campus network topology

• OSPF implements throttles on Link-State Advertisement (LSA) generation and Shortest Path First (SPF) calculations that limit convergence times. The network topology that is suitable for a university campus is the star topology. The HSRP and Rapid PVST+ root should be co-located on the same distribution switches to avoid using the inter-distribution link for transit. 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. This includes PortFast, UplinkFast, BackboneFast, BPDU Guard, BPDU Filter, Root Guard, and Loop Guard. This removes any possibility that a double 802.1Q-tagged packet can hop VLANs. 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. However, the other extreme is also a bad thing. 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. BDPU Guard and Root Guard are tools that can protect against these situations. 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. Support for applications based on Novell IPX, DECnet, AppleTalk, and SNA. Typical campus networks are engineered with oversubscription. Produce a sample addressing plan and present it, plus the underlying design rationale in class. The building blocks of modular networks are easy to replicate, redesign, and expand. This is because this design can be made to be an integration network system, with good Figure 63 OSPF SPF Timer Affects Convergence Time. For this reason, VTP transparent mode is the recommended configuration option. Network designers are now designing campus networks by purchasing separate equipment types (for example, routers, Ethernet switches, and ATM switches) and then linking them. However, when DTP and 802.1Q or ISL negotiation are enabled, considerable time can be spent negotiating trunk settings when a node or interface is restored. However, from a convergence perspective, it is much improved, as shown in Figure 21. The logical topology of the current campus-backbone network at WVCC consists of a hierarchical, mesh architecture with redundant links between buildings. Use the default L3 information for the core nodes and use L3 with L4 information for the distribution nodes. VPTv3 contains many enhancements for security and reliability. Results vary depending on the size of the L2 domain supported by the distribution pair. 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. Recently, local-area networking has been revolutionized by the exploding use of LAN switching at Layer 2 (the data link layer) to increase performance and to provide more bandwidth to meet new data networking applications. Use the following command to disable PAgP negotiation: Additionally, port aggregation should be disabled on interfaces facing end users. Unused VLANs should be manually pruned from trunked interfaces to avoid broadcast propagation. As stated earlier, this problem only occurs in a topology where VLANs span multiple access layer switches in a large L2 domain. 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. Internet worms and denial of service (DoS) attacks have the ability to flood links even in a high-speed campus environment. It is possible to build a topology that does not rely on equal-cost redundant paths to compensate for limited physical fiber connectivity or to reduce cost. 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. The use of triangle rather than square topologies is only a recommendation. The proper configuration and tuning of foundational services is an essential component of a highly available campus network. The minimum goal of high availability network design is to ensure that high-priority, mission-critical data applications and voice/video are never affected by network congestion (see Figure 45). If this cannot be avoided, then tune the ARP aging timer so that it is less than the CAM aging timer. Similarly to the core, the distribution layer is engineered with sufficient bandwidth and capacity so that the complete failure of one of the distribution nodes does not impact the performance of the network from a bandwidth or switching capacity perspective. 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). The network topology that would not … With aggressive HSRP timers (such as those previously recommended in this document), you can minimize this period of traffic loss to approximately 900 milliseconds. This causes yet another convergence event when Access-a end points start forwarding traffic to the primary HSRP peer. •Use VLAN Trunking Protocol (VTP) in transparent mode to reduce the potential for operational error. 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. •L2/L3 distribution with HSRP or GLBP is a tried-and-true design. This provides fast failover from one switch to the backup switch at the distribution layer. For the remainder of this document, the term EtherChannel is used to describe both variants. Additionally, the distribution-to-distribution link is an L3 routed link. Your enterprise can take advantage of the design principles and implementation best practices described in this design guide to implement a network that will provide the optimal performance and flexibility as the business requirements of your network infrastructure evolve. These switches are usually installed to replace shared concentrator hubs and give higher-bandwidth connections to the end user. •Use the Spanning-Tree toolkit to protect against unexpected STP participation. Also known as Three-layered Hierarchical Model, this is the Cisco flagship design for Campus networks. 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. 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. 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. Because of this small amount of memory, the potential for dropped traffic because of Tx-queue starvation is relatively high. The core serves as the backbone for the network, as shown in Figure 2. The high performance collapsed backbone uses layer three switching. After all, this eliminates the dependence of convergence on STP/RSTP. 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. 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. Chapter 5. Additionally, when you remove a direct path of communication for the distribution layer switches, you then become dependent on the access layer for connectivity. Return path traffic is dropped until the SPF timer has expired and normal reroute processing is completed. 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 implementing this topology, be aware that when the primary HSRP peer comes back online and establishes its L3 relationships with the core, it must ARP for all the end points in the L2 domain that it supports. 