Flashcards in Switching Deck (93):
What is the pps rate for a 100BaseX network? Calculate it for the minimum and maximum frame sizes.
Because all of the bit time values are one tenth that of 10 Mbps Ethernet, the pps for 100 Mbps Ethernet is 10 times the 10 Mbps pps values. So Fast Ethernet supports up to 148,800 pps for 64 byte frames and 8,120 pps for 1518 byte frames.
Default etherchannel load balancing method
What are the implications of mixing half-duplex and full-duplex devices? How do you do it?
You need to ensure that your full-duplex devices attach to full-duplex hubs. Otherwise, the full-duplex devices need to run in half-duplex mode. Always attach full duplex together and half duplex together.
What is the smallest Gigabit Ethernet frame size that does not need carrier extension?
The need for the carrier extension bytes is driven by the slotTime. Gigabit Ethernet uses a slot time for 4096 bits. This equates to 512 bytes. Therefore, any frames of 512 bytes or larger do not need carrier extension, whereas all frames less than 512 MUST have carrier extension.
If you attach a multiport repeater (hub) to a bridge port, how many broadcast domains are seen on the hub?
Legacy hubs have all ports in the same collision and broadcast domains, regardless of the internetworking device they attach to
Can a legacy bridge belong to more than one broadcast domain?
Generally, all ports on a legacy bridge belong to the same broadcast domain.
Summarize the three-step process that STP uses to initially converge on an active topology.
Elect a single Root Bridge for the entire bridged network.
Elect one Root Port for every non-Root Bridge.
Elect one Designated Port for every segment.
When running the Spanning-Tree Protocol, every bridge port saves a copy of the best information it has heard. How do bridges decide what constitutes the best information?
Lowest Root BID
Lowest Path Cost to Root Bridge
Lowest Sender BID
Lowest Port ID
Why are Topology Change Notification BPDUs important? Describe the TCN process.
Topology Change Notification BPDUs play an important role in that they help bridges relearn MAC addresses more quickly after a change in the active STP topology. A bridge that detects a topology change sends a TCN BPDU out its Root Port. The Designated Port for this segment acknowledges the TCN BPDU with the TCA flag in the next Configuration BPDU it sends. This bridge also propagates the TCN BPDU out its Root Port. This process continues until the BPDU reaches the Root Bridge. The Root Bridge then sets the TC flag in all Configuration BPDUs sent for twice the Forward Delay period. As other bridges receive the TC flag, they shorten the bridge table aging period to Forward Delay seconds.
How are Root Path Cost values calculated?
Root Path Cost is the cumulative cost of the entire path to the Root Bridge. It is calculated by adding a port's Path Cost value to the BPDUs received on that port.
Assume that you install a new bridge and it contains the lowest BID in the network. Further assume that this devices is running experimental Beta code that contains a severe memory leak and, as a result, reboots every 10 minutes. What effect does this have on the network?
STP is a preemptive protocol that constantly seeks the Root Bridge with the lowest BID. Therefore, in this network, the new bridge wins the Root War, and the entire active topology converges on this bridge every ten minutes. Where links change state during this convergence process, temporary outages of 30–50 seconds occur. When the bridge fails several minutes later, the network converges on the next most attractive Root Bridge and creates another partial network outage for 30–50 seconds.
When do bridges generate Configuration BPDUs?
Every Hello Time seconds on all ports of the Root Bridge (unless there is a Physical-Layer loop.
When a non-Root Bridge receives a Configuration BPDU on its Root Port, it sends an updated version of this BPDU out every Designated Port.
When a Designated Port hears a less attractive BPDU from a neighboring bridge.
When do bridges generate Topology Change Notification BPDUs?
A bridge port is put into the Forwarding state and the bridge has at least one Designated Port.
A port in the Forwarding or Learning states transitions to the Blocking state.
A non-Root Bridge receives a TCN (from a downstream bridge) on a Designated Port.
When is the Root Bridge placement form of STP load balancing most effective? What command(s) are used to implement this approach?
When traffic patterns are well defined and clearly understood.
When is the Port Priority form of STP load balancing useful? What command(s) are used to implement this approach? What makes this technique so confusing?
This form of load balancing is rarely useful. It can only be used with back-to-back switches. It should only be used in early versions of code or when connecting to non-Cisco devices.
What are the components that the default value of Max Age is designed to account for? There is no need to specify the exact formula, just the major components captured in the formula.
The default Max Age value of 20 seconds is designed to take two factors into account: End-to-end BPDU propagation delay and Message Age Overestimate.
