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CCIE Switching
Review Questions for CCIE Switching
131
Computer Networking
Professional
04/16/2010

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Term
What is the default STP bridge priority?
Definition
32768
Term
What is the priority for the VLAN below, assuming other switches are at default or are unknown?
(config)#spanning-tree vlan 100 root primary
Definition
3K: 24576 if other switches are default or unknown.
4K, 6K: 8192 if other switches are default or unknown.
Term
What is the bridge priority if this switch connects to an existing STP?
(config)#spanning-tree vlan 100 root primary
Definition
Current root priority - 4096 = 20480 (if previous root using default of 24576)
Term
What is the bridge priority for the following: (config)#spanning-tree vlan 100 root secondary
Definition

28672 if others are default or unknown

 

Default is 32768

Secondary is 32768 - 4096 = 28672

Root is 32768 - 8192 = 24576

Term
What are the 4 stages of 802.1d STP and how long do they take?
Definition
Blocking (root election): 20 seconds
Listening (path election): 15 seconds
Learning: 15 seconds
Forwarding
Term
802.1D: How does a bridge choose when it has two paths to the root?
Definition
In order of preference:
1) Lowest cost to root
2) Lowest port priority
3) Lowest port number
Term
Who becomes the root bridge in 802.1D?
Definition
Bridge with lowest priority.
If priorities are equal, use lowest MAC.
Term
What is the default cost for 100M link in 802.1D?
Definition
19
Term
What is MaxAge in 802.1D?
Definition
10 * Hello = 20 seconds
This is the duration of root election and the amount of time a switch will ignore bad BPDUs or timeout a root port that has stopped receiving BPDUs.
Term
What etherchannel protocol uses the options "auto" and "desirable"?
Definition
PagP
Term
What etherchannel protocol uses the options "active" and "passive"?
Definition
LCAP -- IEEE 802.1AD
Term
What is the default load balancing scheme for etherchannel?
Definition
Source and destination IP
Term
Will an etherchannel form if one side is configured for PagP and the other is not?
Definition
Yes, unless the "non-silent" option is configured.
Term
What are the three port states in 802.1w?
Definition
Discarding
Learning
Forwarding
Term
What are the two new port roles introduced in 802.1w?
Definition
  1. Alternate (to Root)
  2. Backup (to Designated)

802.1d defined three: root, designated, blocked

Term
What defines a topology change in 802.1w?
Definition
A non-edge port moving to the forwarding state.
Term
What vlans are not eligible for VTP pruning?
Definition
1, 1002-1005

1006 - 4094 are not supported with VTP
Term
Is VTP enabled by default? Is VTP pruning enabled by default?
Definition
VTP is enabled by default, VTP pruning is not.
Term
If one side is configured for "mode trunk" and the other side is "mode dynamic auto", will a trunk form?
Definition
Yes.

