of mission-critical applications such as voice or video. Highly available networks require redundant paths to ensure connectivity in the event of a node or link failure. 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. When it comes to redundancy, however, you can have too much of a good thing. From the perspective of the access layer, at least three sets of redundant links are traversed to another building block, such as the data center. Cisco Campus Network Design In this lesson we’ll take a look at some of the basics of Cisco Campus network 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. •Inline power (POE) for IP telephony and wireless access points, allowing customers to converge voice onto their data network and providing roaming WLAN access for users. Building a Campus network is more than only interconnecting physical network infrastructure devices. If the routing information is not summarized towards the core, Enhanced Interior Gateway Protocol (EIGRP) and Open Shortest Path First (OSPF) require interaction with a potentially large number of peers to converge around a failed node, as shown in Figure 13. It also allows for round robin distribution of default gateways to access layer devices, so the end points can send traffic to one of the two distribution nodes. STP is required to ensure a loop-free topology and to protect the rest of the network from problems created in the access layer. 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 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. Police unwanted traffic flows as close to their sources as possible. •Configure each distribution block as a separate totally stubby OSPF area. Modularity: By separating the various functions that exist on a network into modules, the network is easier to design. •Set trunks to on/on with no negotiate, prune unused VLANs, and use VTP transparent mode. 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. 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. In this configuration, LSAs are isolated to each access layer switch, so that a link flap for one access layer switch is not communicated beyond the distribution pairs. In most cases, VLANs are defined once during switch setup with few, if any, additional modifications to the VLAN database in an access layer switch. © 2020 Cisco and/or its affiliates. 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. It might be a single floor, a building, or even a group of buildings spread over an extended geographic area, Cisco’s hierarchical network design model breaks the complex problem of network design into smaller and more manageable. When packets traverse a network with multiple redundant paths that all use the same input value, a "go to the right" or "go to the left" decision is made for each redundant path. When the distribution layer summarizes towards the core, queries are limited to one hop from the distribution switches, which optimizes EIGRP convergence. In this topology, no VLANs span access layer switches and the distribution layer interconnection is an L3 point-to-point link. When spanning-tree convergence is required, Rapid PVST+ is superior to PVST+ or plain 802.1d. You can also create these channels on interfaces that are on different physical line cards, which provides increased availability because the failure of a single line card does not cause a complete loss of connectivity. In a hierarchical design, the capacity, features, and functionality of a specific device are optimized for its position in the network and the role that it plays. It breaks the complex problem of network design into smaller and more manageable areas. 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. This section describes the best way to build a topology that includes VLANs spanning access layer switches and that depend on STP/RSTP for convergence (see Figure 57). There is a point of diminishing returns when the complexity of configuration and management outweighs any benefit of the added redundancy (see Figure 53). Layer 3 routing protocols are typically deployed in the core-to-core and core-to-distribution layers of the network, and can be used all the way to the access layer. Depending on the version of STP, convergence could take as long as 90 seconds. •EIGRP provides for multiple levels of route summarization and route filtering that map to the multiple tiers of the campus. VTP is an essential component of VLAN Trunking. When you use L3 routed equal-cost redundant paths, vary the input to the CEF hashing algorithm to improve load distribution. See "Using HSRP, VRRP, or GLBP for Default Gateway Redundancy" section for more details on default gateway redundancy. The hierarchical network model stresses redundancy at many levels to remove a single point of failure wherever the consequences of a failure are serious. The time-proven topology that provides the highest availability does not require STP/RSTP convergence. You can use QoS policies to protect mission-critical applications while giving a lower class of service to suspect traffic. In this topology, the CAM table entry ages out on the standby HSRP router. Each level, or tier in the hierarchy is focused on specific set of roles. 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. However, emerging applications like these are built upon the campus foundation. •Use VLAN Trunking Protocol (VTP) in transparent mode to reduce the potential for operational error. The hierarchical campus model implements many L3 equal-cost redundant paths. The subsequent ARP response repopulates the CAM table before the CAM entry is aged out and removed. The following are some of the other key design issues to keep in mind: •Design the core layer as a high-speed, Layer 3 (L3) switching environment utilizing only hardware-accelerated services. The access layer of the network is typically a single point of failure, as shown in Figure 7. 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. Routing protocols are utilized in a hierarchical network design to reroute around a failed link or node. •Use redundant point-to-point L3 interconnections in the core (triangles, not squares) wherever possible, because this design yields the fastest and most deterministic convergence results. Alternatively, you can use Root Guard to protect against an unexpected spanning-tree convergence event caused by the addition of an un-authorized bridge device. If you have an L2 access layer design, redundant supervisors with SSO provide the most benefit. Discussions are divided into designing campus networks, designing WANs, utilizing remote … The Access-b uplink to the backup HSRP peer to Access-b is now a transit link for Access-a traffic, and the Access-b uplink to the primary HSRP peer must now carry traffic for both Access-b (its original intent) and for Access-a. If the attacker has specific knowledge of the 802.1Q native VLAN, a packet could be crafted that when processed, the first or outermost tag is removed when the packet is switched onto the untagged native VLAN. Cisco has developed the Hot Standby Router Protocol (HSRP) to address this need, and the IETF subsequently ratified Virtual Router Redundancy Protocol (VRRP) as the standards-based method of providing default gateway redundancy. Unified Communications services (Cisco Unified Communications Manager, gateways, MTP, and the like). The unexpected side effect is that Access-a traffic goes through Access-b to reach its default gateway. The recommended design is to provide an alternate path to the core, as shown in Figure 11. This L2 looped topology is configuration and management intensive. ), Figure 23 Virtual Trunk Protocol Operation. 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. At the very least, this model requires redundant core and distribution layer switches with redundant uplinks throughout the design. This alternating approach eliminates the always right or always left biased decisions and helps balance the traffic over equal-cost redundant links in the network (see Figure 17). If you do not disable EtherChannel negotiation, then the mismatch between the default states of CatOS and Cisco IOS software can cause as much as seven seconds of loss during link negotiation, as shown in Figure 33. In the data center, you may need a 1:1 ratio. Cisco Campus Network Design Basics In this lesson we’ll take a look at some of the basics of Cisco Campus network design. 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. When HSRP or VRRP are used to provide default gateway redundancy, the backup members of the peer relationship are idle, waiting for a failure event to occur for them to take over and actively forward traffic (see Figure 36). If you are compelled by application requirements to depend on STP to resolve convergence events, use Rapid PVST+. Preemption causes the primary HSRP peer to re-assume the primary role when it comes back online after a failure or maintenance event. EIGRP stub nodes are not able to act as transit nodes and as such, they do not participate in EIGRP query processing. The most challenging and important part of it is the planning and design … When the link from Access-a to the STP root and the HSRP primary switch fails, traffic is lost until the standby HSRP peer takes over as the default gateway.
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