Term
What are the following standards:
802.1d
802.1p
802.1Q
802.2
802.3u
802.3z/802.3ab
802.3ae |
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Definition
802.1d - MAC Bridges
802.1p - Traffic classes/priority/QoS
802.1Q - Virtual bridged LANs/Corrections to MRP
802.2 - LLC
802.3u - Fast Ethernet
802.3z/802.3ab Gigabit Ethernet
802.3ae - Ten-Gigabit Ethernet |
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Term
| What is common across all 802.3, 802.11 and 802.16 IEEE LAN technologies? |
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Definition
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Term
| List all fields within an Ethernet frame that contains additional tags. |
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Definition
Preamble - 7 bytes
SFD - 1 byte
DST - 6 bytes
SRC - 6 bytes
Length or type - 2 bytes
P/Q tag - 4 bytes
Other Tags - Up to 482 bytes
Payload - typically up to 1500 bytes
Pad - variable
FCS - 4 bytes |
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Term
| Describe the Ethernet Preamble field. |
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Definition
This is a recognisable pattern (0xAA typically) that is used by the receiver to recover the clock.
7 bytes |
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Term
| Describe the Ethernet SFD field. |
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Definition
Indicates the end of the preamble and the beginning of the frame.
1 byte |
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Term
| Describe the Ethernet Length/Type field. |
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Definition
doubles as length or type
GE 1536 = type
LE 1500 = length |
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Term
What are the popular Ethernet type values for the following:
IPv4
IPv6
ARP
802.1Q |
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Definition
IPv4 - 0x0800
IPv6 - 0x86DD
ARP - 0x0806
802.1Q - 0x8100 |
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Term
| Describe the Ethernet PQ Tag? |
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Definition
Tag/Protocol ID - 16 bits - defines protocol type
Prio - 3 bits - defines CoS
CFI - 1 bit - used for compatibility with TR, should be 0
VLAN ID - 12 bits - VLAN ID |
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Term
| Describe the Ethernet CRC field. |
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Definition
4 bytes long.
Uses 32 bit divisor on frame and performs one's complement on the remainder.
Receiver performs calculation and will drop frame if result doesn't match the CRC. |
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Term
| What is the minimum Ethernet frame size? |
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Definition
64 bytes, with a minimum payload of 48 bytes (no tags)
Padding may be requird for small frames. |
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Term
| What is the maximum Ethernet frame size? |
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Definition
conventional - 1518 bytes - 1500 MTU.
Jumbo - 9000 bytes.
Envelope (with tags) - 2000 bytes. |
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Term
| What happens to 802.1p if the VLAN field of an 802.1p/q header is set to 0. |
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Definition
| Nothing, 802.1p can be used indepedently of 802.1q. |
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Term
| Why is Ethernet II not 100% efficient? |
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Definition
| Because it has an inter-packet gap of 12 byte times. |
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Term
| Describe how mismatched duplex configurations may degrade performance. |
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Definition
| A half-duplex interface may detect incoming traffic as a collision when there is moderate to heavy traffic in both directions. The exponential backoff will be triggered and the data that caused the collision will be lost. The performance degradation may not be noticed when traffic levels are low. |
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Term
| When might Link-Layer Flow Control be useful? |
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Definition
| If the aggregate traffic rate headed for a station exceeds the station’s link rate, frames start to be stored in the intermediate switches. If this situation persists for a long time, frames may be dropped. |
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Term
| How does Link-Layer Flow Control work? |
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Definition
| Some Ethernet switches (and interfaces) implement flow control by sending special signal frames (Pause messages - 802.3x) between switches and NICs. Flow control signals to the sender that it must slow down its transmission rate. |
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Term
| Why is Link-Layer Flow Control not really used? |
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Definition
| When multiple stations are sending through a switch that is struggling, the switch may send Pause frames to all stations. This is likely to penalise all stations, even if they were not responsible for the high traffic levels. |
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Term
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Definition
| The smallest MTU across the network path comprising all of the links is called the path MTU. |
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Term
| PMTU discovery is required in which version of IP? |
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Definition
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Term
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Definition
| Tunneling, generally speaking, is the idea of carrying lower-layer traffic in higher-layer (or equal-layer) packets. |
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Term
| Describe the original GRE header. |
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Definition
C bit - determines if the optional Checksum field is used.
Reserved (0) - 12 bits.
Version - 3 bits.
Protocol Type - 16 bits.
Checksum (optional) - 16 bits.
Reserved1 (optional) - 16 bits. |
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Term
| Describe the revised GRE header. |
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Definition
C bit - determines if the optional Checksum field is used.
K bit - determines if the optional Key field is used.
S bit - determines if the optional Sequence Number field is used.
Reserved (0) - 12 bits.
Version - 3 bits.
Protocol Type - 16 bits.
Checksum (optional) - 16 bits.
Reserved1 (optional) - 16 bits - present when Checksum is used.
Key (optional) - 32 bits - used to ID packets from the same flow.
Sequence NUmber - 32 bits. |
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Term
| Explain the difference between GRE and PPTP. |
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Definition
GRE tunnels are used within a network infrastructure to carry traffic between ISPs or within an enterprise. Although they can be combined with IPSec, they typically aren't.
PPTP is used between users and their ISP/corporate intranet and are encrypted (e.g. using MPPE. PPTP essentially combines GRE with PPP. |
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