CCNA3 ch 2
Terms in this set (72)
what can be used to manage how a client is assigned a default gateway, and to be able to use an alternate default gateway should the primary default gateway fail.
First Hop Redundancy Protocols
give an example of a redundant device
multilayer switches or routers
A Layer 2 loop can result in three primary issues:
MAC database instability
Multiple frame transmission
what is MAC database instability?
Instability in the content of the MAC address table results from copies of the same frame being received on different ports of the switch. Data forwarding can be impaired when the switch consumes the resources that are coping with instability in the MAC address table.
what is Multiple frame transmission
Multiple copies of unicast frames delivered to destination stations.
do Ethernet frames have a time to live (TTL) attribute?
A loop occurs until it is _______
A host caught in a network loop is not accessible to other hosts on the network. Additionally, because of the constant changes in the MAC address table, the switch does not know out of which port to forward unicast frames.
what is a A broadcast storm
A broadcast storm occurs when there are so many broadcast frames caught in a Layer 2 loop that all available bandwidth is consumed. Consequently, no bandwidth is available for legitimate traffic and the network becomes unavailable for data communication. This is an effective denial of service.
Spanning-Tree Protocol implements the ______ algorithm
how does STP prevent loops?
exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces.
The ______ in a spanning tree is the logical centre and sees all traffic on a network.
what is a BPDU?
BPDUs are data messages that are exchanged across the switches within an extended LAN that uses a spanning tree protocol topology. BPDU packets contain information on ports, addresses, priorities and costs and ensure that the data ends up where it was intended to go. BPDU messages are exchanged across bridges to detect loops in a network topology.
what is a bridge
A bridge is a device that connects two local-area networks (LANs), or two segments of the same LAN that use the same protocol, such as Ethernet
is spanning tree enabled by default?
what does STP use to calculate which links to use in a redundant topology?
STP uses the concepts of a root bridge, port roles, and path costs to calculate which links to use in a redundant topology.
STP ensures that there is only ___ logical path between all destinations on the network by intentionally blocking ______ that could cause a loop
A port is considered blocked when...
user data is prevented from entering or leaving that port. This does not include bridge protocol data unit (BPDU) frames that are used by STP to prevent loops.
STP prevents loops from occurring by configuring a loop-free path through the network using strategically placed "blocking-state" ports. The switches running STP are able to compensate for failures by dynamically unblocking the previously blocked ports and permitting traffic to traverse the alternate paths.
The switch with the _____ BID automatically becomes the root bridge for the STA calculations.
The BID contains what?
a priority value, the MAC address of the sending switch, and an optional extended system ID.
All non-root ports that are still permitted to forward traffic on the network. Designated ports are selected on a per-trunk basis. If one end of a trunk is a root port, the other end is a designated port. All ports on the root bridge are designated ports.
Alternate and backup ports:
Alternate ports and backup ports are configured to be in a blocking state to prevent loops.. Alternate ports are selected only on trunk links where neither end is a root port.
how often does a switch send BPDU frames?
every 2 seconds
each switch maintains local information about what?
its own BID, the root ID, and the path cost to the root.
When adjacent switches receive a BPDU frame what do they do?
they compare the root ID from the BPDU frame with the local root ID. If the root ID in the BPDU is lower than the local root ID, the switch updates the local root ID and the ID in its BPDU messages
what is bridge priority?
The bridge priority is a configurable value that can be used to influence which switch becomes the root bridge. The switch with the lowest priority, which implies the lowest BID, becomes the root bridge because a lower-priority value takes precedence. For example, to ensure that a specific switch is always the root bridge, set the priority to a lower value than the rest of the switches on the network.
The default priority value for all Cisco switches is
A bridge priority of __ takes precedence over all other bridge priorities.
When two switches are configured with the same priority and have the same extended system ID what is used to determine the BID?
the switch having the MAC address with the lowest hexadecimal value will have the lower BID.
A Cisco enhancement of STP
[ [PortFast, UplinkFast, BackboneFast, BPDU guard, BPDU filter, root guard, and loop guard.]]
- high resources needed
- slow convergence
- per vlan tree calculation
RSTP (or IEEE 802.1w):
addresses many convergence issues, but because it still provides a single instance of STP, it does not address the suboptimal traffic flow issues.
