STP/BPDU/MST/PVST/RSTP Flashcards
what are the 5 STP port roles?
root, designated, blocking, alternate, forwarding (host)
what does Root Guard do?
controls where candidate root bridges can be connected and found on a network
How does Root Guard work?
a port can only forward or relay BPDUs, but can’t receive them, disabled by default, enabled per-port, blocks port when superior BPDUs are received
How is Root Guard used?
on ports where you never expect to find a root bridge for a VLAN
What is BPDU Guard?
if any BPDU is received on a port, it puts the port into the errdisable state
How can BPDU Guard be configured?
globally or per-port
What does Loop Guard do?
tracks BPDUs on nondesignated ports. When those BPDUs stop coming, the port is put into loop-inconsistent state and blocks
How does loop guard block ports?
only for the offending VLAN on the port
What does BPDU filtering do?
effectively stops STP on filtered ports
Where is root guard enabled?
all ports where root isn’t expected
where is BPDU guard enabled?
all user ports that have portfast enabled
where is loop guard enabled?
all nondesignated ports, but ok for all ports
can root guard and loop guard be used together
no
can root guard and BPDU guard be used together?
no
should BPDU guard and BPDU filter be used together?
no
RSTP uses which standard?
802.1w
What are the RSTP port roles?
root, designated, alternate, and backup
RSTP root port
port with best root path cost
RSTP designated port
Port with best root path cost on segment
RSTP alternate port
port with alternative path to root less desirable than root
RSTP backup port
port with redundant, but less deisrable connection to a segment. Possibly root candidate
What are the RSTP port states?
discarding, learning, forwarding
RSTP discarding state
incoming frames are dropped (combination of disabled, blocking, and listening)
RSTP learning state
incoming frames dropped, but MACs learned
RSTP forwarding state
frames are forwarded
RSTP BPDU structure
some unused bits are used to allow a port to identify its role and state. Also the BPDUs state that they are V2 BPDUs
When are RSTP BPDUs sent?
every hello interval regardless of whether BPDUs are received from root, allowing any switch to take an active role maintaining the topology
What if RSTP BPDUs aren’t received?
If three are missed in a row, the neighbor is considered down and the data for that neighbor is aged
What is the RSTP default hello interval?
2 seconds
What happens if both 802.1D and RSTP BPDUs are received?
the port will operate under 802.1D rules. If they are received on the same port, it will run 802.1D until the migration delay expires
What are the steps of RSTP convergence?
1- root bridge is elected, 2-the state of every switch port in the STP domain must be brought from blocking state to the appropriate state
What are the 3 RSTP port types?
edge, root, and point to point
RSTP edge port
only a single host connects . If one BPDU is received, it is no longer an edge port
RSTP point to point port
port that connects to another switch and becomes a designated port
How are RSTP point to point ports determined?
automatically by duplex mode in use. Full duplex are point ot point. Half duplex ports use traditional 802.1D convergence is used.
How is RSTP synchronization accomplished?
all non-edge ports are discarding. After the root bridge is identified, the port with superior BPDU becomes root
How does RSTP converge?
if a proposal is received, the recipient isolates, all nonedge ports blocked until proposal sent causing neighbors to synch, which propagates out in waves
How does RSTP identify a topology change?
only when a nonedge port transitions to forwarding
How is an RSTP edge port configured
with portfast
What is RPVST+?
an instance of RSTP running for each VLAN on the switch. Changing from PVST to RPVST+ is disruptive
When looking at the sh spanning tree output, what does P2P denote?
a point to point port type
When looking at the sh spanning tree output, what does P2P peer(STP) denote?
a point to point port type but the neighboring device runs traditional 802.1D STP
What does MSTP do?
Used when there are 1 or more VLANs mapped to a single STP instance
What do you need to determine to use MSTP?
the number of STP instances needed to support the desired topologies and whether to map a set of VLANs to each instance
Can MSTP interoperate with 802.1q and/or PVST+?
yes to both
What does MSTP consist of?
regions and instances
How is an instance defined?
by name, config revision, and instance to vlan mapping table
What is an IST instance?
within a single MST region, an IST instances runs to work out a loop free topology between links where CST meets the region boundary and all switches in the region. It runs at the boundary.
What is an MST instance?
