Transcript Switching
Switching Topic 1 Basic concepts Agenda • • • • Ethernet 802.3 CSMA/CD and duplex Frames and MACs Switching process – Store, forward and buffers • Issues – Collisions and broadcasts – Latency and congestion • Layer 3 switches • Switchport security • Switch boot sequence Ethernet standard IEEE 802.3 • LAN standard • Layer 2 data link (OSI) • CSMA/CD technology for multi-access segments (shared links) • Frames: – Unicast – Broadcast – Multicast • Switches and access points and NICs, twisted pair or fiber (mm or sm) and star topology and pointto-point links CSMA/CD • Carrier sense: – Listen before transmitting, if no traffic transmit message – Keep listening for collisions • Multi-access: – If two devices transmit at the same time, signals collide • Collision detection: – All devices listen for collisions – an increase signal amplitude – Transmitting devices continue to transmit until minimum packet time is reached (jam signal) to ensure that all devices detect the collision – All devices start a back-off algorithm and wait for a random of time (no transmitting) – Back to listening mode – No device has priority to resend • Multi-access hub based, half duplex communications only Duplex • Half duplex – link shared by many hosts via a hub – Data can travel in both directions but only one direction at a time (1 lane bridge) – Uses CSMA/CD to detect and manage collisions – Hub based networks – Lower performance – lots of waiting for the media – 50–60% efficiency • Full duplex – only one host at each end of link – Data can be sent and received at the same time (2 lane bridge) – Uses two pairs of wires (Cat 5e uses four pairs), one to transmit and one to receive – No collisions, sending and receiving done on two separate circuits – CSMA/CD not required, collision detect circuit is disabled – Host is attached to a dedicated switchport – Point-to-point connection – 100% efficiency in both directions (100 Mbps transmit and 100 Mbps receive for FastEthernet) Switchport duplex settings • Auto – Both nodes negotiate the duplex setting to use – Default for FastEthernet ports and 10/100/1000 NICs • Full – Default for 100Base-FX ports and for Gigabit ports • Half – Default mode if auto negotiation fails (unsupported by other host) • Duplex mismatch – Switch configured for full duplex and host only supports half duplex – FCS errors on full duplex port (show interface) – Random ping packets succeed and most fail • Auto-MDIX – Switch detects the cable type for copper Ethernet connections and configures the interface to match – Use either crossover or straight-through cables between hosts and switches and switches and switches – Enabled by default on Cisco® IOS 12.2(18) and later Ethernet frames • Packet is encapsulated into a frame • Frame is transmitted onto the media • Frames use MAC addresses – 48 bits, 12 hex digits, burned into NIC – OUI |Vendor assigned Mac-address-table • MAC address table maps the switchports and the MAC addresses of the hosts connected to the switchport • MAC addresses are learned and added to the MAC address table by checking the source MAC in the header of frames arriving on the switchport • Mappings age out to keep data current • Also called CAM table MAC address table • Demo Switching process • Switch receives an incoming frame through an arriving port • Switch adds source MAC address to MAC address table if not known • Flood, forward or filter? – If a broadcast frame FF-FF-FF-FF-FF-FF, switch forwards out of all ports except arriving port – If unicast frame switch does a lookup on MAC address table for the destination MAC and its associated port • If not found, the frame is broadcast – If the associated port is the same as the arriving port the frame is dropped • Frame is switched to the destination MAC port(s) and forwarded • Uplink ports have multiple MAC addresses associated with them – as all the destinations on the upstream switch are learned from arriving frames they are added to the MAC address table Switch forwarding • Store and forward – high integrity – As a frame arrives it is stored in a buffer until fully received – Switch does error check, computes and verifies CRC value in trailer – If CRC integrity check is successful, MAC address table lookup on destination address for destination port and frame is forwarded, if not then frame is dropped – Store and forward switching is required for QoS analysis for prioritisation – Store and forward is now the only forwarding method on new Cisco® devices • Cut through (fast forward) – fast and low latency – Switch does not perform error checking – Switch buffers first few bytes, determines the destination MAC address, looks up the destination port and begins forwarding through the outgoing port – Faster but frames with errors can be forwarded • Variants – Fragment-free switching • Switch stores the first 64 bytes and does an error check, then starts forwarding Switching symmetry • Symmetric switching – All ports have the same bandwidth – Optimised for distributed traffic load such as peer-topeer desktops • Asymmetric switching – Ports have different bandwidths – More bandwidth dedicated to server switchports and to uplink ports to prevent bottlenecks – Requires memory buffering to match the different data rates Memory buffering • Port-based memory buffering – Arriving frame is queued in the arriving port buffer – Frame is not moved to the destination port until the all frames ahead in the queue are transmitted – Delayed even when the destination port is open • Shared memory buffering – All frames from all ports are stored in a common memory buffer – Frames are linked to their destination port with a map of frame to port links – Frames can be transmitted as soon as the destination