EIGRP can reroute around the failure in 700-1100 ms for the return path traffic. In the data center, servers are commonly dual-attached and L2 connectivity is required, from the host perspective, to support dual attachment. 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.html. This topology raises the following questions: • Where should the root switch be placed? 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. Summarizing using EIGRP or using an area boundary for OSPF are the recommended L3 configurations for the distribution-to-core layer L3 connection. •Tune OSPF hello, dead-interval, and SPF timers to 1, 3, and 1, respectively. The following are additional considerations when comparing EIGRP and OSPF: •Within the campus environment, EIGRP provides for faster convergence and greater flexibility. Failover and convergence work just like HSRP. EIGRP stub nodes are not able to act as transit nodes and as such, they do not participate in EIGRP query processing. Figure 64 Convergence Time with OSPF Totally Stubby Areas. It is not generally practical to provide line rate for every port upstream from the access-to-distribution switch, the distribution-to-core switch, or even for core-to-core links. •Manually prune all VLANS except those needed. Summarization is required to facilitate optimum EIGRP or OSPF convergence. If a hello is not received that contains the port and node information of the sending machine, this indicates a misconfiguration and the port is error-disabled. Sometimes this is undesirable, such as when the switch that is added has been configured to become the STP root for the VLANs to which it is attached. Access layer switches feed up into switches comprising the aggregation layer, also known as … This includes PortFast, BPDU Guard, BPDU Filter, Root Guard, and Loop Guard. Designing a Network Topology In this chapter, you will learn techniques for developing a network topology. Star Topology. 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. Also known as Three-layered Hierarchical Model, this is the Cisco flagship design for Campus networks. With OSPF, you force summarization and limit the diameter of OSPF LSA propagation through the implementation of L2/L3 boundaries or Area Border Routers (ABRs). 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. Figure 1-2 Example of a Campus Network 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. This can result in a bridge between a wireless LAN interface and an Ethernet interface, or between two Ethernet interfaces. Traffic is lost while SSO completes, or indirect detection of the failure occurs. The new MAC address is attached and the packet is forwarded. When making this optimization, there is a trade-off to be considered: when using the Auto/Desirable setting, PAgP is enabled, protecting against misconfiguration and hardware failure scenarios that can cause STP loops to form. •When something goes wrong, how do you find the source of the problem? A robust access layer provides the following key features: •High availability (HA) supported by many hardware and software attributes. As stated earlier, this problem only occurs in a topology where VLANs span multiple access layer switches in a large L2 domain. Instead, it decreases availability by reducing serviceability and determinism. In the campus topology, that is the access layer. To make the individual interfaces passive, where a peering relationship is not desired, enter the following commands: Alternatively, you can make all interfaces passive, and then use the no passive command to enable a routing neighbor relationship on the interfaces where peering is desired. The network topology that is suitable for a university campus is the star topology. Routing protocols are utilized in a hierarchical network design to reroute around a failed link or node. Highly available networks require redundant paths to ensure connectivity in the event of a node or link failure. This promotes scalability and stability. If you require a common, centrally-managed VLAN database, consider using VTP version 3. This provides high-availability for critical user groups. In the past, network designers had only a limited number of hardware options - routers or hubs - when purchasing a technology for their campus networks. 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. However, emerging applications like these are built upon the campus foundation. Note For more details, refer to High Availability Campus Recovery Analysis. Figure 32 Port Aggregation Protocol Operation. The "Campus" is where USERS (employees) connect to the network, along with all of the devices those employees use (e.g. 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. •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. Additional requirements of these designs typically include: This Cisco solution provides manageable switched infrastructure for a campus intranet with over a thousand networked devices. This is shown in the following example: Use either technique to minimize the number of peer relationships between distribution nodes, allowing them to peer only over links intended as transit links. Misconfiguration (mis-matched pairs) or hardware failure can result in unexpected STP behavior. Congestion on a Cisco Catalyst switch interface is not typically caused by oversubscription or an anomaly such as an Internet worm. A campus area network known as (CAN) is used to inter-connect networks in limited geographical locality like university campus, military bases, or organizational campuses etc. Additionally, the distribution-to-distribution link is an L3 routed link. The campus network is mostly owned by an enterprise, university, government, etc. The building blocks of modular networks are easy to replicate, redesign, and expand. 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. A campus network, campus area network, corporate area network or CAN is a computer network made up of an interconnection of local area networks (LANs) within a limited geographical area. 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. Network redundancy and high availability is provided at each layer. The cost for designing of the network system has a limit that cannot be exceeded. •Optimize CEF for best utilization of redundant L3 paths. If you use a topology where spanning-tree convergence is required, then Rapid PVST+ is the best version. CEF is a deterministic algorithm. Traffic is dropped when it arrives on the wrong distribution switch (see Figure 47). EtherChannels are typically deployed between the distribution-to-core and core-to-core interconnections where increased availability and scaled bandwidth are required. When this happens, the router must queue the packets and apply QoS to ensure that important traffic is transmitted first (see Figure 43). •Set hello and dead timers to 1 and 3, respectively. •Efficient network and bandwidth management using software features such as Internet Group Membership Protocol (IGMP) snooping. This can have significant impact on performance. A high-capacity, centralized server farm connects to the backbone and provides internal server resources to users, for example, application, file, print, e-mail, and Domain Name System (DNS) services. This capability facilitates troubleshooting, problem isolation, and network management. Depending on the version of STP, convergence could take as long as 90 seconds. The HSRP and Rapid PVST+ root should be co-located on the same distribution switches to avoid using the inter-distribution link for transit. It is an application of graph theory wherein communicating devices are modeled as nodes and the connections between the devices are modeled as links or lines between the nodes. A Network Diagram showing Network Topology for School. Load balancing, Quality of Service (QoS), and ease of provisioning are key considerations for the distribution layer. Adding and removing VLANs is generally not a frequent network management practice. All rights reserved. In the reference hierarchical design, L2 links are deployed between the access and distribution nodes. PAgP or LACP enable the automatic formation of EtherChannel tunnels between interconnected switches (see Figure 32). Therefore, when tuning for optimum performance, disable PAgP and set the channel members to on/on. When a link or node has failed, an OSPF peer cannot take action until this timer has expired. If the foundation is not rock solid, the performance of applications that depend on network services such as IP telephony, IP video, and wireless communications will eventually suffer. The additional link between the distribution switches is required to support summarization of routing information from the distribution layer towards the core. 