Network should have a minimum starting MTU of at least bytes to support the fabric overlay. MTU values between and are supported along with MTU values larger than though there may be additional configuration and limitations based on the original packet size. Devices in the same routing domain and Layer 2 domain should be configured with a consistent MTU size to support routing protocol adjacencies and packet forwarding without fragmentation. The fabric border nodes serve as the gateway between the SD-Access fabric site and the networks external to the fabric.
The border node is responsible for network virtualization interworking and SGT propagation from the fabric to the rest of the network. This is also necessary so that traffic from outside of the fabric destined for endpoints in the fabric is attracted back to the border nodes. Also possible is the internal border node which registers known networks IP subnets with the fabric control plane node. Packets and frames sourced from inside the fabric and destined outside of the fabric are de-encapsulated by the border node.
This is similar to the behavior used by an edge node except, rather than being connected to endpoints, the border node connects a fabric site to a non-fabric network. Fabric in a Box is an SD-Access construct where the border node, control plane node, and edge node are running on the same fabric node. This may be a single switch, a switch with hardware stacking, or a StackWise Virtual deployment.
SD-Access Extended Nodes provide the ability to extend the enterprise network by providing connectivity to non-carpeted spaces of an enterprise — commonly called the Extended Enterprise.
This allows network connectivity and management of IoT devices and the deployment of traditional enterprise end devices in outdoor and non-carpeted environments such as distribution centers, warehouses, or Campus parking lots.
This feature extends consistent, policy-based automation to Cisco Industrial Ethernet, Catalyst CX Compact, and Digital Building Series switches and enables segmentation for user endpoints and IoT devices connected to these nodes. Using Cisco DNA Center automation, switches in the extended node role are onboarded to their connected edge node using an Extended nodes are discovered using zero-touch Plug-and-Play.
Extended nodes offer a Layer 2 port extension to a fabric edge node while providing segmentation and group-based polices to the endpoints connected to these switches.
Endpoints, including fabric-mode APs, can connect directly to the extended node. Additional design details and supported platforms are discussed in Extended Node Design section below. Fabric WLCs provide additional services for fabric integration such as registering MAC addresses of wireless clients into the host tracking database of the fabric control plane nodes during wireless client join events and supplying fabric edge node RLOC-association updates to the HTDB during client roam events.
From a CAPWAP control plane perspective, AP management traffic is generally lightweight, and it is the client data traffic that is generally the larger bandwidth consumer. Wireless standards have allowed larger and larger data rates for wireless clients, resulting in more and more client data that is tunneled back to the WLC. The requires a larger WLC with multiple high-bandwidth interfaces to support the increase in client traffic. In non-fabric wireless deployments, wired and wireless traffic have different enforcement points in the network.
Quality of service and security are addressed by the WLC when it bridges the wireless traffic onto the wired network. For wired traffic, enforcement is addressed by the first-hop access layer switch. This paradigm shifts entirely with SD-Access Wireless.
Data traffic from the wireless endpoints is tunneled to the first-hop fabric edge node where security and policy can be applied at the same point as with wired traffic. Typically, fabric WLCs connect to a shared services network though a distribution block or data center network that is connected outside the fabric and fabric border, and the WLC management IP address exists in the global routing table. This avoids the need for route leaking or fusion routing a multi-VRF device selectively sharing routing information to establish connectivity between the WLCs and the APs.
Each fabric site must have a WLC unique to that site. Further latency details are covered in the section below. Strategies on connecting the fabric to shared services and details on route leaking and fusion routing are discussed in the External Connectivity and VRF-Aware Peer sections below.
Fabric access points operate in local mode. This generally means that the WLC is deployed in the same physical site as the access points. If this latency requirement is meant through dedicated dark fiber or other very low latency circuits between the physical sites and the WLCs deployed physically elsewhere such as in a centralized data center, WLCs and APs may be in different physical locations as shown later in Figure A maximum RTT of 20ms between these devices is crucial.
