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 topology raises the following questions: • Where should the root switch be placed? The design principles and implementation best practices described in this document are tried-and-true lessons learned over time. Support for network virtualisation e.g. A hierarchical design avoids the need for a fully-meshed network in which all network nodes are interconnected. The following configuration examples show how to enable UDLD for CatOS and Cisco IOS software. There is a remote possibility that an attacker can create a double 802.1Q-encapsulated packet. The borderless switched network should also deliver current and future need. When using the on/on setting, PAgP is not enabled on members of the bundle. •Control peering across access layer links (passive interfaces). In a test environment using a typical IP addressing scheme of one subnet per VLAN and two VLANs per access switch using the RFC1918 private address space, the default L3 algorithm provided about one-third to two-thirds utilization. •Traffic is dropped until the MaxAge timer expires and until the listening and learning states are completed. The modular design makes the network more scalable and manageable by promoting deterministic traffic patterns. In a typical hierarchical model, the individual building blocks are interconnected using a core layer. It includes the following topics: •Layer 2 Redundancy—Spanning Tree Protocol Versions, •Protecting Against One-Way Communication with UniDirectional Link Detection, •Link Aggregation—EtherChannel Protocol and 802.3ad, •Using HSRP, VRRP, or GLBP for Default Gateway Redundancy, •Ensuring Connectivity in Case of Failure, •Tuning Load Balancing with Cisco Express Forwarding. In a topology with three switches, the answer depends on many factors. This section summarizes the campus network design recommendations, and includes the following topics: The following are the design recommendations for Layer 3 foundation services: •Design for deterministic convergence—triangles, not squares. (See Figure 24.). 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. Failover and convergence work just like HSRP. 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. Congestion on a Cisco Catalyst switch interface is not typically caused by oversubscription or an anomaly such as an Internet worm. 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). Cisco has identified several modules, including the enterprise campus… Protecting against double failures by using three redundant links or three redundant nodes in the hierarchical design does not increase availability. 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). number of L3 hopes and expected future growth, Convergence time e.g. Examples of functions recommended to be located in a services block include: There might be multiple services blocks depending on the scale of the network, the level of geographic redundancy required, and other operational and physical factors, As described in Cisco’s Enterprise Campus 3.0 Architecture, http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/campover.html#wp708979, The Data Center block of a campus network also known as “ server farm” can be considered as another block of the campus LAN that uses the same hierarchical design model, however in the data center there are some factors and design requirements that are different from a normal access-distribution switches design such as port capacity, ~0% of oversubsecription and more specialised services can be introduced like firewalling and loadbalcing services. If a supervisor fails on a non-redundant node, the links fail and the network converges around the outage through the second core or distribution node. This promotes scalability and stability. This allows for the failure or removal of one of the distribution nodes without affecting end point connectivity to the default gateway. However, in the core of the network a "less is more" approach should be taken. When the algorithm was changed to include L4 information, nearly full utilization was achieved with the same topology and traffic pattern (see Figure 31). You should enable the following additional STP features to protect against soft failures and rogue devices: Enable either Root Guard or BPDU Guard on access layer ports. In general, when you avoid STP/RSTP, convergence can be predictable, bounded, and reliably tuned. If you build a topology using triangles, with equal-cost paths to all redundant nodes, you can avoid timer-based, non-deterministic convergence. Another alternative is to configure both ends of the trunk to desirable. The core needs to be fast and extremely resilient because every building block depends on it for connectivity. Monitor redundant link utilization in the hierarchical model and take steps to tune both L2 (EtherChannel) and L3 (CEF) links to avoid under-utilization. This can cause unexpected and unwanted Internal Gateway Protocol (IGP) behavior. A link between the two distribution nodes is also required. When the distribution layer summarizes towards the core, queries are limited to one hop from the distribution switches, which optimizes EIGRP convergence. As demand increases or capacity is reduced, best-effort traffic may also be affected. The benefits of dynamic propagation of VLAN information across the network are not worth the potential for unexpected behavior due to operational error. •Tune EtherChannel and CEF load balancing to ensure optimum utilization of redundant, equal-cost links. 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. Classify and mark applications as close to their sources as technically and administratively feasible. If an L3 link between the distribution nodes is not present, return traffic (from the core to the access layer) could be dropped if an access layer link fails and the distribution nodes are not interconnected with an L3 link, as shown in Figure 14. The access-distribution block consists of two of the three hierarchical tiers within the multi-layer campus architecture: the access and distribution layers. The most challenging and important part of it is the planning and design phases where different technical variables and technologies need to be considered that could even effect the product selection and the design entirely. However, fully-routed access layer designs are not often deployed today. This design is less than optimal from a convergence perspective. The content of this book focuses on the prepare phase, plan phase, and design phases of the PPDIOO process as applied to building an enterprise campus network. However, the other extreme is also a bad thing. Figure 41 GLBP with STP Blocking Distribution-to-Distribution Link. Also, peering and adjacency issues exist with a fully-meshed design, making routing complex to configure and difficult to scale. NSF/SSO provide the most benefit in environments where single points of failure exist. The use of triangle rather than square topologies is only a recommendation. Default gateway redundancy is an important component in convergence in a hierarchical network design. 