"mode trunk" will send DTP (but it doesn't require it to form a trunk).
Term
If one side is configured for "mode trunk, nonnegotiate" and the other side is "mode dynamic auto", will a trunk form?
Definition
No. "dynamic auto" will not initiate DTP but it requires that DTP succeed.  "non-negotiate" means other sides will not send DTP.
Term
Which IEEE standard defines STP?
Definition
802.1D
Term
In 802.1D, what is the port called that is receiving the best BPDUs?
Definition
The Root Port is the port on the switch that receives the best BPDUs.
Term
In 802.1D, what is the name of the port that can send the best BPDU on the segment?
Definition
The Designated Port can send the best BPDU on the segment
Term
In STP, what is the name of any port that is not designated or root and receives better BPDUs than it can send?
Definition
A blocked port.
Term
What is the function of the Blocking phase in STP?
Definition
The root is elected in the blocking phase
Term
Can bridges learn MAC addresses in the Blocking phase of STP?
Definition
No, MAC addresses are not learned in the blocking phase.
Term
In what phase of STP does path election occur?
Definition
Path election occurs in the Listening Phase of STP
Term
At what link speed is the 16 bit short mode cost inaccurate for STP?
Definition
Cost is 10 for 10G (which is inaccurate as it is higher than the 1G cost of 4).
Term
What is the cost of a 100M link in 32 bit Long Mode Cost?
Definition
In Long Mode, the cost of a 100M link is 200000.
Term
What does the command "spanning-tree pathcost method long" do?
Definition
Changes to 32 bit path cost
Term
What determines which switch will forward frames to/from this segment when two switches are connected to the same downstream LAN?
Definition
Lowest Root Path Cost
Lowest Sender Bridge ID
Lowest Sender Port ID
Term
How long is the fwd-delay timer and what 802.1D phase does it occur in?
Definition
15 seconds, Learning phase
Term
In a stable STP topology, who originates BPDUs?
Definition
Only the root originates BPDUs. non-root switches forward those BPDUs.
Term
In what STP phase are MAC addresses learned and put in address tables?
Definition
The Learning Phase
Term
In STP, when will a non-root bridge send a BPDU?
Definition
When the topology changes (link down), the directly connected bridge sends a TCN BPDU to the root.
Term
What does a designated bridge do when it receives a TCN BPDU from a downstream switch?
Definition
The designated bridge sends a TCN-ACK to the downstream bridge and a TCN BPDU to the root bridge.
Term
What is the ultimate purpose of a TCN BPDU in STP?
Definition
To accelerate the MAC aging process for a period of time.
Term
What does the root do when it receives a TCN BPDU?
Definition
The root starts sending BPDUs with the TC bit set.
Term
What do bridges do when they are receiving TC BPDUs from the root?
Definition
They age out MAC address table entries after fwd-delay time (15 seconds) and relearn MACs for fwd-delay+macage = 35 seconds
Term
When a switch stops receiving BPDUs on its root port, how long does it wait before it recalculates STP?
Definition
MaxAge (20 seconds)
Term
What is the IEEE standard for RSTP?
Definition
802.1w
It's "W"ascally rapid.
Term
What is the normal bridge ID?
Definition
Bridge priority (2 bytes) + MAC (6 bytes)
Term
What is the extended bridge ID?
Definition
Bridge Priority (4 bits) + VLAN (12 bits) + MAC (6 bytes)
Term
In 802.1D, how many STPs will you have if you have 10 VLANs.
Definition
One. 802.1D define a Common Spanning Tree (CST) for all VLANs.
Term
In 802.1D, who sets the timers for BPDUs?
Definition
The root switch. Settings on non-root switches don't matter.
Term
In 802.1D, BPDUs are tagged or untagged?
Definition
STP BPDUs are untagged.
Term
In 802.1w, what kind of port is blocked because it receives better BPDUs from another bridge on the same segment?
