ggered updates, 129 unidirectional links, 101-103 tromboning, defined, 527 Unified Communications Manager, 350 troubleshooting unified fabric, 265 ASA firewalls, 509 uniq command (NX-OS), 278 bridge assurance, 103 unshielded twisted pair (UTP), 5 broadcast storms, 82-86 upgrading converged networks, 620-626 networking equipment, 722-725 CSU/DSU, 371-374 NX-OS, 312-316 dial peers, 569 UplinkFast feature (spanning tree), 93, 97 duplex mismatch, 100 US West, 352 failures, 700-704 username command, 439-442 Frame Relay, 403-407 UTC (Coordinated Universal Time), 691 loopback tests and, 366-369 UTP (unshielded twisted pair), 5 MAC address table instability, 86 phone registration, 567 problem isolation and, 403 SIP issues, 570 VAD (voice activity detection), 563 T1, 366-369 Variable Length Subnet Masks (VLSM), 132, TFTP service, 568 unidirectional links, 101-103 VCs (virtual circuits) VoIP issues, 567-570 defined, 387 wireless networks, 521 Frame Relay service considerations, 390 trunk encapsulation command, 42 subinterfaces and, 391, 401 trunk negotiation, 40 troubleshooting, 403-407 trunk ports, 26 types of, 387 trunking VDC (Virtual Device Context), 115, 283-287, defined, 37 protocols supporting, 38 vdc combined-hostname command, 287 trunks vdc command, 283 configuring, 42-47 vdc-admin role, 442 defined, 26, 37 vdc-operator role, 442 EtherChannels and, 64 verifying mutex support, 305 functionality, 38-42 virtual circuits (see VCs) interswitch, 630 Virtual Device Context (VDC), 115, 283-287, network design considerations, 641 tunnels virtual LANs (see VLANs) defined, 167 virtual Port-Channel (see vPC) Virtual Private Networks (see VPNs) GRE, 167-179 VPN, 167 Virtual Router Redundancy Protocol (VRRP), Turbo ACLs, 423, 424 type of service (TOS) field, 578-581 Virtual Routing and Forwarding (VRF), 115- 117, 207, 1,281-283 virtual servers in network designs, 648 virtual switch link (VSL), 256 UDLD (Unidirectional Link Detection), 103 Virtual Switching System (see VSS) UDP packets, 524, 575 Virtual Teletype Terminal (VTY), 