Fabric-mode APs continue to support the same wireless media services that traditional APs support such as applying AVC, quality of service QoS , and other wireless policies. They must be directly connected to the fabric edge node or extended node switch in the fabric site. For their data plane, Fabric APs establish a VXLAN tunnel to their first-hop fabric edge switch where wireless client traffic is terminated and placed on the wired network.
Fabric APs are considered a special case wired host. As a wired host , access points have a dedicated EID-space and are registered with the control plane node. It is a common EID-space prefix space and common virtual network for all fabric APs within a fabric site. The assignment to this overlay virtual network allows management simplification by using a single subnet to cover the AP infrastructure at a fabric site. To enable wireless controller functionality without a hardware WLC in distributed branches and small campuses, the Cisco Catalyst Embedded Wireless Controller is available for Catalyst Series switches as a software package on switches running in Install mode.
The wireless control plane of the embedded controller operates like a hardware WLC. Fabric in a Box deployments operating in StackWise Virtual do not support the embedded wireless controller functionality and should use a hardware-based or virtual WLC Catalyst CL.
They are an SD-Access construct that defines how Cisco DNA Center will automate the border node configuration for the connections between fabric sites or between a fabric site and the external world. Once in native IP, they are forwarded using traditional routing and switching modalities. IP-based transits are provisioned with VRF-lite to connect to the upstream device. Transit control planes nodes are a fabric role construct supported in SD-Access for Distributed Campus.
It operates in the same manner as a site-local control plane node except it services the entire fabric. Transit control plane nodes are only required when using SD-Access transits. Each fabric site will have their own site-local control plane nodes for intra-site communication, and the entire domain will use the transit control plane nodes for inter-site communication.
Transit control plane nodes provide the following functions:. This creates an aggregate HTDB for all fabric sites connected to the transit. It is an organization scope that consists of multiple fabric sites and their associated transits.
The concept behind a fabric domain is to show certain geographic portions of the network together on the screen. For example, an administrator managing a fabric site in San Jose, California, USA and another fabric site in Research Triangle Park, North Carolina, USA, which are approximately 3, miles 4, kilometers apart, would likely place these fabric sites in different fabric domains unless they were connected to each other with the same transit.
Figure 13 shows three fabric domains. The large text Fabrics represents fabric domains and not fabric sites which are shown Figure Both East Coast and West Coast have a number of fabric sites, three 3 and fourteen 14 respectively, in their domain along with a number of control plane nodes and borders nodes.
It is not uncommon to have hundreds of sites under a single fabric domain. A fabric site is composed of a unique set of devices operating in a fabric role along with the intermediate nodes used to connect those devices. At minimum, a fabric site must have a control plane node and an edge node, and to allow communication to other destinations outside of the fabric site, a border node. Fourteen 14 fabric sites have been created.
Each site has its own independent set of control plane nodes, border nodes, and edge nodes along with a WLC. LAN Design Principles. Device Role Design Principles. Feature-Specific Design Requirements. Wireless Design. External Connectivity. Security Policy Considerations. Multidimensional Considerations. Any successful design or system is based on a foundation of solid design theory and principles.
Designing an SD-Access network or fabric site as a component of the overall enterprise LAN design model is no different than designing any large networking system. The use of a guiding set of fundamental engineering principles ensures that the design provides a balance of availability, security, flexibility, and manageability required to meet current and future technology needs. This section provides design guidelines that are built upon these balanced principles to allow an SD-Access network architect to build the fabric using next-generation products and technologies.
These principles allow for simplified application integration and the network solutions to be seamlessly built on a modular, extensible, and highly-available foundation design that can provide continuous, secure, and deterministic network operations. This section will begin by discussing LAN design principles, discusses design principles covering specific device roles, feature-specific design considerations, wireless design, external connectivity, security policy design, and multidimensional considerations.