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. Operational resiliency: Enables resiliency capabilities to the next level, providing complete network availability even during planned network outages using In Service Software Upgrade (ISSU) features. Figure 45 QoS Mitigation of Tx-Queue Starvation. 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. VLANs (and thus broadcast domains) are … 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. UDLD detects these physical misconfigurations and disables the ports in question. •Negotiation of 802.1Q or ISL encapsulation: –Negotiate—Negotiate ISL or 802.1Q encapsulation with peer, –No negotiate—Always use hard-set encapsulation. 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. 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. Figure 26 Double 802.1Q-Encapsulated Packets. Load balancing, Quality of Service (QoS), and ease of provisioning are key considerations for the distribution layer. Figure 52 Traffic Flooding on Single Interface. 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. Additionally, when you remove a direct path of communication for the distribution layer switches, you then become dependent on the access layer for connectivity. Even though bandwidth capacity has increased to 1 Gbps, multiples of 1 Gbps, and even 10 Gbps, it is still impractical to provide enough bandwidth to run an entire access layer switch full of ports at line rate at the same time. Under normal circumstances, the network should provide an adequate level of service for all network traffic, including lower priority best-effort traffic. Depending on the version of STP, convergence could take as long as 90 seconds. 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. If the design is modified to support VLANs spanning access layer switches the fast convergence benefit of the design can not be realized. For example, by default, the Windows XP Home Networking Wizard bridges together all the interfaces on the machine. 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. This helps prevent the VLAN hopping attack by making it difficult to correctly tag a packet. This tuning can save seconds of outage when restoring a failed link or node. 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. •Operating system high-availability features, such as Link Aggregation (EtherChannel or 802.3ad), which provide higher effective bandwidth while reducing complexity. Figure 8 Triangle and Square Network Topologies. • What are the implications of STP/RSTP convergence? On links between a CatOS device and a Cisco IOS software device, you should disable PAgP negotiation if EtherChannel tunnels are not required. 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. When an end point ARPs for its default gateway, the virtual MACs are checked out on a round-robin basis. 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… Additionally, you can use QoS to reduce the priority of unwanted traffic. 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. •Routed Access—This option is interesting from a convergence performance perspective, but is not yet widely deployed. Only use BPDU Guard if you are able to intervene and re-enable error-disabled ports. The campus network, as defined for the purposes of the enterprise design guides, consists of the integrated elements that comprise the set of services used by a group of users and end … As illustrated in Figure 3-8, the Cisco SONA provides an enterprise-wide framework that integrates the entire network—campus… The following are best practices to use when deploying multiple VLANs on a single switch-to-switch interconnection or trunk: •Deploy VLANs on the interconnection between access and distribution layers. You can achieve load balancing on the uplinks from the access layer to the distribution layer in many ways, but the easiest way is to use GLBP. 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. Summarization is required to facilitate optimum EIGRP or OSPF convergence. In a hierarchical campus network if a virtual switch mechanism was not used at the distribution layer such as Cisco VSS, then the distribution layer switches need to provide the FHRP service e.g. This traffic flooding can have a performance impact on the connected end stations because they may receive a large amount of traffic that is not intended for them. Adding and removing VLANs is generally not a frequent network management practice. 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. If you want the best convergence available and you can ensure that no VLAN will need to span multiple access layer switches, then using a routed access layer topology is a viable design alternative. According to Cisco Medianet QoS campus design, the primary role of QoS in medianet campus networks is not to control latency or jitter (as it is in the WAN/VPN), but to manage packet loss. In this topology, SSO provides for protection against supervisor hardware or software failure with 1-3 seconds of packet loss and no network convergence. Recommend correcting if you have the opportunity. This section describes the foundation technologies used in the campus network and the recommended configurations. While this is not optimum, it is also not detrimental from the perspective of outbound traffic. For the VMDC version "2.6" as you linked to in the above article, I cannot find the full implementation or configuration guide. 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. The behavior of the outbound traffic from the access layer to the rest of the network was described in the previous example (Figure 55). 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. Use Loop Guard to protect the network from a soft failure where physical connectivity and packet forwarding are intact but STP (BPDU generation, forwarding, and evaluation) fails. 