Multi-Layer Switching Layers 1, 2, and 3
Cisco Hierarchical Model • Access Layer • Workgroup • Access layer aggregation and L3/L4 services • Distribution Layer • Services, Server Farms • ACLs, Queues; policy-based connectivity • Core Layer • Rapid Packet Switching • Optimal connectivity between blocks • NOTE: Different Layers require difference devices
Distribution Layer Functions • Address or area aggregation • Departmental or workgroup access • Broadcast/multicast domain definition • VLAN routing • Any media transitions that need to occur • Security • Can be a redistribution point between routing domains or the demarcation between static and dynamic routing protocols • Can be point at which remote sites access the corporate network.
Access Layer Functions • Shared bandwidth • Switched bandwidth • MAC-layer filtering • Microsegmentation • The way the layers are implemented depends on the needs of the network being designed • For a network to function optimally and maintain scalability as growth occurs, hierarchy must be maintained.
Resource Usage • Two common types of broadcasts that poll the network • IP Address Resolution Protocol (ARP) requests • NetBIOS name requests. • normally propagated across an entire subnet and expect the target device to respond directly to the broadcast. • Multicast traffic can also consume a large amount of bandwidth. • Multicast traffic is propagated to a specific group of users • can consume most, if not all, of the network resources. • An example of a multicast implementation is the Cisco IP/TV solution, which uses multicast packets to transport multimedia such as audio and video.
80/20 RuleNow Reversed • No more than 20 percent of the network traffic should move across the backbone of the network. • Now 80% goes across backbone because • Server Farms • Internet • Improve Network Performance by: • Moving resources such as applications, software programs, and files from one server to another to contain traffic locally within a workgroup. • Moving users logically, if not physically, so that the workgroups reflect the actual traffic patterns. • Adding servers so that users can access them locally without having to cross the backbone.
Designing for New Realities • Fast convergence • This requirement stipulates that the network must be able to adapt very quickly to changes in the network topology • Deterministic paths • Alows for a device or an administrator to make decision based on the desirability of a path • Redundancy • Ensures network is available at all times • Scalability • Changes can be made without radical topology changes • Centralized Services – e.g. Server Farms
New Realities • New 20/80 rule • Multicasting • Multiprotocol Support
Services • Local services • On local LAN • Remote services • May be close but are on different LAN • Enterprise services • Services common to all users -- e-mail, Internet access, and videoconferencing • Place in common subnet close to backbone
Layer 2 Switching • PDU is FRAME • Workgroup Connectivity & network segmentation • Hardware-based bridging • Wire-speed performance • High Speed Scalability • Low Latency • MAC Address • Low Cost • Some of characteristics of legacy bridging • Broadcast domain • Scaling and performance issues
Layer 3 SwitchingHardware-based routing – place switch any where • Packet forwarding is handled by specialized hardware ASICs. • goal is to capture the speed of switching and the scalability of routing. • Layer 3 switch acts on a packet as a router would • Determining the forwarding path based on Layer 3 information • Validating the integrity the L 3 header via checksum • Verifying packet expiration and updates accordingly • Processing and responding to any option information • Updating forwarding statistics in the Management Information Base (MIB) • Applying security controls if required • Implementing quality of service (QoS
Layer 4 Switching • Layer 3 hardware-based routing that accounts for Layer 4 control information • ability to make forwarding decisions based on L4 parameters such as port number as well as MAC address or source/destination IP address • Control Layer 4 switching • Extended ACL lists • NetFlow Switching, utilized on the Cisco 7200 and 7500 router platforms • Prioritize traffic by type of application • Requires high-speed performance switch with extensive memory to support tables and table processing
Multilayer Switching • Combines Layer 2 switching and Layer 3 routing functionality • moves campus traffic at wire speed and at same time satisfies Layer 3 routing requirements • Accelerates routing performance through the use of dedicated ASICs. • MLS can operate at Layer 3 or 4. • When operating as a Layer 3 switch, the switch caches flows based on IP addresses. • When operating as a Layer 4 switch, the switch caches conversations based on source address, destination address, source port, and destination port
Network Building Blocks • Campus elements: • Switch block • Core block • Contributing variables: • Server block • WAN block • Mainframe block • Internet connectivity
Switch Block • Contains both router and switch functionality • The distribution device can be one of the following: • switch and external router combination • Multi-layer switch • A switch may support one or more subnets. • subnet must reside within one broadcast domain. • all stations residing in or ports configured on the same VLAN are assigned network addresses within the same subnet. • The broadcast-isolation feature of VLANs is the characteristic that allows VLANs to be identified with subnets.
Spanning Tree • Access devices have redundant connections, or uplinks, to the distribution switch to maintain resiliency. • Spanning-Tree Protocol allows these redundant links to exist while preventing undesirable loops in the switch block. • The Spanning-Tree Protocol terminates at the boundary of the switch block
Scaling the Switch Block • Depends on different types and patterns of traffic • Amount of Layer 3 switching capacity at the distribution layer • Number of users per access-layer switch • Extent to which subnets need to traverse geographical locations within the network • Size to which the Spanning-Tree domains should be allowed to grow • Sizing the switch block involves two main factors: • Traffic types and behavior • Size and number of workgroups
The Core Traffic • The core can consist of one subnet; • For resiliency and load balancing, at least two subnets are configured. • Because VLANs terminate at the distribution device, core links are not trunk links and traffic is routed across the core. • core links do not carry multiple VLANs per link. • One or more switches make up a core subnet • Two basic core designs: • Collapsed core • Dual core
Layer 3 Backbone Scaling • Fast convergence • Automatic load balancing • Elimination of peering problems • Performance Issues