Flashcards
Chapter 6: Traditional STP
Cards in this deck (36)
- 1Describes transparent bridge operation■ The bridge has no initial knowledge of any end device’s location
■ The bridge constantly update its bridging table
■ If a frame arrives with the broadcast address, the bridge flood the frame
■ If a frame arrives with a unknown destination address (unknown unicast) it's treats the frame as if it were a broadcast
■ Frames forwarded across the bridge cannot be modified by the bridge itself. Therefore, the bridging process is effectively transparent . - 2Explains a bridging loop153
- 3Discusses BPDUs- Data messages are exchanged
in the form of BPDU
- SW send BPDU with the SRC add of their unique MAC add of the port itself
- The SW is unaware of the DST add, so BPDU frames are sent with the well-known DST add STP multicast address 01-80-c2-00-00-00 - 4Two types of BPDU exist■ Configuration BPDU , used for spanning tree computation
■ Topology Change Notification (TCN) BPDU , used to announce changes in the network topology - 5Configuration BPDU Message Content
- 6Configuration BPDU Message Content 2
- 7Root Bridge- Reference point
- Best BID - 8Bridge ID- Identifies it to other switches
- 8-byte value - 9BID Values■ Bridge Priority (2 bytes ): Value of 0 to 65,535, defaults to 32,768 (or 0x8000)
■ MAC Address (6 bytes ): MAC add value used by a SW can come from Supervisor module, backplane, or a pool of 1024 add that are assigned to every supervisor or backplane depending of a sw model and it cannot change it - 10Root Port- Only one root port on each nonroot switch
- concept of cost, evaluating root path cost - 11Roth path cost (1 per segment)- Cumulative cost value of all the links leading to the root bridge
- only the root path cost is carried inside the BPDU
- As the root path cost travels along, other switches can modify its value to make it cumulative - 12Path cost- The path cost, however,
is not contained in the BPDU. It is known only to the local switch where the port (or
“path” to a neighboring switch) resides.
- 1-byte value - 13STP patch cost values4 Mbps - 250
10 Mbps - 100
16 Mbps - 62
45 Mbps - 39
100 Mbps - 19
155 Mbps - 14
622 Mbps - 6
1 Gbps - 4
10 Gbps - 2 - 14Electing Designated Port (1 per segment)- Remove the possibility of bridging loops
- DP in each network segment
- Elected based on the lowest cumulative RPC to the RB
- If a neighboring switch on a shared LAN segment sends a BPDU announcing a lower root path cost, the neighbor must have the designated port. - 15STP Tiebraker1. Lowest root bridge ID
2. Lowest root path cost to root bridge
3. Lowest sender bridge ID
4. Lowest sender port ID - 16STP States■ Disabled
■ Blocking
■ Listening
■ Learning
■ Forwarding - 17STP States and Port Activity
- 18STP Timers■ Hello timer
■ Forward Delay timer
■ Max (Maximum) Age timer - 19Hello Timer- The hello time value configured in the root bridge switch determines the
hello time for all nonroot switches
- However, all switches have a locally configured hello time that is used to time TCN BPDUs when they are retransmitted
(2 segundos) - 20STP Timers List
- 21¿How does a SW announce a change in the active network topology?switches send a TCN BPDU
- 22Topology Change Notification BPDU Message Content
- 23¿When a SW send a TCN BPDU?When a port on an active SW comes UP or DOWN
- 24¿From what port a TCN BPDU is sent?By the Root Port
- 25¿TCN BPDU carries data?The TCN BPDU carries no data about the change but informs recipients only that a change has occurred
- 26¿What type of port wont sent a TCN BPDU?The switch will not send TCN BPDUs if the port has been configured with PortFast enabled
- 27¿What is the frequency of TCN BPDU?The switch continues sending TCN BPDUs every hello time interval until it gets an
acknowledgment from its upstream neighbor. As the upstream neighbors receive the TCN BPDU, they propagate it on toward the root bridge and send their own acknowledgments - 28¿What happen when the RB receive a TCN BPDU?It also sends out an acknowledgment.
However, the root bridge sets the Topology Change flag in its Configuration BPDU,
which is relayed to every other bridge in the network - 29¿What happen when a non-rb receive a Config BPDU with the TC flag?It cause all other bridges to shorten their bridge table aging times from the default (300 seconds) to the forward delay value (default 15 seconds)
- 30¿Wich is the duration time of this condition?This condition lasts for the sum of the
forward delay and the max age (default 15 + 20 seconds). - 31¿How much topology changes exist?• Direct topology changes
• Indirect topology changes
• Insignificant topology changes - 32Direct topology changes (Link failure)- A direct topology change is one that can be detected on a switch interface
- If a trunk link suddenly goes down the switch on each end of the link can IMMEDIATELY detect a link failure.
- Total time loss of connectivity. Forward delay period (15 sec), or 30 sec total. - 33Indirect Topology Changes (absence of BPDU)- No data (including BPDUs) can
pass between those switches.
- STP can detect and recover from indirect failures, thanks to timer mechanisms (Max Age timer)
- After the Max Age timer expires, no other BPDU is available to refresh the “best” entry, so it is flushed. Switch C now must wait to hear from the Root again on any of its ports. - 34Indirect Topology timers- Max age timer expired (20 seconds)
- Configuration BPDU was received (2 seconds)
- Listening (15 seconds) and Learning (15 seconds) states
- 52 seconds elapse if the default timer values are used. - 35Insignificant Topology Changes- Constant state of flushing bridge tables when PCs power off and on
- Ports towards host
- Cosmetics topology changes
- No actual topology change occurred
- You can enable the STP PortFast
- This means more unknown unicast - 36Types of STP
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- 300-115 CCNP SWITCH 300-115 (Part III: Working with Redundant Links) Flashcards on Chapter 6: Traditional STP, created by Luis Domínguez on 02/04/2018.
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