- You will see based off of device types and OS.
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 10.10.1.1 00.11.12.13.14.15
- 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)
- Sends or receives in one direction at a time.
- 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
- Fast Forward Mode
- Forwards the frame as soon as it gets the destination MAC address. No error check.
- 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)
- All ports share the memory space, frames are stored as needed.
- Stays in the same queue regardless of inbound/outbound priority.
- A set number and type of port.
- Purchase a chassis, then line cards.
- 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?”
- “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?
- What does ‘no switchport’ do?
- Creates a routed port.