ICND2 Section 1 - LAN Switching Flashcards
3 problems caused by not using STP
- Broadcast Storms
- MAC table instability
- Multiple copies of the same frame delivered to a host.
What types of packets are processed by a port in Blocking?
STP control traffic only
4 general steps for a switched network to get from nothing to full STP convergence
- Elect a root switch
- Select the root ports
- Select the Designated ports
- Select the Blocked ports
What does BPDU stand for?
Bridge Protocol Data Unit
What is the BID?
How is it calculated?
Bridge ID
2 byte priority field + 6 byte system ID (BIA)
For STP, is higher or lower priority better?
Lower
In STP, if the priority is equal, what is used to determine the root bridge?
The lowest System ID, aka MAC address used in the BID
How is cost to the root bridge calculated?
The sum of all ports exited to reach the root bridge
How does a switch determine it’s neighbor’s cost to get to the root bridge?
It’s a value listed in the neighbor’s BPDU root cost.
What is the default bridge priority?
32768
STP IEEE standard?
802.1D
Time interval for BPDUs
2 seconds
4 STP port states
Listening
Learning
Forwarding
Blocking
Roughly speaking, what is used to calculate STP port cost
Port Speed
What is a switch doing while a port is in Listening
Listening for and processing BPDUs, trying to see if it gets any info that would cause it to go to a Blocking state.
Does a switch populate the MAC table while in Listening?
No
What is a switch doing in Learning?
Learning source MAC addresses and populating the MAC address table, preparatory to beginning Forwarding.
Default STP port costs for the following speeds: 10M 100M 1G 10G
10M = 100 100M = 19 1G = 4 10G = 2
When a non-root switch receives a Hello BPDU, what does it change before forwarding it on?
It adds it’s root cost to the root cost present in the BPDU.
What are the 3 types of BPDUs?
Hello BPDU
CBDPU - Configuration BPDU
TCN BPDU - Topology Change Notification BPDU
Command to configure an etherchannel
conf t
interface int
channel-group group number mode on
Command to show the status of an etherchannel
show etherchannel group number
What are the 2 protocols that can be used to create dynamic etherchannels?
PAgP
LACP
What does PAgP stand for?
Port Aggregation Protocol
What does LACP stand for?
Link Aggregation Control Protocol
What is the IEEE standard for PAgP and LACP?
PAgP is Cisco proprietary
LACP is 802.1ad
Syntax to enable PAgP on a interface
conf t
int gi1/0
channel-group group number mode {desirable | auto}
Syntax to enable LACP on a interface
conf t
int gi1/0
channel-group group number mode {active | passive}
In PAgP configurations, what does the keyword desirable tell the switch to do vs auto?
It tells the switch “use this protocol and begin negotiations”
versus “use this protocol and wait for the other switch to begin negotiations.”
In LACP configurations, what does the keyword active tell the switch to do vs passive?
It tells the switch “use this protocol and begin negotiations”
versus “use this protocol and wait for the other switch to begin negotiations.”
What happens with an etherchannel where both sides are set to either auto or passive?
It won’t come up, because both sides are waiting for the other to begin negotiations.
