100-490 RSTECH flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

EtherType field

A 2-byte field in an Ethernet II frame that identifies the upper-layer protocol carried in the payload, such as 0x0800 for IPv4 or 0x86DD for IPv6.

Native VLAN on trunk

The single VLAN whose frames cross an 802.1Q trunk untagged. Both ends of the trunk must agree on it, or frames leak between VLANs and CDP reports a mismatch.

Root bridge election

Switches compare bridge IDs (priority plus MAC). The lowest wins. Default priority is 32768, adjustable in increments of 4096, so lowering priority forces a switch to become root.

Administrative distance

A trust rating for route sources on a Cisco router. Lower is preferred: connected 0, static 1, EIGRP 90, OSPF 110, RIP 120. The lowest-distance route enters the routing table.

Usable hosts per subnet

Computed as 2 to the power of host bits, minus 2, because the all-zeros network address and all-ones broadcast address cannot be assigned to hosts.

Access versus trunk port

An access port belongs to one VLAN and sends untagged frames to end devices. A trunk port carries multiple VLANs using 802.1Q tags between switches.

Default gateway function

The router IP a host sends to when the destination is outside its own subnet. Without it, hosts reach only local-subnet addresses.

MAC address structure

48 bits expressed as 12 hexadecimal digits. The first 24 bits are the OUI assigned to the manufacturer; the last 24 bits uniquely identify the interface.

Configuration register 0x2142

A boot setting that makes the router ignore the saved startup configuration, starting with an empty configuration. It is used during password recovery, then reset to 0x2102.

Saving the running configuration

The command copy running-config startup-config writes the active configuration in RAM to NVRAM so it survives a reload. write memory performs the same action.

show ip interface brief

Displays each interface with its IP address, Layer 1 status, and Layer 2 protocol status. Status up with protocol down typically indicates a Layer 2 mismatch.

show version output

Reports the IOS image name, uptime, installed memory, interface inventory, and the current configuration register value, useful for inventory and boot troubleshooting.

Privileged EXEC mode

Entered with the enable command and shown by the # prompt. It grants access to show commands, copy, reload, and entry into global configuration mode.

Global configuration mode

Entered with configure terminal from privileged EXEC. Changes made here affect the whole device, such as hostname, routing, and ACL definitions.

CDP neighbor discovery

Cisco Discovery Protocol, enabled by default, advertises platform, IP address, and interface details to directly connected Cisco devices. show cdp neighbors detail displays them.

show mac address-table

Lists learned MAC addresses mapped to switch ports and VLANs. Entries age out by default after 300 seconds of inactivity.

Interface speed and duplex

Autonegotiation is the default. If one side is hard-set, the other must be hard-set to the same values; otherwise duplex mismatch causes errors and slow throughput.

Boot system command

A global configuration command that specifies which IOS image in flash the router loads at startup, overriding the default first-image selection.

Bottom-up troubleshooting

Start at the physical layer and work upward through data link, network, and application. It quickly isolates cabling and link problems before examining addressing or services.

Duplex mismatch symptoms

CRC and input errors, late collisions, and slow throughput under load while simple pings still succeed, usually caused by one side hard-set and the other auto-negotiating.

Traceroute operation

Sends packets with incrementing time-to-live values. Each router that decrements TTL to zero replies with ICMP time exceeded, revealing the path hop by hop.

show interfaces error counters

CRC and input errors indicate cabling or duplex faults; late collisions indicate duplex mismatch or cable runs exceeding allowed length; output drops indicate congestion.

MAC address flapping

The same MAC appears on different switch ports in succession, a classic sign of a Layer 2 loop or miswiring. Spanning tree or port security usually resolves it.

Incomplete ARP entry

An ARP entry with no resolved MAC means the destination IP is on the local subnet but is not answering, often due to a down host, wrong VLAN, or firewall.

show ip route codes

The legend identifies route sources: C connected, S static, O OSPF, D EIGRP, R RIP, and * candidate default. A missing entry means no known path.

debug command caution

Debug output is generated for every matching event and can overload the CPU on a busy device. Restrict it with ACLs and disable it with undebug all when finished.

Standard versus extended ACL

Standard ACLs (1-99) match only source IP and are placed near the destination. Extended ACLs (100-199) match source, destination, protocol, and port and go near the source.

Implicit deny any

Every Cisco ACL ends with an invisible deny statement for all traffic. An ACL that matches nothing else blocks the remaining packets, so at least one permit is required.

Port security violation modes

Protect silently drops offending frames; restrict drops and logs with a counter; shutdown puts the port in err-disabled state, requiring shutdown then no shutdown to recover.

DHCP DORA process

Four messages: Discover broadcast by the client, Offer from the server, Request from the client, and Acknowledge from the server, which finalizes the lease.

DHCP relay with ip helper-address

Configured on a router interface to forward client DHCP broadcasts as unicasts to a server on another subnet, since broadcasts do not cross router interfaces.

NAT inside local address

The private IP address of an internal host before translation, as seen inside the network. It is translated to an inside global address for traffic leaving the outside interface.