Definition
Alternate port (offers alternate path to root)
Term
In 802.1w, what kind of port receive better BPDUs from another port on its own bridge on the same segment?
Definition
A Backup port is a backup for the designated port.
Term
What are the three link types in 802.1w?
Definition
1) Edge: single host, no STP participation, same as portfast
2) Shared: connect to other switch via hub (rarely used)
3) point-to-point: Designated port
Term
How often do 802.1s bridges send BPDUs?
Definition
Every 2 seconds.
Term
Does an RSTP bridge need to receive a BPDU from the root in order to send one?
Definition
No, RTSP bridges forward BPDUs every 2 seconds regardless of what the root does.
Term
How many BPDUs in a row can be missed before an RSTP bridge ages out BPDU information from that neighbor?
Definition
3 missed Hellos.
Term
RSTP BPDUs are marked as what version?
Definition
Version 2
Term
In RSTP, what does a bridge do if it receives an inferior BPDU on a port?
Definition
An RSTP bridge will immediately send a BPDU with root info when an inferior BPDU is received.
Term
In STP, what does a bridge do if it receives an inferior BPDU on a port?
Definition
It will wait 20 seconds (MaxAge) before aging out BPDU info and transitioning to Listening phase.
Term
Which protocol sends a BPDU with proposal bit when it comes up?
Definition
RSTP
Term
What two things does an RSTP switch do when it receives a better BPDU from a new neighbor?
Definition
1) Block its former root port
2) Explicitly authorizes neighbor to put port in forwarding state by sending proposal BPDU with agree bit set.
Term
In RSTP, is a link going down a topology change?
Definition
No (but it is in STP. Only a non-edge port transitioning to forwarding is considered a topology change in RSTP.
Term
What happens when an RSTP bridge detects a topology change in RSTP?
Definition
1) Bridge starts TCWile Timer (2x hello = 4 seconds) on designated and root ports.
2) Flushes MAC associated with these ports
3) Sends BPDUs with TC bit.
4) Neighbors receiving BPDU with TC bit flood it.
Term
What must all switches agree on in the same MST region?
Definition
1) MST config name
2) rev number
3) instance to VLAN maping table
Term
What is a region in MST?
Definition
A group of bridges with the same MST configuration.
Term
How many MST instances are allowed per region?
Definition
16
Term
What instances sends and receives BPDUs in MST?
Definition
Only Instance 0 (Internal Spanning Tree-IST).
Term
What do MST BPDUs contain?
Definition
A digest of VLAN to instance mapping.
Term
How many VLANs can be mapped in MST?
Definition
Up to 4096.
Term
What is the IEEE standard for MST?
Definition
802.1s
Term
What happens when a BPDU is received on a portfast port?
Definition
Portfast functionality is lost and the port begins STP.
Term
What STP features allows a switch to immediately change to a redundant link if the directly-connected root port fails?
Definition
Uplink Fast
Term
What STP stages does Uplink fast skip?
Definition
Listening and Learning.
Term
What does configuring Uplink Fast do to priority and port cost?
Definition
Increases priority to 49152 and increment port cost by 3000 to discourage downstream switches from going through it.
Term
In uplink fast, how does downstream switch tell upstream switch that addresses are reachable on a new link?
Definition
Downstream switch sends dummy multicast to 0100.0ccd.cdcd for each MAC in the CAM table at a rate of 150 per second.
Term
To be effective, where should Uplink Fast be configured?
Definition
On all switches, all VLANs.
Term
What are two indications of an indirect link failure?
Definition
Missed Hello or inferior bpdu
Term
What is the convergence delay for an indirect link failure with and without backbone fast configured?
Definition