port is idle – Larger frames are transmitted with fewer dropped frames as memory is allocated dynamically Collision Issues • Shared media environments have the potential for collisions – All connections on a hub belong to one collision domain – Don’t use hubs (200% reduced to 50% efficiency) • Host connecting to a switch is a dedicated connection – – – – An individual collision domain, a microsegment There is no potential for collisions Separate wires are used to transmit and receive 24 port switch has 24 collision domains • Switches increase the number of collision domains (and reduce the size of collision domains ) • Switches improve efficiency as all bandwidth is available to the host Broadcast issues • Many protocols must broadcast – ARP (who has 192.168.1.1?) to determine a destination host MAC – DHCP (are you a DHCP server?) to locate a DHCP server • Switches forward broadcast frames – Broadcasts are sent through all switchports including links to other switches except the originating switchport • All hosts receive and process broadcasts – Bandwidth used up – CPU processing time used up • As more switches and hosts are added there are more broadcasts on the network – More than 20% broadcast traffic on a host and the network is too large • Too much broadcast traffic reduces performance, uses bandwidth and CPU cycles • Routers divide networks and define broadcast domains – Routers do not forward broadcasts Segmentation • Segmentation is creating a boundary around a physical grouping of hosts • Routers segment the broadcast domain – Creating smaller broadcast domains reduces broadcast traffic and makes more bandwidth and processing available to applications – Each router interface connects to a different LAN network (different broadcast domain) • Switches segment the collision domain – Reduces the size of the collision domain – Each switchport connects to a different segment (collision domain) Broadcast and collision domains Latency • Latency or delay is the time a frame or packet takes to travel from the source to the destination • Sources of latency: – NIC delay – time to encode and transmit signals or receive and decode frames – Propagation delay – time for a signal to move down the link to the destination – Transmission delay – time it takes the switch to process, buffer and forward the frame • Switches have less latency than routers because: – Routers have more complex and processor intensive functions (ACLS and routing) – Routers strip frame headers to read packet headers • Switches support high forwarding rates – By using ASICS application specific integrated circuits to provide hardware support for wire speed • Access layer switches can be oversubscribed – Full bandwidth on all ports is more than the internal forwarding rate Congestion • Causes of network congestion: – More powerful hosts which send and process data at higher rates – Increasing volumes of network traffic: • due in part to broadcast traffic • due in part to 80/20 rule changing to 20/80 • now 80% of resources are located outside the LAN and require crossing the core • High bandwidth applications – such as desktop publishing, engineering design, video on demand, e-learning and streaming video (video and multimedia) Network bottlenecks • How many ports are required for hosts? For uplinks? • What speed is the host sending at? • 48 ports running at 1 Gbps in full duplex requires an internal forwarding rate of 96 Gbps – What is the internal throughput of the device? – Can it handle the anticipated traffic loads considering its placement in the network? • Latency is greater on routers but routers split broadcast domains • Do the maths and aggregate multiple links Security issues • • • • • • Limits the number of valid MAC addresses allowed on the port – Port will not forward traffic from disallowed addresses – Authorised MAC address is assured full bandwidth on the port Static secure MAC addresses: Manually configured in address table Dynamic secure MAC addresses: Learned dynamically (removed when switch restarts) Sticky secure MAC addresses: Dynamically learn MAC addresses and saved to the running configuration Security violation mode: – If more than the maximum allowed MAC addresses attempts to access the interface OR, if an address learned or configured on a secure interface is seen on another secure interface in the same VLAN a violation occurs Actions taken when violation occurs: – Protect: drop frame and no notification sent – Restrict: drop frame and send notification, SNMP trap or syslog message – Shutdown: interface is disabled and LED turns off, SNMP trap and syslog message sent and violation counter incremented. (Release with shutdown and no shutdown commands). Layer 3 switching • Layer 3 switches can examine IP addresses and route traffic at switch speeds – Layer 3 switches can route between VLANs • Layer 3 switching is faster than routing • Layer 3 switches do not support WAN interfaces • Layer 3 switches do not support advanced routing functions such as remote access connections VPNs Switch boot sequence • Loads boot loader from ROM • Boot loader – – – – – initialises CPU registers performs POST initialises flash file system loads the default IOS image into memory initialises interfaces with commands from config.text stored in flash • POST completes – SYST LED blinks green or amber if POST fails • Boot loader provides a command line to format flash file system, reinstall IOS image or recover a password Agenda • • • • Ethernet 802.3 CSMA/CD and duplex Frames and MACs Switching process – Store, forward and buffers • Issues – Collisions and broadcasts – Latency and congestion • Layer 3 switches • Switchport security • Switch boot sequence Switching Topic 1 Basic concepts