Wednesday, February 10, 2016

CCNAv1 Chapter 5 Notes

Chapter 5
  • 0xA9

MAC Addresses:
  • You will see based off of device types and OS.
    • 00-60-2F-3A-07-BC
    • 00:60:2F:3A:07:BC
    • 00.60.2F.3A.07.BC

Multicast MAC Address
  • Always starts with “01-00-5E”
  • Convert the last 23 bits of the IP multicast group address to 6 hex digits.

  • Uses 2 messages:
    • ARP Request
    • ARP Reply
  • IPv4 sending to a host on a local network:
    • PC’s & Switches / Routers have ARP tables.
  • If no entry found in the local table (PC) send ARP request.
  • IPv4 sending to host not on the local network it means
  • Sending unit will use the mac address of the DGW.

  • Entries stored in the ARP table for various amounts of time. Example: Windows OS stores for two minutes but that entry is used again before the time expires, the time is extended to ten minutes.

Cisco Router Command - ‘Show IP ARP’
  • Layer 2 Switch: Maintains a table
    • Called a mac address table (mat)
    • 1 Entry per Port (if in use)
  • Port 12

  • Would there ever be more than one entrance per port? Yes
    • Why? Only if there is another switch attached to that port.

  • Port 12 A0.B1.C2.D3.E4.F5

Duplex Settings (Switch)
  • Full
  • Half
    • Sends or receives in one direction at a time.
  • Auto

  • Auto-MDIX
    • On by default for Cisco iOS
    • Port identifies what cable is in use and configures the interface to accept it.
Cisco Switch Frame Forwarding
  • Store and Forward
    • (Current Cisco Models default)
    • Switch waits until it has the entire frame, uses CRP (error checking) then forwards.
  • Cut-Through Switching
  1. Fast Forward Mode
  • Forwards the frame as soon as it gets the destination MAC address. No error check.
  1. Fragment-Free
  • Waits until it has the first 64 bits.

Switch Memory Buffering 10/5/15
  • When a switch receives a frame, it has to have a place to put it. It shows how it stores it while it’s dealing with it.
  • Two methods of dealing with memory buffering…
    • Port Based
      • Frames are put in a queue that’s linked to the incoming and outgoing port.
      • Each transaction requires two queues. (In queue, Out queue)
    • Shared
      • All ports share the memory space, frames are stored as needed.
      • Stays in the same queue regardless of inbound/outbound priority.

Switch Configuration
  • Fixed
    • A set number and type of port.
  • Modular
    • Purchase a chassis, then line cards.
  • Stackable
    • They connect via a special cable-- treated as one switch.

  • When starting out (What switch type should I get?)
    • “What do I need?”
    • “Will there be expansion?”
    • “Costs?”
    • “Rack units?”

  • Define: Port Density
    • The number of ports.

  • Define: Rack Unit
    • A measurement of height.

  • Multi-Layer Switch
    • Combines a switch with a router.
    • Layer 2 and/or Layer 3
      • Layer 3 uses CEF (Cisco Express Forwarding) that contains an FIB (Forwarding Information Base)
    • The FIB is similar to a routing table.
    • Also uses adjacency tables to maintain the layer two next-hop addresses.

  • Three types of Layer 3 Interfaces (Definitions)
    • SVI (Switch Virtual Interface) - Logical interface associated with a vlan
    • Router Port - Physical port configured as a router port.
    • Layer 3 Etherchannel - Logical interface with a bundle of routed ports.

Store-and-Forward: Store-and-Forward switching will wait until the entire frame has arrived prior to forwarding it.
  • This method stores the entire frame in memory.
  • Once the frame is in memory, the switch checks the destination address, source address, and the CRC. If no errors are present, the frame is forwarded to the appropriate port.
  • This process ensures that the destination network is not affected by corrupted or truncated frames.

Cut-Through: Cut-Through switching will begin forwarding the frame as soon as the destination address is identified.
  • The difference between this and Store-and-Forward is that Store-and-Forward receives the whole frame before forwarding.
  • Since frame errors cannot be detected by reading only the destination address, Cut-Through may impact network performance by forwarding corrupted or truncated frames.
  • These bad frames can create broadcast storms where in several devices on the network respond to the corrupted frames simultaneously.

Ethernet Frame Header
#1           2                   3                    4                    5                    6                    7


Start of Frame Delimiter

Destination Address (MAC)

Source Address (MAC)

46 to 1500

802.2 Header & Data

Frame Check Sequence

When a PC sends a message to another device on another network, it uses the default gateway (which is to a router in this diagram). If the device was on the same network it would have the mac address of the device it’s sending to.
  • Two facts about fixed configuration switches:
    • Has a set number of ports. (Can’t be changed)
    • Can be stackable.

  • What does adding a line card do to a switch?
    • Adds more ports / expansion to a switch.

  • What is asymmetric switching?
    • e

  • What does ‘no switchport’ do?
    • Creates a routed port.

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