The MST instance (MSTI) runs alongside the IST and represents an STP instance
How many MST instances can be in a region?
16 - IST gets MSTI 0, 1-15 are available for use. MSTIs are locally significant to the MST region
How do you implement MST?
MST attributes must be manually entereed on each switch in the region. They don’ t propagate like VTP.
What are the steps to configure MST?
1- enable MST, 2-enter MST config mode, 3-assign region name and region configuration rev, 4-map vlans to an MSTI, 5-show pending changes, 6- commit by exiting
What is the IEEE standard for STP?
802.1D
How are BPDUs spread?
multicast to 01-80-c2-00-00-00
What are the 2 types of BPDU?
configuration and TCN
How often are BPDUs sent?
2 seconds (def)
What is a bridge ID made up of?
bridge priority and MAC
What is the bridge priority range?
0-65535 (def 32768) lower=better
In a converged fabric, which switch(es) send config BPDUs?
root
How is root port decided upon?
lowest cumulative cost to root
What is the cost of a 10 Mbps link?
100
What is the cost of a 100Mbps link?
19
What is the cost of a 1Gbps link?
4
What is the cost of a 10Gbps link?
2
How many designated ports are there?
1 per segment
If there are 2 ports with same root cost, how is the tie broken for designated port?
1-lowest root bridge ID, 2-lowest root path cost, 3-lowest bridgeID, 4-lowest port ID
What are the 5 STP states?
disabled, blocking, listening, learning, forwarding
What is the STP disabled state?
admin down
What is the STP blocking state?
Can hear only BPDUs
what is the STP listening state?
can send and receive BPDUs
What is the STP learning state?
after forwarding delay, port can learn new MAC addresses
What is the STP forwarding state?
after another forwarding delay, the port can send and receive data frames, collect MAC addresses, and send and receove BPDUs
What are the 5 steps to manualy configure STP?
1-ID link path costs, 2-ID roodt bridge, 3-select root port (1/switch), 4-select designated port (1/segment), 5-ID blocking port
What is the STP hello timer?
Time between BPDUs sent by root bridge. Default of 2 seconds
What is the STP forward delay timer?
The time that a port spends in both listening and learning states . Default 15 seconds
What is the STP Max Age timer?
The time interval that a switch stores a BPDU before discarding it. Default 20 seconds
What is the default network diameter?
7 switches from root bridge
Where should you change the network diameter value?
Cisco recommends only on the root bridge
What is a topology change?
When a port moves into forwarding or from fowarding or learning to blocking.
What happens when the topology changes?
A TCN BPDU is sent out of the switch’s root port. The switch will continue sending TCN’s every hello interval until ack’d by upstream neighbor. The root bridge will send a Config BPDU to all switches
What happens when switches receive a configuration BPDU?
They shorten their bridge table aging times from default (300sec) to the fowarding delay (15 sec)
What is a direct topology change?
One that can be detected on a switch interface (ie. Up/down)
What is an indirect topology change?
The link status stays up, but something between them has failed or is filtering traffic
What is an insignificant topology change?
A host port goes up or down
What is common spanning tree (CST)?
Single instance of STP for all VLANs. BPDUs are sent over trunks using the native VLAN with untagged frames. Dot1q based
What is Per VLAN STP (PVST)?
Cisco proprietary. 1 instance of STP/VLAN. Requires ISL instead of dot1q
What is PVST plus (PVST+)?
allows devices to interoperate with PVST and CST. Can use both dot1q and ISL
What is the port ID?
16 bits (8b port priority, 8b port number)
what is the port number range?
0-255
what is the port priority range?
0-255 (lower is better)
Where should STP timers be configured and why?
on root, because the root bridge propagates timers with config BPDU
How do you automatically configure STP timers
use the spanning-tree root macro
What is portfast?
almost eliminates listening and learning states for single hosts on an access link
What does the switchport host macro do?
enables portfast, sets the port to access and disables PAgP
What does uplinkfast do?
enables switches at the end of the spanning tree branches to have a functioning root port while keeping 1 or more redundant potential ports in blocking mode. When the main fails, the other comes up.
What are the gotchas for uplinkfast?
should be used on access switches, can’t be used on root, keeps enabled switch from becoming root by raising priority of switch and all ports, making them undesirable