What are the components that the default value of Forwarding Delay is designed to account for? There is no need to specify the exact formula, just the major components captured in the formula.
he default Forward Delay value of 15 seconds is designed to take four factors into account: End-to-End BPDU Propagation Delay, Message Age Overestimate, Maximum Transmission Halt Delay, and Maximum Frame Lifetime.
What are the main considerations when lowering the Hello Time from the default of two seconds to one second?
Lowering the Hello Time value can allow you to improve convergence time by lowering Max Age or Forward Delay (you have to do this separately) but also doubles the load that STP places on your network. Notice that load here refers to both the load of Configuration BPDU traffic and, more importantly, Spanning Tree CPU load on the switches themselves.
Where should PortFast be utilized? What does it change about the STP algorithm?
In general, PortFast should only be used on end-station ports. It allows a port to immediately move into the Forwarding state when it initializes. Other than that, the processing is the same. When using redundant NICs that toggle link state, it can also be useful for links to servers.
Where should UplinkFast be utilized? In addition to altering the local bridging table to reflect the new Root Port after a failover situation, what other issue must UplinkFast address?
UplinkFast should only be utilized in leaf-node, wiring closet switches. After a failover, UplinkFast must generate dummy multicast packets to update bridging tables throughout the network in addition to updating its own bridging table.
Where should BackboneFast be utilized?
To work correctly, BackboneFast must be enabled on every switch in a given Layer 2 domain.
Where is PVST+ useful?
PVST+ is useful when you are trying to connect traditional PVST Catalyst devices with 801.Q switches that only support a single instance of the Spanning-Tree Protocol.
Can MST regions be connected to PVST regions?
MST and PVST regions cannot be connected through trunk links (MST switches only support 802.1Q trunks, and PVST switches only support ISL trunks). However, the two types of switches can be connected through access (non-trunk) links (although this is rarely useful).
Can you disable STP on a per-port basis?
STP cannot be disabled on a per-port basis on Layer 2 Catalyst equipment such as the 4000s, 5000s, and 6000s. In fact, some Layer 3 Catalyst switches (Sup III with NFFC) require that STP be disabled for the entire device (all VLANs).
Why is it important to use a separate management VLAN?
It is important to use a separate management VLAN to prevent CPU overload. If the CPU does overload as a result of excessive broadcast or multicast traffic, the Spanning Tree information can become out-of-date. When this occurs, it becomes possible that a bridging loop could open. If this loop forms in the management VLAN, remaining CPU resources are quickly and completely exhausted. This can spread throughout the network and create a network-wide outage.
What happens if UplinkFast sends the fake multicast frames to the usual Cisco multicast address of 01-00-0C-CC-CC-CC?
If UplinkFast sends the dummy frames to the usual Cisco multicast address of 01-00-0C-CC-CC-CC, older, non-UplinkFast-aware Cisco Layer-2 devices do not flood the frames. Therefore, this does not update bridging tables through the network.
What happens in a traffic loading situation for EtherChannel when two servers pass files between each other?
All of the traffic between the servers crosses the same segment. This happens because the servers use the same MAC address for all of the frames. The ECB performs an X-OR on the MAC addresses and comes up with the same result every time.
If you have access to equipment, attempt to configure a two-segment EtherChannel where one end is set to transport only VLANs 1–10 and the other end of the segment is set to transport all VLANs. What gets established?
Nothing. Both ends of an EtherChannel must be configured to pass the same set of VLANs on all interfaces.
What is the difference between routing and Layer 3 switching?
In one sense, nothing. In another sense, the term routing implies that the forwarding is software-based where the term Layer 3 switching implies that hardware-based forwarding is used. In both cases, general-purpose CPUs are used to handle control plane functions (such as routing protocols and configuration).
Can the router-on-a-stick approach to inter-VLAN routing also support inter-VLAN bridging?
Yes. Simply configure a bridge-group on multiple subinterfaces.
Does MLS eliminate the need for a router?
No. Because MLS is a routing switch Layer 3 switching technique, it relies on caching information learned from the actions of a real router. The router must therefore be present to handle the first packet of every flow and perform the actual access list processing.
Does MLS require a router that runs the router-based NetFlow mechanism?
No. Other than the fact that MLS and NetFlow on the routers can both be used for detailed data collection, the two mechanisms are completely separate. A router doing MLS processing does not need to be running router NetFlow.
In MLS, does the router create the shortcut entry and download it to the Layer 3 CAM table located in the Catalyst's NFFC or MSFC/PFC?
