Ch18-Configuring IPv4 Addresses and Static Routes Flashcards
On an IP host, the IP address of some router to which the host sends packets when the packet’s destination address is on a sunset other than the local subnet
default gateway/router
A list of IP addresses of neighbors on the same VLAN, along with their MAC addresses, as kept in memory by hosts and routers
ARP table
A list of routes in a router, with each route listing the destination subnet and mask, the routers interface out which to forward packets destined for the subnet, and as needed, the next-hop routers IP address
routing table
In an IP routing table, part of a routing table entry that refers to the local interface out which the local router should forward packets that match the route.
Outgoing interface
Exit Interface
Egress interface
One of the virtual interfaces on a single physical interface
sub-interface
A configuration concept inside Cisco switches, used as an interface between IOS running on the switch and a VLAN supported inside the switch, so that the switch can assign an IP address and send packets into that VLAN
VLAN interface
A LAN switch that can also perform Layer3 routing functions.
Layer 3 switch
On a router, an IP route added to the routing table when the router interface is both up and has an IP address configured. The route is for the subnet that can be calculated given the configured IP address and mask
connected route
On a router, the route that is considered to match all packets that are not otherwise matches by some more specific route
default route
An IP route on a router created by the user configuring the details of that route on the local router
static route
A route with a /32 mask, which by virtue of this mask represents a route to a single host IP address
host route
A static IP route that uses a higher administrative distance than other routes, typically routes learned by a routing protocol. As a result, the router will not use the static route if the routing protocol route has been learned, but will use the static route if the routing protocol fails to learn the route
floating static route
A route for a classful network
network route
In Cisco routers, a means for one router to choose between multiple routes to reach the same subnet when those routes are learned by different routing protocols. A measure of the “trustworthiness” of learned routes. The lower its value, the better the source of the routing information
administrative distance
Command reference:
Global command to create a sub interface and to enter configuration mode for that sub interface
interface int-type-num.subint
Command reference:
A subinterface cmmand that tells the router to use 802.1Q trunking, for a particular VLAN, and with the native keyword, to not encapsulate in a trunking header.
encapsulation dot1q vland-id [native]
Command reference:
A command on Cisco switches that enables the switch to support IP routing
sdm prefer lanbase-routing
Command reference:
Global command that enables or disables the routing of IPv4 packets on a router or Layer 3 switch
[no] ip routing
Command reference:
Global command on a Layer 3 switch to create a VLAN interface and to enter configuration mode for that VLAN interface
interface vlan vlan-id
Command reference:
Global configuration command that creates a static route
ip route address mask egress-int [distance] [permanent]
Command reference:
List the router’s entire routing table
show ip route
Command reference:
A subinterface cmmand that tells the router to use 802.1Q trunkin, for a particular VLAN, and with the native keyword, to not encapsulate in a trunking header.
Command reference:
Lists a subset of the routing table
show ip route [connected | static | rip]
Command reference:
Lists detailed information about the route that a router matches for the given IP address
show ip route ip-addr
Command reference:
Lists VLAN configuration and statistics for VLAN trunks configured on routers
show vlans
Command reference:
Lists the routers IPv4 ARP cache
show ip arp
show arp
Command reference:
Removes all dynamically learned ARP table entries, or if the command lists an IP address, removes the entry for that address only
clear ip arp [ip-addr]
Which command produced the following output?
show ip route
Which commands would have produced the following output?
show arp / show ip arp
Consider a host attempting to send a packet to a destination on its local subnet. The host will perform two actions prior to sending that packet. What will those two actions be?
- Determine host destination MAC address (check its ARP table and ARP if necessary)
- Encapsulate packet in a frame with the destination MAC address of the destination host.
Consider a host attempting to send a packet to a destination that is not on its local subnet. The host will perform two actions prior to sending that packet. What will those two actions be?
- Determine the default gateway destination MAC address (ARP table, ARP if necessary).
- Encapsulate the packet in a frame with the default gateway’s MAC address as the destination MAC address.
A router has just received a frame with a packet destined for a device on another subnet. The router must now decide to foward or filter that frame. As part of the decision proces, what two actions will the router take?
- Examine the frame for errors (inspect FCS).
- Ensure the destination MAC address is, in fact, the router’s MAC address (or a multicast or broadcast address).
If the frame has no errors, and the destination MAC address belongs to the router, the frame will be processed. Otherwise, it will be filtered.
A router has received a frame with a packet destined for another subnet. The router has processed the frame and de-encapsulated the packet. What is the router’s next step?
Examine the destination IP address of the packet, and attempt to find a suitable match for that destination in the routing table.
A router has matched the destination IP address of an incoming packet with a suitable entry in its routing table. What information is supplied by that routing table entry?
The exit interface (and possibly the next-hop router) that will be used to transmit the packet toward its destination.
A router is prepared to send a packet out a specific exit interface. What steps will the router take to accomplish this?
- If the exit interface is a LAN interface, the router must look to its ARP table (or ARP) to determine the destination’s MAC address.
- Encapsulate the packet in a frame appropriate for the exit interface
A router interface was configured with the intention of creating a connected route. What conditons must be satisfied in order for the router to add this route to its routing table?
The interface must be in an “up/up” state, and the interface must have an assigned IP address.
Multiple VLANs are in use within an organization. What options exist for routing between those VLANs?
- A router with a physical interface for each and every VLAN.
- A router with VLAN trunking enabled, connected to a trunking port on a switch (ROAS)
- Use of a Layer 3 switch
Consider a design which uses a router-on-a-stick topolgy using multiple subinterfaces. What configuration possibilities are available on the router to account for the native VLAN?
- Configure an additional subinterface with an IP address and an encapsulation command noting that VLAN as native:
int g0/0.99 encapsulation dot1q 99 native IP address 10.1.99.1 255.255.255.0
- Configure the physical interface with an IP address, but without an encapsulation command. The router will assume the physical address is using the native VLAN.
An expected route does not appear in the routing table. What possible conditions would cause this?
- Is the outgoing interface up/up?
- Is the next-hop router not reachable
- Does a better route with lower administrative distance exist?
An incorrect static route appears in the routing table. What conditions could cause this?
- Is there a subnetting math error in the configuration of the route?
- Is the next-hop IP correct, and does it reference an IP address on the neighbor router?
- Is the outgoing interface correct?