Underlay Network Design. Overlay Network Design. Shared Services Design. The following LAN design principles apply to networks of any size and scale. This section looks at underlay network, overlay network, shared services and services blocks, DHCP in the Fabric along with latency requirements for the network.
Layer 3 Routed Access Introduction. Enterprise Campus Architecture. About Layer 3 Routed Access. Having a well-designed underlay network ensures the stability, performance, and efficient utilization of the SD-Access network. Whether using LAN Automation or deploying the network manually, the underlay networks for the fabric have the following general design requirements:. Enabling a campus and branch wide MTU of ensures that Ethernet jumbo frames can be transported without fragmentation inside the fabric.
Combining point-to-point links with the recommended physical topology design provides fast convergence in the event of a link failure. The fast convergence is a benefit of quick link failure detection triggering immediate use of alternate topology entries preexisting in the routing and forwarding table.
Implement the point-to-point links using optical technology as optical fiber interfaces are not subject to the same electromagnetic interference EMI as copper links. Copper interfaces can be used, though optical ones are preferred. ECMP-aware routing protocols should be used to take advantage of the parallel-cost links and to provide redundant forwarding paths for resiliency. Routing protocols use the absence of Hello packets to determine if an adjacent neighbor is down commonly called Hold Timer or Dead Timer.
Thus, the ability to detect liveliness in a neighbor is based on the frequency of Hello packets. Each Hello packet is processed by the routing protocol adding to the overhead and rapid Hello messages creates an inefficient balance between liveliness and churn.
BFD provides low-overhead, sub-second detection of failures in the forwarding path between devices and can be set a uniform rate across a network using different routing protocols that may have variable Hello timers. NSF-aware IGP routing protocols should be used to minimize the amount of time that a network is unavailable following a switchover. These addresses also be propagated throughout the fabric site.
Reachability between loopback address RLOCs cannot use the default route. Although there are many alternative routing protocols, the IS-IS routing protocol offers operational advantages such as neighbor establishment without IP protocol dependencies, peering capability using loopback addresses, and agnostic treatment of IPv4, IPv6, and non-IP traffic. Manual underlays are also supported and allow variations from the automated underlay deployment for example, a different IGP could be chosen , though the underlay design principles still apply.
For campus designs requiring simplified configuration, common end-to-end troubleshooting tools, and the fastest convergence, a design using Layer 3 switches in the access layer routed access in combination with Layer 3 switching at the distribution layer and core layers provides the most rapid convergence of data and control plane traffic flows. Enterprise Campus Architecture Introduction. Hierarchical network models are the foundation for modern network architectures.
This allows network systems, both large and small, simple and complex, to be designed and built using modularized components. These components are then assembled in a structured and hierarchical manner while allowing each piece component, module, and hierarchical point in the network to be designed with some independence from overall design. Modules or blocks can operate semi-independently of other elements, which in turn provides higher availability to the entire system. By dividing the Campus system into subsystems and assembling them into a clear order, a higher degree of stability, flexibility, and manageability is achieved for the individual pieces of the network and the campus deployment as a whole.
These hierarchical and modular networks models are referred to as the Cisco Enterprise Architecture Model and have been the foundation for building highly available, scalable, and deterministic networks for nearly two decades. The Enterprise Architecture Model separates the network into different functional areas called modules or blocks designed with hierarchical structures.
The Enterprise Campus is traditionally defined with a three-tier hierarchy composed of the Core, Distribution, and Access Layers. In smaller networks, two-tiers are common with core and distribution collapsed into a single layer collapsed core. The key idea is that each element in the hierarchy has a specific set of functions and services that it offers. The same key idea is referenced later in the fabric control plane node and border node design section.
The access layer represents the network edge where traffic enters or exits the campus network towards users, devices, and endpoints. The primary function of an access layer switch is to provide network access to the users and endpoint devices such as PCs, printers, access points, telepresence units, and IP phones. The distribution layer is the interface between the access and the core providing multiple, equal cost paths to the core, intelligent switching and routing, and aggregation of Layer 2 and Layer 3 boundaries.