- fast convergence
- medium resources needed
- all vlan calculation
The lowest BID value is determined by the combination of what three fields?
a priority value, the MAC address of the sending switch, and an optional extended system ID.
[supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.]
- fast convergence
- very high resources needed
- per vlan tree calculation
The IEEE 802.1s standard, and cisco inspired by MISTP.
- medium or high resources needed
- fast convergence
- per instance tree calculation
To reduce the number of required STP instances, MSTP maps multiple VLANs that have the same traffic flow requirements into the same spanning tree instance.
The Cisco implementation of MSTP, which provides up to 16 instances of RSTP (802.1w) and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each instance supports Port Fast, BPDU guard, BPDU filter, root guard, and loop guard. The CPU and memory requirements of this version are less than those of Rapid PVST+, but more than those of RSTP.
what is the default spanning tree mode for Cisco Catalyst switches?
PVST+, which is enabled on all ports.
For each VLAN in a switched network, PVST+ performs four steps to provide a loop-free logical network topology what are they?
1. Elects one root bridge:
Only one switch can act as the root bridge (for a given VLAN). The root bridge is the switch with the lowest bridge ID. On the root bridge, all ports are designated ports (in particular, no root ports).
2. Selects the root port on each non-root bridge:
STP establishes one root port on each non-root bridge. The root port is the lowest-cost path from the nonroot bridge to the root bridge, indicating the direction of the best path to the root bridge. Root ports are normally in the forwarding state.
3.Selects the designated port on each segment:
On each link, STP establishes one designated port. The designated port is selected on the switch that has the lowest-cost path to the root bridge. Designated ports are normally in the forwarding state, forwarding traffic for the segment.
4. The remaining ports in the switched network are alternate ports:
Alternate ports normally remain in the blocking state, to logically break the loop topology. When a port is in the blocking state, it does not forward traffic, but can still process received BPDU messages.
what is An RSTP edge port?
An RSTP edge port is a switch port that is never intended to be connected to another switch device. It immediately transitions to the forwarding state when enabled.
what is a Point-to-Point Link?
Point-to-Point Link: A port operating in full-duplex mode typically connects a switch to a switch and is a candidate for rapid transition to a forwarding state.
A port operating in half-duplex mode connects a switch to a hub that attaches multiple devices.
what is one way to repair a spanning tree problem?
One way to correct spanning tree failure is to manually remove redundant links in the switched network, either physically or through configuration, until all loops are eliminated from the topology. When the loops are broken, the traffic and CPU loads should quickly drop to normal levels, and connectivity to devices should be restored.
why use a virtual router.
what is HSRP?
- HSRP defines a group of routers one active and one standby
- Virtual IP and MAC are shared between the 2 routers
- HSRP is cisco proprietary
- VRRP is a standard protocol
To verify HRSP state, use the _______ command
what is Gateway Load Balancing Protocol (GLBP)
GLBP is a Cisco proprietary solution to allow automatic selection and simultaneous use of multiple available gateways in addition to automatic failover between those gateways
what are First hop redundancy protocols and what can they do?
First hop redundancy protocols; such as HSRP, VRRP, and GLBP provide alternate default gateways for hosts in the switched environment.
what are the 5 port states used by STP?
explain the blocking state used by STP
The port is an alternate port and does not participate in frame for- warding. The port receives BPDU frames to determine the location and root ID of the root bridge switch and what port roles each switch port should assume in the final active STP topology.
explain the listening state used by STP
Listens for the path to the root. STP has determined that the port can participate in frame forwarding according to the BPDU frames that the switch has received thus far. At this point, the switch port not only receives BPDU frames, but it also transmits its own BPDU frames and informs adjacent switches that the switch port is preparing to participate in the active topology.
explain the learning state used by STP
Learns the MAC addresses. The port prepares to participate in frame forwarding and begins to populate the MAC address table
explain the forwarding state used by STP
The port is considered part of the active topology. It forwards data frames and sends and receives BPDU frames. it learns mac addresses
• RSTP is the preferred protocol for preventing Layer 2 loops in a switched network environment.
• With Rapid PVST+, an independent instance of RSTP runs for each VLAN.