No. Many people are of the opinion that MLS is simply a router running router NetFlow that learns a flow and then ships the results of this flow to a Catalyst. This is not the case. First, if it were the case, the flow would probably be over before the information could be learned by the Catalyst. Second, the NFFC learns the cache information totally by itself. It only needs to know the MAC address and VLAN information of the router (it learns this via MLSP).
What is MHSRP? How is it useful?
MHSRP stands for Multigroup Hot Standby Router Protocol. It is a technique that creates two (or more) shared IP addresses for the same IP subnet. It is most useful for load balancing default gateway traffic.
Ethernet process by which devices attached to the same cable negotiate their speed and the duplex settings over the cable.
Define half duplex
Ethernet feature in which a NIC or Ethernet port can only transmit or receive at the same instant in time, but not both. Half duplex is required when a possibility of collisions exists.
Define full duplex
Ethernet feature in which a NIC or Ethernet port can both transmit and receive at the same instant in time. It can be used only when there is no possibility of collisions. Loopback circuitry on NIC cards is disabled to use full duplex.
Define crossover cable
Copper cable with RJ-45 connectors in which a twisted pair at pins 1,2 on the first end of the cable is connected to pins 3,6 on the other end, with a second pair connected to pins 3,6 on the first end and pins 1,2 on the other end.
Define straight-through cable
Copper cable with RJ-45 connectors in which the wire at pin 1 on one end is connected to pin 1 on the other end; the wire at pin 2 is connected to pin 2 on the other end; and so on.
Define loopback circuitry
A feature of Ethernet NICs. When the NIC transmits an electrical signal, it “loops” the transmitted electrical current back onto the receive pair. By doing so, if another NIC transmits a frame at the same time, the NIC can detect the overlapping received electrical signals, and sense that a collision has occurred.
Define I/G bit
The most significant bit in the most significant byte of an Ethernet MAC address, its value implies that the address is a unicast MAC address (binary 0) or not (binary 1).
Define U/L bit
The second most significant bit in the most significant byte of an Ethernet MAC address, a value of binary 0 implies that the address is a Universally Administered Address (UAA) (also known as Burned-In Address [BIA]), and a value of binary 1 implies that the MAC address is a locally configured address.
Carrier sense multiple access with collision detection. A media-access mechanism where devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit simultaneously, a collision occurs and is detected by all colliding devices. This collision subsequently causes each device to delay retransmissions of the collided frame for some random length of time.
A method of collecting traffic received on a switch port or a VLAN and sending it to specific destination ports on the same switch.
A method of collecting traffic received on a switch port or a VLAN and sending it to specific destination ports on a switch other than the one on which it was received.
Define remote VLAN
The destination VLAN for an RSPAN session.
Define monitor session
The command used to initialize a SPAN or RSPAN session on a Catalyst switch.
Define VLAN filtering
Removing unwanted VLANs from a Layer 2 path.
Define encapsulation replication
Regeneration of the Layer 2 encapsulation removed from frames forwarded in a SPAN session.
A group of devices on one or more LANs that are configured (using management software) so that they can communicate as if they were attached to the same wire, when, in fact, they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, they are extremely flexible.
Define broadcast domain
A set of all devices that receive broadcast frames originating from any device within the set. Devices in the same VLAN are in the same broadcast domain.
A Cisco-proprietary protocol used to dynamically negotiate whether the devices on an Ethernet segment want to form a trunk and, if so, which type (ISL or 802.1Q).
Define VTP pruning
VTP process that prevents the flow of broadcasts and unknown unicast Ethernet frames in a VLAN from being sent to switches that have no ports in that VLAN
The IEEE standardized protocol for VLAN trunking.
Cisco-proprietary VLAN trunking protocol.
Define native VLAN
The one VLAN on an 802.1Q trunk for which the endpoints do not add the 4-byte 802.1Q tag when transmitting frames in that VLAN.
The process of taking a PDU from some other source and placing a header in front of the original PDU, and possibly a trailer behind it.
Define private VLAN
A Cisco switch feature that allows separation of ports as if they were in separate VLANs, while allowing the use of a single IP subnet for all ports.
Define promiscuous port
With private VLANs, a port that can send and receive frames with all other ports in the private VLAN.
Define community VLAN
With private VLANs, a secondary VLAN in which the ports can send and receive frames with each other, but not with ports in other secondary VLANS.
Define isolated VLAN
With private VLANs, a secondary VLAN in which the ports can send and receive frames only with promiscuous ports in the primary VLAN.
