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 Rule Now 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 -- , 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 Switching Hardware-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