642-811 Building Cisco Multilayer Switched Networks (BCMSN) Practice Exams

Demo Question 2.

Answer: B, D

Explanation: UplinkFast provides fast convergence in the network access layer after a spanning-tree topology change using uplink groups. An uplink group is a set of ports (per VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is forwarding) and a set of blocked ports (not including self-looped ports). The uplink group provides an alternate path in case the currently forwarding link fails. NoteUplinkFast is most useful in wiring-closet switches with a limited number of active VLANs. This enhancement might not be useful for other types of applications and should not be enabled on backbone or distribution layer switches. Figure 9-1 shows an example UplinkFast network topology. SwitchA, the root switch, is connected directly to SwitchB over link L1 and to SwitchC over link L2. The port on SwitchC that is connected to Switch B over linkL3 is in blocking state. Figure9-1: UplinkFast Example Before Direct Link Failure If SwitchC detects a link failure on the currently active link L2 (a direct link failure), UplinkFast unblocks the blocked port on SwitchC and transitions it to the forwarding state immediately, without transitioning the port through the listening and learning states (as shown in Figure 9-2). This switchover takes approximately oneto fiveseconds. Figure9-2: UplinkFast Example After Direct Link Failure As soon as the switch transitions the alternate port to the forwarding state, the switch begins transmitting dummy multicast frames on that port, one for each entry in the local EARL table (except those entries associated with the failed root port). By default, approximately 15dummy multicast frames are transmitted per 100milliseconds. Each dummy multicast frame uses the station address in the EARL table entry as its source MAC address and a dummymulticast address (01-00-0C. CD. CD. CD. as the destination MAC address. Switches receiving these dummy multicast frames immediately update their EARL table entries for each source MAC address to use the new port, allowing the switches to begin using the new path almost immediately. If connectivity on the original root port is restored, the switch waits for a period equal to twice the forward delay time plus 5seconds before transitioning the port to the forwarding state in order to allow the neighbor port time to transition through the listening and learning states to the forwarding state. Reference: http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/rel_5_4/config/stp_enha.htm#xtocid25869