The Core layer is the backbone interconnecting all the layers and ultimately providing access to the compute and data storage services located in the data center and access to other services and modules throughout the network. It ties the Campus together with high bandwidth, low latency, and fast convergence. For additional details on the Enterprise Campus Architecture Model, please see:. In typical hierarchical design, the access layer switch is configured as a Layer 2 switch that forwards traffic on high speed trunk ports to the distribution switches.
The distribution switches are configured to support both Layer 2 switching on their downstream trunks and Layer 3 switching on their upstream ports towards the core of the network. The function of the distribution switch in this design is to provide boundary functions between the bridged Layer 2 portion of the campus and the routed Layer 3 portion, including support for the default gateway, Layer 3 policy control, and all required multicast services.
Layer 2 access networks provide the flexibility to allow applications that require Layer 2 connectivity to extend across multiple wiring closets. This design does come with the overhead of Spanning-Tree Protocol STP to ensure loops are not created when there are redundant Layer 2 paths in the network.
The stability of and availability for the access switches is layered on multiple protocol interactions in a Layer 2 switched access deployment. Trunking protocols ensure VLANs are spanned and forwarded to the proper switches throughout the system.
While all of this can come together in an organized, deterministic, and accurate way, there is much overhead involved both in protocols and administration, and ultimately, spanning-tree is the protocol pulling all the desperate pieces together. All the other protocols and their interactions rely on STP to provide a loop-free path within the redundant Layer 2 links.
If a convergence problem occurs in STP, all the other technologies listed above can be impacted. The hierarchical Campus, whether Layer 2 switched or Layer 3 routed access, calls for a full mesh equal-cost routing paths leveraging Layer 3 forwarding in the core and distribution layers of the network to provide the most reliable and fastest converging design for those layers.
An alternative to Layer 2 access model described above is to move the Layer 3 demarcation boundary to the access layer. Layer 2 uplink trunks on the Access switches are replaced with Layer 3 point-to-point routed links. This brings the advantages of equal cost path routing to the Access layer. Using routing protocols for redundancy and failover provides significant convergence improvement over spanning-tree protocol used in Layer 2 designs. Traffic is forwarded with both entries using equal-cost multi-path ECMP routing.
In the event of a failure of an adjacent link or neighbor, the switch hardware and software immediately remove the forwarding entry associated with the lost neighbor. However, the switch still has a remaining valid route and associated CEF forwarding entry. With an active and valid route, traffic is still forwarded. The result is a simpler overall network configuration and operation, dynamic load balancing, faster convergence, and a single set of troubleshooting tools such as ping and traceroute.
Layer 3 routed access is defined by Layer 3 point-to-point routed links between devices in the Campus hierarchy. SVIs and trunk ports between the layers still have an underlying reliance on Layer 2 protocol interactions. SD-Access networks start with the foundation of a well-design, highly available Layer 3 routed access foundation. For optimum convergence at the core and distribution layer, build triangles, not squares, to take advantage of equal-cost redundant paths for the best deterministic convergence.
Square topologies should be avoided. As illustrated in Figure 16, Core switch peer devices should be cross linked to each other. Distribution switches within the same distribution block should be crosslinked to each other and connected to each core switch. Access switches should be connected to each distribution switch within a distribution block, though they do not need to be cross-linked to each other. The interior gateway routing IGP routing protocol should be fully featured and support Non-Stop Forwarding, Bidirectional Forwarding Detection, and equal cost multi-path.
Point-to-point links should be optimized with BFD, a hard-coded carrier-delay and load-interval, enabled for multicast forwarding, and CEF should be optimized to avoid polarization and under-utilized redundant paths. It is the virtualization of two physical switches into a single logical switch from a control and management plane perspective.
It provides the potential to eliminate spanning tree, first hop redundancy protocol needs, along with multiple touch points to configure those technologies. Using Multichassis EtherChannel MEC , bandwidth can be effectively doubled with minimized convergence timers using stateful and graceful recovery.
