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Implementing Cisco Enterprise Network Core Technologies (ENCOR) v1.3 Quiz Questions and Answers

Answer :
  • 3

Explanation :

Unlike other spanning tree protocols, in which all the spanning tree instances are independent, MST establishes and maintains IST, CIST, and CST spanning trees: -> An IST is the spanning tree that runs in an MST region. Within each MST region, MST maintains multiple spanning tree instances. Instance 0 is a special instance for a region, known as the IST. All other MST instances are numbered from 1 to 4094. In the case for this question, there will be the 2 defined MST instances, and the special 0 instance, for a total of 3 instances. The IST is the only spanning tree instance that sends and receives BPDUs. All of the other spanning tree instance information is contained in MSTP records (M- records), which are encapsulated within MST BPDUs. Because the MST BPDU carries information for all instances, the number of BPDUs that need to be processed to support multiple spanning tree instances is usernamcantly reduced. All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root bridge ID, root path cost, and so forth. By default, all VLANs are assigned to the IST. An MST instance is local to the region; for example, MST instance 1 in region A is independent of MST instance 1 in region B, even if regions A and B are interconnected. -> A CIST is a collection of the ISTs in each MST region. -> The CST interconnects the MST regions and single spanning trees. Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/spantree.html
Answer :
  • Elects a root switch

Explanation :

The first step taken by the Spanning-Tree Protocol (STP) is to elect a root bridge (switch). The root bridge keeps the STP database. The bridge ID is used to select the root bridge in the network. The bridge ID is a combination of the priority of the bridge (switch) and the MAC address. If two switches or bridges have the same priority value, the switch with the lowest MAC address will have the lowest priority and become the root bridge. Once the STP process is complete (after switches go through the learning and listening stages), STP disables redundant loops in the network. STP does not set a priority for each redundant link. The network administrator can manually set the priority of a switch. STP then uses the priority and the switch's MAC address to calculate the bridge ID, which is used to select the root bridge. STP does not first select a designated switch for each switched segment. The first goal of STP is to select a root bridge for a switched segment (VLAN).
Answer :
  • 802.1d does not understand RSTP BPDUs because they are different versions, but when a RSTP switch receives an 802.1d BPDU, it responds with an 802.1d BPDU and eventually the two switches run 802.1d to communicate.

Explanation :

For backward compatibility with 802.1D switches, RSTP selectively sends 802.1D configuration BPDUs and TCN BPDUs on a per-port basis. When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch processes all BPDUs received on that port and ignores the protocol type. If the switch receives an 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is connected to an 802.1D switch and starts using only 802.1D BPDUs. However, if the RSTP switch is using 802.1D BPDUs on a port and receives an RSTP BPDU after the timer has expired, it restarts the timer and starts using RSTP BPDUs on that port. Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/spantree.html
Answer :
  • Source address

Explanation :

The Unicast RPF feature helps to mitigate problems that are caused by the introduction of malformed or forged (spoofed) IP source addresses into a network by discarding IP packets that lack a verifiable IP source address. For example, a number of common types of denial-of-service (DoS) attacks, including Smurf and Tribal Flood Network (TFN), can take advantage of forged or rapidly changing source IP addresses to allow attackers to thwart efforts to locate or filter the attacks. For Internet service providers (ISPs) that provide public access, Unicast RPF deflects such attacks by forwarding only packets that have source addresses that are valid and consistent with the IP routing table. This action protects the network of the ISP, its customer, and the rest of the Internet.
Answer :
  • The switch connected to the phone provides the VLAN

Explanation :

When using auxiliary VLANs, the switch connected to the phone provides the VLAN to the phone. IP telephones typically have a built in 3-port 10/100 hub. One port internally attaches to the phone, one port is attached to the switch access port, and the other is used to connect to the workstation. The PC attached to the switch port via the IP phone is unaware of the presence of the phone Objective: Layer 2 Technologies - Sub-Objective: Configure and verify VLANs - References: Cisco > Home > Products and Services > Cisco Interfaces and Modules > Cisco Network Modules > Product Literature > Data Sheets > Cisco Catalyst 6500 Series Switches - Cisco > Cisco IOS Interface and Hardware Component Command Reference > squelch through system jumbomtu > switchport voice vlan
Answer :
  • The port goes into root-inconsistent state and stops forwarding traffic.

Explanation :

The root guard ensures that the port on which root guard is enabled is the designated port. Normally, root bridge ports are all designated ports, unless two or more ports of the root bridge are connected together. If the bridge receives superior STP Bridge Protocol Data Units (BPDUs) on a root guard-enabled port, root guard moves this port to a root-inconsistent STP state. This root-inconsistent state is effectively equal to a listening state. No traffic is forIarded across this port. In this way, the root guard enforces the position of the root bridge. Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96b.shtml
Answer :
  • security violation

Explanation :

There are various reasons for the interface to go into errdisable. The reason can be: -> Duplex mismatch -> Port channel misconfiguration -> BPDU guard violation -> UniDirectional Link Detection (UDLD) condition -> Late-collision detection -> Link-flap detection -> Security violation -> Port Aggregation Protocol (PAgP) flap -> Layer 2 Tunneling Protocol (L2TP) guard -> DHCP snooping rate-limit -> Incorrect GBIC / Small Form-Factor Pluggable (SFP) module or cable -> Address Resolution Protocol (ARP) inspection -> Inline power Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00806cd87b.shtml