Download Multi-Layer Switching

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
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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
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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
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
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
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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:
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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
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
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Fast convergence
Automatic load balancing
Elimination of peering problems
Performance Issues