A mechanism in which VLAN information can extend over another set of 802.1Q trunks by tunneling the original 802.1Q traffic with another 802.1Q tag. It allows a service provider to support transparent VLAN services with multiple customers, even if the customers use overlapping VLAN numbers.
A convention often used as the data link protocol over Cable in which Ethernet is used as the data link protocol, but with PPP being encapsulated inside Ethernet. The combination gives the data link features of both Ethernet and PPP, in particular, the ability to forward the layer 2 Ethernet frames to the correct router, plus PPP authentication function of CHAP.
Digital subscriber line, a common Internet service type for residential and business customers.
A single instance of STP that is applied to multiple VLANs, typically when using the 802.1Q trunking standard.
Defined in IEEE 802.1d, a protocol used on LAN bridges and switches to dynamically define a logical network topology that allows all devices to be reached, but prevents the formation of loops.
Defined in IEEE 802.1s, a specification for multiple STP instances when using 802.1Q trunks
Defined in IEEE 802.1w, a specification to enhance the 802.1d standard to improve the speed of STP convergence.
Define Hello timer
An STP timer that dictates the interval at which the Root switch generates and sends Hello BPDUs.
Define Maxage timer
An STP timer that dictates how long a switch should wait when it ceases to hear Hellos.
Define forward delay timer
An STP timer that dictates how long a port should stay in the listening state and the learning state.
Define blocking state
An 802.1d STP port state in which the port does not send or receive frames, except for listening for received Hello BPDUs.
Define forwarding state
An 802.1d STP port state in which the port sends and receives frames.
Define listening state
An 802.1d STP transitory port state in which the port does not send or receive frames, and does not learn MAC addresses, but does wait for STP convergence and for CAM flushing by the switches in the network.
Define learning state
An 802.1d STP transitory port state in which the port does not send or receive frames, but does learn the source MAC addresses from incoming frames.
Define disabled state
An 802.1d STP transitory port state in which the port does not send or receive frames, but does learn the source MAC addresses from incoming frames.
Define alternate state
An 802.1w RSTP port state in which the port is not the Root Port but is available to become the root port if the current root port goes down.
Define discarding state
An 802.1w RSTP port state in which the port is not forwarding or receiving; covers 802.1d port states disabled, blocking, and listening.
Define backup state
An 802.1w RSTP port state in which the port is an alternative Designated Port on some LAN segment.
Define root port
The single port on each nonroot switch upon which the best Hello BPDU is received.
Define Designated port
With Spanning Tree Protocol, the single port on each LAN segment from which the best Hello BPDU is forwarded.
Define superior BPDU
Jargon used by STP mostly when discussing the root election process; refers to a Hello with a lower bridge ID. Sometimes refers to a Hello with the same bridge ID as another, but with better values for the tiebreakers in the election process.
A Cisco-proprietary STP implementation, created many years before IEEE 802.1s and 802.1w, that speeds convergence and allows for one STP instance for each VLAN.
The combination of PVST+ and Rapid Spanning Tree. It provides subsecond convergence time and is compatible with PVST+ and MSTP.
Cisco-proprietary STP feature in which an access layer switch is configured to be unlikely to become Root or to become a transit switch. Also, convergence upon the loss of the switch’s Root Port takes place in a few seconds.
Cisco-proprietary STP feature in which switches use messaging to confirm the loss of Hello BPDUs in a switch’s Root Port, to avoid having to wait for maxage to expire, resulting in faster convergence.
Cisco-proprietary STP feature in which a switch port, known to not have a bridge or switch attached to it, transitions from disabled to forwarding state without using any intermediate states.
Define root guard
Cisco-proprietary STP feature in which a switch port monitors for incoming superior Hellos, and reacts to a superior Hello to prevent any switch connected to that port from becoming root
Define BPDU guard
Cisco-proprietary STP feature in which a switch port monitors for STP BPDUs of any kind, err-disabling the port upon receipt of any BPDU.
A protection against problems caused by unidirectional links between two switches. Uses messaging between switches to detect the loop, err-disabling the port when the link is unidirectional.
Define Loop guard
Protects against problems caused by unidirectional links between two switches. Watches for loss of received Hello BPDUs, in which case it transitions to a loop-inconsistent state instead of transitioning to a forwarding state.
Defined in IEEE 802.1AD, defines a messaging protocol used to negotiate the dynamic creation of PortChannels (EtherChannels) and to choose which ports can be placed into an EtherChannel.