50 seconds without backbone fast

30 seconds with backbone fast

 

  • Switch that receives inferior BPDU on root port will switch to alternate port immediately (not wait 10 X Hello timer = 20 seconds) and begin listening and learning.
  • Switch that misses a BPDU on root port will send RLQ query (all the way to root) to see if path to root exists first.
Term
What feature allows multiple links to be treated as a single link from an STP perspective?
Definition
PortChannels
Term
What must bundled ports have in common?
Definition
Speed, duplex and interface type
Term
What must bundled trunk ports have in common?
Definition
native vlan, allowed vlans, encapsulation, DTP mode, STP port cost per VLAN
Term
What must non-trunk bundled ports have in common?
Definition
All ports in same VLAN
Term
For bundled routed ports, where is the iP address configured
Definition
IP address is configured on port channel interface (not physical interface)
Term
What are the limitations of an etherchannel participating in a SPAN sessions?
Definition
An etherchannel can be the source port for SPAN, but not the destination
Term
What will happen if a physical port that is part of an etherchannel is assigned as the destination of a SPAN session?
Definition
The port will be removed from the group when span is on.
Term
In an etherchannel, what link(s) does STP run on?
Definition
STP runs on the first physical interface
Term
How can traffic be load-balanced on an etherchannel?
Definition
L2/L3 and/or L4 headers (depends on switch and code version)
Term
What fields would not be a good choice for load balancing on an access switch etherchannel?
Definition
Destination IP and MAC (because most traffic from endpoints goes to default gateway which has same MAC and IP).
Term
How is inbound multicast and broadcast traffic handled on an etherchannel?
Definition
Inbound multicast and broadcast are not forwarded back out any other links on the etherchannel?
Term
What command would statically configure a port channel without using a dynamic protocol?
Definition
channel-group 1 on
Term
What is the LACP equivalent of PagP's "auto" setting in the channel-group 1 auto command?
Definition
Like PagP's "auto", LACP's "passive" waits for the other side to send a message first.
Term
Does "channel-group 1 active" enable LACP or PagP?
Definition
"active" is for LACP
Term
Does "channel-group 1 desirable" enable LACP or PagP?
Definition
"desirable" is a PagP setting
Term
What are the recommended settings for PagP and LACP?
Definition
"desirable" (PagP) and "active" (LACP) will ensure the portchannel gets formed dynamically
Term
What does IEEE 802.1AD define?
Definition
Link Aggregation Control Protocol (LACP)
Term
If a port is configured for "channel-group 1 desirable", what will the switch do if no response is received to pagp messages?
Definition
It will form a port-channel anyway after 15 seconds of silence.
Term
"channel-group 1 desirable non-silent", what will the switch do if no response is received to pagp messages?
Definition
The switch must receive pagp from the other side before it will form portchannel
Term
How many links can be configured and how many used at any one time for a LACP portchannel?
Definition
16 can be defined but only 8 will be used at one time (others are hot standby)
Term
Which switch makes decisions about which ports are used in LACP portchannels?
Definition
Switch with the lowest system priority
Term
How are ports selected in LACP?
Definition
Lowest priority or lowest port number if priority the same.
Term
How does global bpduguard config defer from port config?
Definition
Global config "spanning-tree portfast bpduguard disable" only applies to portfast ports. Port config "spanning-tree bpduguard enable" applies regardless of portfast
Term
What happens on a port where rootguard is configured when a superior bpdu is received?
Definition
Port is put in root inconsistent state and all traffic is blocked.
Term
If a rootguard port stops receiving superior bpdus, what happens?
Definition
If superior bpdu stops, root guard allows STP to continue (no err-recovery config required)
Term
What are three STP protection mechanisms for access switches?
Definition
bpduguard
bpdu filter
root guard
Term
What are two STP protection features for trunk ports?
Definition
Loop guard
UDLD
Term
Where should loop guard be deployed?
Definition
Root and alternate root ports on trunk ports
Term
How does loop guard prevent loops?
Definition
Prevents root ports from going through STP if BPDUs stop. Port will be moved to loop inconsistent state.
Term
Does loop guard affect the whole port?
Definition
No, loop guard only affects VLANs.
Term
Why is UDLD better on port channels?
Definition
UDLD only impacts a single link. Loopguard impacts the whole port channel.
Term
Name two advantages of loopguard over UDLD
Definition
Loopguard is faster and can detect problems with STP.
UDLD detects problems with UDLD packets (STP may be broken when UDLD is not)
Term
How does UDLD work?
Definition
Each switch sends an L2 UDLD frame, expects an echo.
Term
What's the difference between normal and aggressive UDLD mode?
Definition
Normally, UDLD only disables itself. Aggressive UDLD disables both sides.
Term
Is VTP pruning enabled or disabled by default?
Definition
VTP Pruning is disabled by default.
Term
What 4 errors are indicators of duplex mismatch?
Definition
FCS, CRC, alignment, or runts
Term
In RSTP and 802.1D, what ports send BPDUs?
Definition
Only Designated Ports. BPDUs are never sent on root ports or blocked ports.
Term
What comes first, STP, PagP/LACP, DTP?
Definition
STP doesn't run until port type is decided, so: DTP -> PagP/LACP -> STP
Term
What is the minimum amount of information that must be configured for a VTP update to be sent?
Definition
At least one VTP server must have a domain name configured.
Term
True or false: VTP sends updates out all ports.
Definition
False -- VTPp sends updates out all active trunk interfaces only.
Term
How can VTP DOS attacks be mitigated?
Definition
Mitigate VTP DoS attacks by configuring VTP passwords (MD5).
Term
Can a VTP server updates its VLAN config with a VTP advertisement from a client?
Definition
Yes.
Term
Which trunk protocol adds a 26 byte header and 4 byte CRC?
Definition
ISL
Term
Which trunk protocol uses the source address of the trunking device and a multicast destination address?
Definition
ISL
Term
Which trunk protocol inserts a 4 byte header/tag?
Definition
802.1Q
Term
What does an Ether frametype of 0x8100 indicate?
Definition
Frametype 0x8100 indicates that an 802.1Q header follows.
Term
Which switchport mode always trunks and sends DTP to help other side trunk?
Definition
switchport mode trunk
Term
What switchport mode initiates DTP and only trunks if DTP succeeds?
Definition
switchport mode dynamic desirable
Term
What does "switchport nonnegotiate" do?
Definition
Disables DTP
Term
What switchport mode never trunks and sends DTP to help other side not trunk either?
Definition
switchport mode access
Term
Which trunk protocol supports extended range VLANs today?
Definition
Both ISL and 802.1Q support extended range VLANs (in the past, ISL only did normal range but it has changed.)
Term
What is the range of extended VLANs?
Definition
1006 - 4094
Term
Do CDP and VTP pass inside QinQ tunnels?
Definition
Yes, CDP and VTP pass in QinQ tunnels
Term
Will an 802.1Q trunk form if the native VLAN is different?
Definition
No, native VLAN must match for 1Q trunk to form.
Term
Is VLAN 4095 user configurable?
Definition
No, like VLAN 0, VLAN 4095 is reserved for system use.
Term
What does switchport mode dot1qtunnel do?
Definition
Sets up a Q-in-Q tunnel
Term
What 3 PagP mode combinations will result in the formation of an Etherchannel?
Definition
auto-desirable
desirable-desirable
on-on
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