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. NSF/SSO provide the most benefit in environments where single points of failure exist. This model also requires a redundant distribution pair supporting each distribution building block. This allows for the failure or removal of one of the distribution nodes without affecting end point connectivity to the default gateway. 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. Data Network. This is a benefit, however it makes this design less flexible than other configurations. As a result, some redundant links are underutilized and the network is said to be experiencing CEF polarization (see Figure 16). However, no VLAN exists across multiple access layer switches. 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. 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). A campus network is a building or group of buildings all connected into one enterprise network that consists of many local-area networks (LANs). The hierarchical network model stresses redundancy at many levels to remove a single point of failure wherever the consequences of a failure are serious. If you follow the rules, you can achieve deterministic convergence. A campus is generally a portion of a company (or the whole company) that is constrained to a fixed geographic area. At first glance, this appears to be a serious risk. 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 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. Customize every detail by using smart and dynamic toolkits. If you have an L2 access layer design, redundant supervisors with SSO provide the most benefit. 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. Default gateway redundancy is an important component in convergence in a hierarchical network design. Every participant node is directly … Routing integrates these switched networks, and provides the security, stability, and control needed to build functional and scalable networks. 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. They might also upgrade wiring to meet the requirements of emerging applications. Also, peering and adjacency issues exist with a fully-meshed design, making routing complex to configure and difficult to scale. The logical topology of the current campus-backbone network at WVCC consists of a hierarchical, mesh architecture with redundant links between buildings. 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. •Tune GLBP/HSRP preempt delay to avoid black holes. Figure 54 Removal of L2 Distribution-to-Distribution Link. If you have a routed access layer design, redundant supervisors with NSF with SSO provide the most benefit. Switches or workstations running a version of STP are commonly introduced into a network. The hierarchical campus model implements many L3 equal-cost redundant paths. 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. With currently available hardware switching platforms, CPU resources are not as scarce in a campus environment as they might be in a WAN environment. As a result, no better than 1.65 seconds of convergence time can be achieved in the event of an access layer to distribution layer uplink failure or primary distribution node failure (see Figure 63). Both distribution nodes can forward return path traffic from the rest of the network towards the access layer for devices attached to all members of the stack or chain. •Use UDLD to protect against one-way up/up connections. 2. Campus network devices can currently provide a high level of availability within the individual nodes. 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. Additionally, you can use QoS to reduce the priority of unwanted traffic. 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. This allows the network to converge in 60-200 milliseconds for EIGRP and OSPF. A tree … In a 40-node access layer test, recovery times of up to four seconds were measured for all flows to be re-established during this convergence event. •MST—Provides up to 16 instances of RSTP (802.1w) and combines many VLANS with the same physical and logical topology into a common RSTP instance. Multilayer switches that provide Layer 2 and 3 functionality, for example, are now appearing in the marketplace. 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. The benefits of dynamic propagation of VLAN information across the network are not worth the potential for unexpected behavior due to operational error. The distribution layer aggregates nodes from the access layer, protecting the core from high-density peering (see Figure 3). Use L3 and L4 (UDP/TCP port) information as input to hashing algorithms. This occurs because the ARP and CAM aging timers are different. 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). CEF determines the longest path match for the destination address using a hardware lookup. •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. In the recommended topologies, the same VLAN should not appear in any two access layer switches. In a typical hierarchical model, the individual building blocks are interconnected using a core layer. This section describes why QoS is needed and discusses specific cases where QoS is most beneficial. This redundant L3 peering has no benefit from an HA perspective, and only adds load in terms of memory, routing protocol update overhead, and complexity. The distribute list allows only the default route ( to be advertised to the access layer nodes. 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). Under normal circumstances, the network should provide an adequate level of service for all network traffic, including lower priority best-effort traffic. Without careful consideration, discontinuous VLAN/subnets, routing black holes, and active/active HSRP/GLPB situations can exist. Use whichever technique requires the fewest lines of configuration or is the easiest for you to manage. 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. Figure 34 First Hop Default Gateway Redundancy. This behavior caused a considerable amount of traffic being dropped; more than 40 seconds in the tested topology. •Do not extend area 0 to the edge switch. Chapter 5. Return path traffic is dropped until the SPF timer has expired and normal reroute processing is completed. The enterprise edge functional area filters traffic from the edge modules and routes it into the enterprise campus. 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. It has a root node and all other nodes are connected to it forming a hierarchy. For example, by default, the Windows XP Home Networking Wizard bridges together all the interfaces on the machine. 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… •Traffic is dropped until the MaxAge timer expires and until the listening and learning states are completed. While it is tempting to reduce cost by reducing links between the distribution nodes to the core in a partial mesh design, the complexity and convergence tradeoffs related to this design are ultimately far more expensive. 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