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 -- 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 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