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. 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. 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 access layer is the first point of entry into the network for edge devices, end stations, and IP phones (see Figure 5). The behavior of the outbound traffic from the access layer to the rest of the network was described in the previous example (Figure 55). This helps prevent the VLAN hopping attack by making it difficult to correctly tag a packet. 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. 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. Feel free to export, print, and share your diagrams. If you require a common, centrally-managed VLAN database, consider using VTP version 3. 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. STP lets the network deterministically block interfaces and provide a loop-free topology in a network with redundant links (see Figure 18). Figure 21 PVST+ and Rapid PVST+ Performance. 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. The distribute list allows only the default route (0.0.0.0) to be advertised to the access layer nodes. If the L2/L3 boundary is in the access layer of the network, a design in which a routing protocol is running in the access layer, then NSF with SSO provides an increased level of availability. There should be no need to redesign the whole network each time a module is added or removed. Figure 8 shows both triangle and square network topologies. 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 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). IGMP snooping helps control multicast packet flooding for multicast applications. •Traffic is dropped until the MaxAge timer expires and until the listening and learning states are completed. A minimal configuration in the core reduces configuration complexity limiting the possibility for operational error. Most campus networks feature a high-performance, switched backbone, called the campus backbone, that connects buildings and different parts of the campus. 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. However, this approach can cause its own set of problems (see Figure 54), including the following: •Traffic is dropped until HSRP becomes active. Adding and removing VLANs is generally not a frequent network management practice. Figure 8 Triangle and Square Network Topologies. •Control route propagation to access layer using distribute lists. GLBP provides HSRP-like redundancy and failure protection. UDLD monitors hello messages to ensure that a response is received from the destination device, as shown in Figure 29. This promotes scalability and stability. 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. When this happens, the router must queue the packets and apply QoS to ensure that important traffic is transmitted first (see Figure 43). There are many reasons why STP/RSTP convergence should be avoided for the most deterministic and highly available network topology. 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. Read More •Only span VLANs across multiple access layer switches if you must. This L2 looped topology is configuration and management intensive. On links between a CatOS device and a Cisco IOS software device, you should disable PAgP negotiation if EtherChannel tunnels are not required. UDLD detects these physical misconfigurations and disables the ports in question. With currently available hardware switching platforms, CPU resources are not as scarce in a campus environment as they might be in a WAN environment. The following configuration snippets demonstrate how EIGRP was configured to achieve sub-200ms convergence for link and node failure scenarios. Campus networks generally use LAN technologies, such as Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), Fast Ethernet, Gigabit Ethernet, and Asynchronous Transfer Mode (ATM). In the configuration example below, summary routes are sent towards the core: When summarization is used, the distribution nodes interact with a bounded number of routing peers when converging around a link or node failure. Unused VLANs should be manually pruned from trunked interfaces to avoid broadcast propagation. 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. Suppose that each "node" represents 100 computer workstations. While this is not optimum, it is also not detrimental from the perspective of outbound traffic. However, a routed access layer topology is not a panacea. The default state for PAgP in CatOS is desirable, meaning that a CatOS switch tries to negotiate an EtherChannel. 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. 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. 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. 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. When designing a campus network, the network engineer needs to plan the optimal use of the highly redundant devices. (See Figure 24.). Summarizing using EIGRP or using an area boundary for OSPF are the recommended L3 configurations for the distribution-to-core layer L3 connection. CatOS devices should have PAgP set to off when connecting to a Cisco IOS software device if EtherChannels are not configured. It shows these modules and illustrates that the campus infrastructure module has three submodules: Building access submodule: Located within a campus building, this submodule contains end-user workstations and IP phones connected to switches or wireless access points. The star topology makes a network robust, and it has a faster performance. If you build a topology using triangles, with equal-cost paths to all redundant nodes, you can avoid timer-based, non-deterministic convergence. Approximately 50 percent of the hosts are not affected by the convergence event because their traffic is not flowing over the link or through the failed node. Figure 1-18 Sample Medium Campus Network Topology Large Campus Network Design Large campus networks are any installation of more than 2000 end users. Cisco introduced the hierarchical design model, which uses a layered approach to network design in 1999 (see Figure 1). 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 primary HSRP peer remains active and also forwards outbound traffic for its half of the stack. The hub-and-spoke topology design provides a more efficient operation for IP Multicast in the distribution layer because there is now a single logical designated router to forward IP Multicast packets to a given VLAN in the … •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. While this negotiation is happening, traffic is dropped because the link is up from an L2 perspective. Load balancing, Quality of Service (QoS), and ease of provisioning are key considerations for the distribution layer. A Network Diagram showing Network Topology for School. 