Chapter 1 Packet Forwarding Flashcards
What is unknown Unicast flooding?
When a packet contains a destination MAC address that is not in the switch’s MAC address table. The switch forwards the packet to every switch port.
What is a VLAN?
VLAN provide logical segmentation by creating multiple broadcast domains on the same network switch.
What are the four fields that are added to the 802.1Q packet header?
- Tag protocol identifier (TPID): this is a 16 bit field set to 0x8100 to identify the packet as an 802.1Q.
- Priority code point (PCP): This 3-bit field indicates a class of service (CoS) as part of layer 2 QoS between switches.
- Drop eligible indicator (DEI): This is 1-bit field indicates whether the packet can be dropped when there is a bandwidth contention.
- VLAN identifier (VLAN ID): This 12-bit field specifies the VLAN associated with a network packet.
How many unique VLANs can be created from the 12bits of VLAN ID?
4094 Unique VLANs.
What is VLAN 0?
It is reserved for 802.1P traffic and cannot be modified or deleted.
What is VLAN 1 ?
It is the default VLAN. It cannot be modified or deleted.
What are the normal VLAN range?
VLANs 2 to 1001. They can be added, deleted or modified as necessary.
What are the reserved VLAN?
VLANs 2 to 1005 are reserved and cannot be deleted.
What are the extended VLANs?
VLAN 1006 to 4094. They can be added, modified and deleted.
What is an Access port?
Access port is designed to access only one VLAN.
What tag is not included on an access port?
The 802.1Q tags are not included on packets transmitted or received on access ports.
What is a Trunk port?
Trunk ports can carry multiple VLANs between a switch and another switch.
What is a Native VLAN?
Any traffic that is advertised or received on a trunk port without the 802.1Q VLAN tag is associated to the native VLAN.
What is the purpose of MAC address Table?
It is responsible for identifying the switch ports and VLANs with which a device is associated.
How does switches build MAC address table?
Switches build MAC address table by examining the source MAC address for traffic that it receives.
Where does MAC address table resides?
MAC address tables resides in CAM. The CAM uses high speed memory that is faster than typical computer RAM due to its search techniques.
What are the two main methods of layer 3 forwarding?
- Forwarding traffic to devices on the same subnet.
2. Forwarding traffic to devices on a different subnet.
What is the purpose of ARP table?
ARP table provides a method of mapping Layer 3 IP Address to Layer 2 MAC address by storing the IP of a host and it’s corresponding MAC address.
What is contained in the ARP table?
ARP table contains entries for remote devices that the host has communicated with recently and are on the same IP network segment.
Which commands route packets between VLANs?
SW1# Configure Terminal
SW1(config)# interface vlan 10
SW1(config-if)# IP address 10.10.10.1 255.255.255.0
SW1(config-if)# ipv6 address 2001:db8:10::1/64
SW1(config-if)# no shutdown
SW1(config-if)# interface VLAN 99
SW1(config-if)#Ip address 10.99.99.1 255.255.255.0
SW1(config-if)#ipv6 address 2001:db8:99::1/64
SW1(config-if)# no shutdown
What is a process switching?
Process switching also referred to as software switching or slow path, is a switching mechanism in which the general purpose CPU on a router is in charge of packet switching.
Types of packets that require soft handling?
- Packets sourced or destined to the router( using control traffic or routing protocols)
- Packets that are too complex for the hardware to handle ( that is IP packets with IP options)
- Packets that require extra information that is not currently known ( for example ARP)
What is the difference between software and hardware switching?
Software switching is significantly slower than switching done in hardware.
How is the routing table built?
The routing table also known as Routing Information Base (RIB), is built from information obtained from DYNAMIC routing protocols, DIRECTLY connected and STATIC routes.
What is Cisco Express Forwarding (CEF)?
CEF is a Cisco proprietary default switching mechanism on most Cisco platforms that do all their packet switching using the general-purpose CPU.
What is Ternary Content Addressable Memory (TCAM)?
TCAM allows for the matching and evaluation of a packet on more than one field. It is the extension of the CAM architecture but enhanced to allow for upper- layer processing.
What three results does TCAM search provide?
Zero (0) for true
One (1) for false
X for do not care
Why does TCAM operates on hardware ?
To provide faster processing and scalability than process switching. This allows for some features like ACL to process at the same speed regardless of whether there 10 entries or 500.
What is a centralised forwarding?
When a Route Processor(RP) engine is equipped with a forwarding engine so that it can make all the packet switching decisions.
What is a distributed architecture?
Distributed architecture is when a line card is equipped with forwarding engines so that they can make packet switching decisions without the intervention of Route Processor.
What are the components of software CEF?
- Forwarding Information Base: The FIB is built directly from the routing table and contains the next-hop IP address for each destination in the network.
- Adjacency Table: The adjacency table, also known as the Adjacency Information Base, Contains the directly connected next-hop IP addresses and their corresponding next-hop MAC addresses. It is populated with data from the ARP table or other Layer 2 protocol tables.
What is a TTL used for?
The TTL is a layer 3 loop prevention mechanism that reduces a packets TTL field by 1 for every layer 3 hop.
If a router received a packet with a TTL of 0, the packet is discarded.
What is a route processor (RP)?
RP is responsible for learning the network topology and building the route table (RIB)
What is a Stateful Switchover?
SSO is a redundancy feature that allows a Cisco router with two RPs to synchronise router configuration and control plane state information.
What is checkpointing?
The process of mirroring information between two RPs.
CEF is composed of which components?
- Forward Information Base
2. Adjacency Table
What are the characteristics of a layer 2 switch access port?
- Access mode will is the default operating mode for a port on a layer 2 switch.
- An access port is associated with a single VLAN.
Which packet forwarding process uses ternary content addressable memory (TCAM)?
Hardware CEF
What is a feature of a Cisco Switch SVI?
An SVI is configured by setting it as a VLAN interface.
Which Cisco router component performs process switching of packets?
The general-purpose CPU.
What is the difference between a routed switch port and a trunk port on a multilayer switch?
A router port is assigned an IP address.
What would be the source of the entries in the ARP table of a particular IPv4 host?
Devices that the host has communicated with recently that are on the same IP Network segment.
Which is the correct description of a native VLAN on a Cisco switch?
The VLAN that contains any untagged network traffic on a trunk port.
Which router memory component is used by CEF to make packet forwarding decisions?
Forwarding Information Base(FIB)
Where does a Cisco switch store the MAC address table entries?
CAM
Which CEF component enables multiple actions to be performed in addition to allowing or dropping traffic?
TCAM
What is a feature of virtual LANs ?
Logical segmentation is provided by creating multiple broadcast domains on a single switch.
What are two characteristics of a layer 2 Ethernet trunk ports?
- Trunk ports carry multiple VLANS
2. Trunk ports connect a switch to other switches or other network devices.