• RSTP supports a new port type: an alternate port in discarding state.
• There are no blocking ports. RSTP defines port states as discarding, learning, or forwarding.
• RSTP (802.1w) supersedes STP (802.1D) while retaining backward compatibility
• RSTP keeps the same BPDU format as IEEE 802.1D, except that the version field is set to 2 to indicate RSTP, and the flags field uses all 8 bits.
explain Hot Standby Router Protocol (HSRP)
- designed to allow for transparent failover of a first hop IPv4 device
HSRP provides high network availability by providing first-hop routing redundancy for IPv4 hosts on networks configured with an IPv4 default gateway address. HSRP is used in a group of routers for selecting an active device and a standby device. In a group of device interfaces, the active device is the device that is used for routing packets; the standby device is the device that takes over when the active device fails, or when preset conditions are met. The function of the HSRP standby router is to monitor the operational status of the HSRP group and to quickly assume packet-forwarding responsibility if the active router fails.
Virtual Router Redundancy Protocol version 2 (VRRPv2):
A nonproprietary election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on an IPv4 LAN. This allows several routers on a multi-access link to use the same virtual IPv4 address. A VRRP router is configured to run the VRRP protocol in conjunction with one or more other routers attached to a LAN. In a VRRP configuration, one router is elected as the virtual router master, with the other routers acting as backups, in case the virtual router master fails.
Each switch has a Bridge ID (BID) that is made up of a priority value, an extended system ID, and the MAC address of the switch.
All switches in the network take part in the election process. After a switch boots up, it sends out BPDU frames containing the switch BID and the root ID every 2 seconds. By default, the root ID matches the local BID for all switches on the network. The root ID identifies the root bridge on the network. Initially, each switch identifies itself as the root bridge after bootup.
Lets look at it this way, when switches A, B, C and D are on the same network or broadcast domain boots up, the switches will forward their Bridge Protocol Data Unit (BPDU) frames to neighbouring switches. All switches in the network or broadcast domain will read the root ID information from the BPDU frame of all their neighbours.
After reviewing the entire root ID's from the BPDU received from each switch, the switch with the lowest BID ends up being identified as the Root Bridge for the spanning tree process. It may not be an adjacent switch, but any other switch in the broadcast domain.
Root Ports - Switch ports closest to the root bridge with the lowest cost path.
Designated Ports - All non-root ports that are still permitted to forward traffic on the network.
Non-designated ports - All ports configured to be in a blocking state to prevent loops.
Designing a network to use multiple paths between switches to ensure that there is no single point of failure
what are some issues a broadcast storm can cause?
prevent a switch from accurately forwarding frames.
Because of high processing demands during a broadcast storm, communication can fail between end stations in the broadcast domain.
During the implementation of Spanning Tree Protocol, all switches are rebooted by the network administrator. What is the first step of the spanning-tree election process?
All the switches send out BPDUs advertising themselves as the root-bridge.
After the election of the root bridge has been completed, how will switches find the best paths to the root bridge?
Each switch will analyse the sum of all port costs to reach the root and use the path with the lowest cost.
When PVST is running over a switched network, which port state can participate in BPDU frame forwarding based on BPDUs received, but does not forward data frames?
What are expectations of configuring PortFast on a switchport?
The switch port should never receive BPDUs from end stations that are connected to the port.
The switch port immediately transitions from the blocking to the forwarding state.
Which port states are used by Rapid PVST+
An administrator is troubleshooting a switch and wants to verify whether it is a root bridge. What command can be used to do this?
What is the initial approach that should be used to troubleshoot a broadcast storm in a switched network?
Manually remove redundant links in the switched network
When first hop redundancy protocols are used, which items will be shared by a set of routers that are presenting the illusion of being a single router?
A network administrator is overseeing the implementation of first hop redundancy protocols. Which of the following protocols will not be able to function with multivendor devices?
what allows multiple VLANs to run in a single spanning tree instance.
List the three steps that an FHRP initiates during a router failover process.
The standby router stops receiving hello messages from the forwarding router.
The standby router assumes the role of the forwarding router.
The new forwarding router assumes the IP and MAC address of the virtual router.
what is a Cisco enhancement of RSTP that provides a spanning tree instance for each VLAN.