In traditional networks, StackWise virtual is positioned in the distribution layer and in collapsed core environments to help VLANs span multiple access layer switches, to provide flexibility for applications and services requiring Layer 2 adjacency, and to provide Layer 2 redundancy. The distribution and collapsed core layers are no longer required to service the Layer 2 adjacency and Layer 2 redundancy needs with the boundary shifted.
In a Layer 3 routed access environment, two separate, physical switches are best used in all situations except those that may require Layer 2 redundancy.
For example, at the access layer, if physical hardware stacking is not available in the deployed platform, StackWise Virtual can be used to provide Layer 2 redundancy to the downstream endpoints.
StackWise Virtual can provide multiple, redundant 1- and Gigabit Ethernet connections common on downstream devices. In the SD-Access fabric, the overlay networks are used for transporting user traffic across the fabric. The fabric encapsulation also carries scalable group information used for traffic segmentation inside the overlay VNs.
Consider the following in the design when deploying virtual networks:. In general, if devices need to communicate with each other, they should be placed in the same virtual network. If communication is required between different virtual networks, use an external firewall or other device to enable inter-VN communication. Virtual Network provides the same behavior and isolation as VRFs.
Using SGTs also enables scalable deployment of policy without having to do cumbersome updates for these policies based on IP addresses. Subnets are sized according to the services that they support, versus being constrained by the location of a gateway. Enabling the optional broadcast flooding Layer 2 flooding feature can limit the subnet size based on the additional bandwidth and endpoint processing requirements for the traffic mix within a specific deployment.
Avoid overlapping address space so that the additional operational complexity of adding a network address translation NAT device is not required for shared services communication.
Services Block Design. Shared Services Routing Table. As campus network designs utilize more application-based services, migrate to controller-based WLAN environments, and continue to integrate more sophisticated Unified Communications, it is essential to integrate these services into the campus smoothly while providing for the appropriate degree of operational change management and fault isolation.
And this must be done while continuing to maintain a flexible and scalable design. A services block provides for this through the centralization of servers and services for the Enterprise Campus. The services block serves a central purpose in the campus design: it isolates or separates specific functions into dedicated services switches allowing for cleaner operational processes and configuration management. It also provides a centralized location for applying network security services and policies such as NAC, IPS, or firewall.
The services block is not necessarily a single entity. 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.
One services block may service an entire deployment, or each area, building, or site may have its own block. The services block does not just mean putting more boxes in the network. Services blocks are delineated by the services block switch. The goal of the services block switch is to provide Layer 3 access to the remainder of the enterprise network and Layer 2 redundancy for the servers, controllers, and applications in the services block.
This allows the services block to keep its VLANs distinct from the remainder of the network stack such as the access layer switches which will have different VLANs. These Ethernet connections should be distributed among different modular line cards or switch stack members as much as possible to ensure that the failure of a single line card or switch does not result in total failure of the services to remainder of the network.
Terminating on different modules within a single Catalyst and Nexus modular switch or different switch stack members provides redundancy and ensures that connectivity between the services block switch and the service block resources are maintained in the rare event of a failure. The key advantage of using link aggregation is design performance, reliability, and simplicity. With the Ethernet bundle comprising up to eight links, link aggregation provides very high traffic bandwidth between the controller, servers, applications, and the remainder of the network.
If any of the individual ports fail, traffic is automatically migrated to one of the other ports. If at least one port is functioning, the system continues to operate, remain connected to the network, and is able to continue to send and receive data. When connecting wireless controllers to the services block using link aggregation, one of three approaches can be used:.
The links are spread across the physical switches. This is the recommended option. This is a variation of first option and is recommended only if the existing physical wiring will not allow for Option 1. If the survivability requirements for these locations necessitate network access, connectivity, and services in the event of egress circuit failure or unavailability, then a services block should be deployed at each physical location with these requirements.
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