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. 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. This promotes scalability and stability. Tuning of Cisco Express Forwarding (CEF) equal-cost path selection is required to prevent CEF polarization, in which redundant links may be underutilized. This removes the possibility of flooding asymmetrically-routed return path traffic to all ports. 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. They are based on common use cases or engineering system priorities. This removes any possibility that a double 802.1Q-tagged packet can hop VLANs. While this negotiation is happening, traffic is dropped because the link is up from an L2 perspective. In Figure 53, the addition of a single switch to a very basic topology adds several orders of magnitude in complexity. For optimum core layer convergence, build triangles, not squares, to take advantage of equal-cost redundant paths for the best deterministic convergence. 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. When an indirect failure is detected and STP/RSTP converges, the distribution nodes reestablish their HSRP relationships and the primary HSRP peer preempts. Two types of trunks are currently available: 802.1Q is the Institute of Electrical and Electronics Engineers (IEEE) standard implementation. Dynamic Trunk Protocol (DTP) runs over switch interconnections and allows them to form a trunking interface. Typically deployed as a pair of L3 switches, the distribution layer uses L3 switching for its connectivity to the core of the network and L2 services for its connectivity to the access layer. Figure 32 Port Aggregation Protocol Operation. You must consider the additional IP address consumption for the point-to-point links between the access layer and distribution layer. Campus network devices can currently provide a high level of availability within the individual nodes. This occurs because the ARP and CAM aging timers are different. However, a routed access layer topology is not a panacea. An L3 link is required between the distribution nodes. Otherwise, multiple convergence events can occur for a single failure and undesirable traffic paths are taken after the spanning tree converges. Figure 4 Distribution Layer—High Availability. Note For more information, refer to the QoS SRND (http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html), which provides configuration examples and a detailed explanation of other technical issues. Traffic is flooded out the same interface that would be used normally, so the end result is the same. VRRP is the logical choice when interoperability with a non-Cisco device is required. ARP processing is rate limited in Cisco IOS software and in hardware to protect the CPU against DoS attacks that might overrun the CPU with an extraordinary number of ARP requests. •Assign the native VLAN to an unused ID or use the Tagged Native VLAN option to avoid VLAN hopping. This occurred while the access layer switch was load sharing over the equal-cost paths on both uplinks to the distribution layer, and the recovering distribution node was unable to forward the traffic being sent its way (see Figure 64). The switches in the access layer are connected to two separate distribution layer switches for redundancy. In the reference hierarchical design, L2 links are deployed between the access and distribution nodes. You can use BPDU Guard to stop all bridge devices, such as switches, from being added to your network. •Consider EIGRP/Routing in the access layer. The following configuration example shows how to change the 802.1Q native VLAN to something other than 1 (the default). There are quite a few spelling errors. Use L3 and L4 (UDP/TCP port) information as input to hashing algorithms. If the connection between the distribution layer switches is an L3 connection, then there are no loops and all uplinks actively forward traffic. Many redundant paths are provided in the recommended network topology. •Efficient network and bandwidth management using software features such as Internet Group Membership Protocol (IGMP) snooping. 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. Unused VLANs should be manually pruned from trunked interfaces to avoid broadcast propagation. The principal advantages of this model are its hierarchical structure and its modularity. The following are general design considerations: •Use HSRP or GLBP for default gateway redundancy (sub-second timers). The Cisco enterprise campus architecture divides the enterprise network into physical, logical, and functional areas while leveraging the hierarchical design. 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. STP lets the network deterministically block interfaces and provide a loop-free topology in a network with redundant links (see Figure 18). Police unwanted traffic flows as close to their sources as possible. Channel partners are absorbing a new take on Cisco campus network design … Get validated design guidance on our open, software-driven approach to deploy a digital-ready network. 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. The following are additional considerations when comparing EIGRP and OSPF: •Within the campus environment, EIGRP provides for faster convergence and greater flexibility. 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. Each level, or tier in the hierarchy is focused on specific set of roles. Figure 21 PVST+ and Rapid PVST+ Performance. Additionally, the distribution-to-distribution link is an L3 routed link. 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. PAgP or LACP enable the automatic formation of EtherChannel tunnels between interconnected switches (see Figure 32). 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. •Avoid daisy chaining stackable switches; stacks are good, StackWise and chassis solutions are better. Finally, VTP transparent mode should be used because the need for a shared VLAN database is lessened given current hierarchical network design. Another technique used multiple HSRP groups on a single interface and used DHCP to alternate between the multiple default gateways. This is also called a 'collapsed backbone' design for medium campus networks. 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. L3 recovery using NSF happens after the SSO convergence event, minimizing L3 disruption and convergence. 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