CCNP ENCOR v8 Chapters 1 – 5: L2 Redundancy Test Online

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Switches that perform Layer 3 switching process packets at Layer 2 hardware speeds while providing the scalability of routing.

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A switch routed port is a Layer 3 port that requires removal of Layer 2 port functionality with the no switchport interface configuration command. VLANs are associated with switch Layer 2 access and trunk ports.

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802.1Q is a standard trunking protocol where untagged frames are placed on the native VLAN. The native VLAN must be configured the same at both ends of the trunk link.

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CEF switching on Cisco hardware-based routers uses forwarding engines implemented in specialized ASICs to forward packets. Cisco software-based routers use the general-purpose CPU for packet switching. InterVLAN routing using subinterfaces is not a CEF technology, and route processors using stateful switchover is a redundancy technology used by routers.

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CAM is used by ARP to store MAC and IPv4 address pairs. CEF uses a FIB.

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In order to forward packets, Cisco Express Forwarding (CEF) requires the MAC address of the next hop provided by ARP.

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Spanning Tree Protocol (STP) is required to ensure correct network operation when designing a network with multiple interconnected Layer 2 switches or using redundant links to eliminate single points of failure between Layer 2 switches. Routing is a Layer 3 function and does not relate to STP. VLANs do reduce the number of broadcast domains but relate to Layer 3 subnets, not STP.

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By default, all switches will have the same priority value, so the election of the root bridge will be based on the lowest MAC address.

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After a Cisco switch boots, it will send out BPDUs containing its individual BID and the root ID for the network. By default, the initial root ID at bootup will be the ID of that individual switch. After a root bridge is elected, port states and paths are chosen.

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After the election of a root bridge has occurred, each switch will have to determine the best path to the root bridge from its location. The path is determined by summing the individual port costs along the path from each switch port to the root bridge.

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The three components that are combined to form a bridge ID are bridge priority, extended system ID, and MAC address.

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Switches learn MAC addresses at the learning and forwarding port states. They receive and process BPDUs at the blocking, listening, learning, and forwarding port states.

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Although the spanning-tree vlan 10 root primary command will ensure a switch will have a bridge priority value lower than other bridges introduced to the network, the spanning-tree vlan 10 priority 0 command ensures the bridge priority takes precedence over all other priorities.

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The switch priority is a value between 0-61,440 in increments of 4,096. Switch priority is initially set to 32,768 (the default). Therefore, Switch_1 can become the root bridge if the priority is set to a value between 0-28,672.

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The optional diameter keyword in the spanning-tree vlan vlan-id root {primary | secondary} [diameter diameter] command allows for tuning of the STP convergence. The diameter keyword should reference the maximum number of Layer 2 hops that a switch can be from the root bridge and modify the timers accordingly. The timers do not need to be modified on other switches because they are carried throughout the topology through the root bridge BPDUs.

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The spanning-tree bpduguard default command will enable BPDU guard on all switch ports that have PortFast-enabled. This will put the port in an error-disabled state if a BPDU from another switch is received on a PortFast-enabled interface.

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Root guard in conjunction with PortFast, and BPDU guard is used to prevent an STP manipulation attack.

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MST uses a special STP instance called the internal spanning tree (IST), which is always the first instance, instance 0. The IST runs on all switch port interfaces for switches in the MST region, regardless of the VLANs associated with the ports.

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The portfast feature is enabled on a specific access port with the spanning-tree portfast command. One of the benefits of the STP portfast feature, is that the access ports bypass the earlier 802.1D STP states (learning and listening) and forward traffic immediately.

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LACP fast advertises an LACP packet every second and after three seconds of not receiving an LACP packet, removes the link from the EtherChannel bundle. LACP port priority allows an LACP enabled switch to select which member interfaces are active within a port-channel that has more member interfaces than the maximum allowed. LACP system priority allows an LACP enabled switch to be designated as the master switch for a port-channel. LACP min-links is used to configure a required minimum number of physical connections that must be active in order for the EtherChannel to stay active.

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The mst version number and region name must match for all switches in the same MST region. These parameters can be adjusted through MST configuration.

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In MST configuration, an MST instance priority can be defined in one of two methods: spanning-tree mst instance-number priority priority , where the priority is a value between 0 and 61,440, in increments of 4096 spanning-tree mst instance-number root { primary | secondary }[ diameter diameter ], where the primary keyword sets the priority to 24,576, and the secondary keyword sets the priority to 28,672

Hint

SW1 is running MST, which is IEEE 802.1S standard. IEEE 802.1W is the standard for RSTP. SW1 is the root bridge for instance 1. The instance 1 priority is set as 24576. If it is set through the secondary keyword, it would be 28672. If the portfast feature is enabled on a port, the port type would be P2p Edge.

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As shown in the output, the interface STP cost is derived from the media speed of the interface using the long-mode method. Using this method, the range of the interface path cost is from 1 to 200000000. A lower path cost represents higher-speed transmission bandwidth.

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VLAN assignment to the IST and trunk link pruning are two common misconfigurations within the MST region. A misconfigured region name or misconfigured revision number will cause the failure in forming the MST region.

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The PVST simulation mechanism sends out PVST+ including RSTP BPDUs, one for each VLAN, using the information derived from the internal spanning tree (IST). This requires a mapping of one topology in IST to multiple VLANs toward the PVST link.

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The three VTP modes are server, client, and transparent. In server VTP mode, the switch can create, modify, and delete VLANs and send this information on to other switches that are in the same VTP domain. Switches in transparent VTP mode can do the same except that information is not transmitted to other switches.

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A switch in VTP server mode can create, modify, and delete VLANs as well as transmit that information (if the switch has the highest VTP configuration revision number) to other switches in the same VTP domain.

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Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol used for dynamically negotiating a trunk connection between two Cisco switches.

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Because EtherChannel creates one logical link out of a number of physical ports, STP will treat the one logical link as an individual connection. When a physical port that is part of the logical link fails, the link will stay active and STP will not recalculate.

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An EtherChannel link can only be created between two switches or between an EtherChannel-enabled server and a switch. Traffic cannot be sent to two different switches through the same EtherChannel link.

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The guidelines for configuring an EtherChannel link are as follows:

• Interfaces which form an EtherChannel can be physically discontiguous, and on different modules.
• Interfaces in an EtherChannel have to operate at the same speed and in the same duplex mode.
• Interfaces in the EtherChannel must be assigned to the same VLAN, or be configured as a trunk.
• Interfaces in the EtherChannel have to support the same allowed range of VLANs.

Hint

There are five DTP mode combinations that will result in successful trunk link negotiation between two switches. dynamic auto and dynamic desirable dynamic desirable and dynamic desirable trunk and trunk trunk and dynamic auto trunk and dynamic desirable

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In order to create a loop-free topology, 802.1w RSTP has the following port roles: root, designated, alternate, edge, and backup. RSTP is able to actively confirm that a port can safely transition to the forwarding state without having to rely on a timer configuration.

Hint

To form a single path through a Layer 2 network, Layer 2 devices communicate with one another by the use of bridge protocol data units (BPDUs) as part of the Spanning Tree Protocol (STP). An attacker manipulates STP by spoofing the root bridge so that a device that is controlled by the attacker becomes a root bridge. The attacker can then obtain more information about the network or networks. BPDU guard can be used on ports that connect to user devices that are configured with PortFast so that BPDUs will not be accepted through that port.

Hint

Besides the VTP domain name and mode needing to be configured, the switch must connect to other switches in the same VTP domain through a trunk in order to transmit/receive VTP information.