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. Considerable outages can be experienced when distribution nodes are restored with totally stubby areas. This topology raises the following questions: • Where should the root switch be placed? 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. Although individual purchase decisions might seem harmless, network designers must not forget that this separate equipment still works together to form a network. 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. The benefits of dynamic propagation of VLAN information across the network are not worth the potential for unexpected behavior due to operational error. Data Network. Trunking protocols allow network node interconnections (uplinks) to carry multiple VLANS through a single physical link, as shown in Figure 22. 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. Higher-end switches provide uplinks to the building distribution module. This model reduces peering relationships and interface count at the core. 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. Access layer switches feed up into switches comprising the aggregation layer, also known as … In the first case, the standby HSRP peer can go active as it loses connectivity to its primary peer, forwarding traffic outbound for the devices that still have connectivity to it. This is also called a 'collapsed backbone' design for medium campus networks. •EIGRP provides for multiple levels of route summarization and route filtering that map to the multiple tiers of the campus. 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). It might be a single floor, a building, or even a group of … An enterprise can have more than one campus. This design is less than optimal from a convergence perspective. 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 rule-of-thumb recommendation for oversubscription is 20:1 for access ports on the access-to-distribution uplink. Figure 12 Recommended Topology (Links Between Two Distribution Nodes). Additionally, because both EIGRP and OSPF load share over equal-cost paths, this provides a benefit similar to GLBP. It is therefore recommended that only links intended for transit traffic be used to establish routing neighbor or peer relationships. Cisco switches let you tune the hashing algorithm used to select the specific EtherChannel link on which a packet is transmitted. Similarly to the L2/L3 distribution layer topology, NSF with SSO provides 1-3 seconds of packet loss without network convergence compared to total outage until a failed supervisor is physically replaced for the routed access topology. Only use L2 looped topologies if it cannot be avoided. Figure 13 Convergence Around a Failed Node. Millisecond timers can reliably be implemented to achieve sub-second (800 ms) convergence based on HSRP/GLBP failover. For example, each building distribution submodule should have two equal-cost paths to the campus backbone. This can have significant impact on performance. Highly available networks require redundant paths to ensure connectivity in the event of a node or link failure. –By default, one of the possible adjacencies is selected by a hardware hash where the packet source and destination IP address are used. When considering core topologies, it is important to consider the benefits of topologies with point-to-point links. The Enterprise Composite Network Model is a blueprint that network designers can use to simplify the complexity of a large internetwork. This provides high-availability for critical user groups. In the topology shown in Figure 57, the following convergence times can be observed: •With PVST+ (with UplinkFast)—Up to 5 seconds, •With Rapid PVST+ (address by the protocol)—1 second. Diagram of the topology of the data network backbone for the University of Kansas . 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. 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. Rapid PVST+ greatly improves the detection of indirect failures (L2 distribution-to-distribution link) or link up (uplink) restoration events. 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. To conserve memory and optimize performance at the access layer, configure a distribute list outbound on the distribution switch and apply it to all interfaces facing the access layer. One important factor to take into account when tuning HSRP is its preemptive behavior. However, the traffic in this attack scenario is in a single direction and no return traffic can be switched by this mechanism. Before the development of GLBP, methods used to utilize uplinks more efficiently were difficult to implement and manage. The following are general design considerations: •Use HSRP or GLBP for default gateway redundancy (sub-second timers). Figure 27 Mitigating Double-Tagged Packet Attacks. EIGRP stub nodes are not able to act as transit nodes and as such, they do not participate in EIGRP query processing. 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). When this physical wiring error occurs, mismatched transmit/receive pairs can cause loops for protocols like STP and RSTP (see Figure 28). The core needs to be fast and extremely resilient because every building block depends on it for connectivity. You can use the powerful Edraw campus network design software to create network system drawings. The campus network construction in the application of network technology is the important branch of LAN technology to build and management. HSRP and VRRP with Cisco enhancements both provide a robust method of backing up the default gateway, and can provide sub-second failover to the redundant distribution switch when tuned properly. Link up/down topology changes can be propagated almost immediately to the underlying protocols. This behavior caused a considerable amount of traffic being dropped; more than 40 seconds in the tested topology. A campus network is a building or group of buildings all connected into one enterprise network that consists of many local-area networks(LANs). A loopback cable is not required to ensure connectivity because traffic can pass over the distribution-to-distribution interconnection, as shown in Figure 49. In the distribution layer, change the default CEF load balancing behavior and use L3 and L4 information as input into the CEF hashing algorithm. 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