Routing Fundamentals Flashcards
In routing, what are the packet forwarding mechanisms
Process switching
Fast switching
Cisco Express Forwarding (CEF)
What is process switching
Legacy, rarely used
Packets are forwarded to control plane where CPU matches destination address with entry in routing table
Slow.
(think of this as solving a problem by doing math long hand even if it is an identical problem)
What is fast switching
Common
Uses cache to store next hop information
If it is not there, it is process switched
(think of this as solving a math problem long hand and remembering the answer for subsequent problems)
What is CEF
Most recent
CEF builds a Forward information Base (FIB) and adjacency table
Table entries are not packet triggered like fast switching but change triggered when something changes in the network topology
(think of this as solving every possible problem ahead of time in a spreadsheet)
How does a router forward a frame
De-encapsulates layer 2 frame header and trailer to inspect layer 3 packet
Examines IP address to find best path
If path found, encapsulates the L3 packet into new L2 frame and forwards it to correct exit interface
How does RIP determine best path
Hop count
How does OSPF determine best path
Ciscos cost based on cumulative BW from source to destination
How does EIGRP determine best path
BW
Delay
Load
Reliability
What happens when a router has two or more paths to destination with equal cost metrics
Forwards packet using both paths equally
How do you configure a default static route
Router (config)# ip route 0.0.0.0 0.0.0.0 (ip add | exit int)
What is a floating static route
Used to provide a backup path to the primate static or dynamic route in the event of a failutre
Configure with higher AD
What is the administrative distance
If multiple routes exist on same network, the route uses the one with the lowest AD
How is floating static route configure
Same as static route but number on the end for AD
What is a distance vector protocol
Does not have knowledge of entire path to destination
Uses routers as ‘sign posts’ along path to destination
What is a link state protocol
Has complete view of topology or network
Like having a map
What are the main distant vector protocols
Rip v1
Rip v2
IGRP
EIGRP
What are the main link state protocols
OSPF
IS-IS
What is the main exterior gateway protocol
BGP
What is the AD of a connected interface
0
What is the AD of a static route
1
What is the AD of EIGRP
90
What is the AD of OSPF
110
What is the AD of RIPv2
120
What is the difference between RIP and RIPv2
RIPv2 uses classless addressing instead of classful
RIPv2 can send multicast instead of broadcast
How is rip configured
router rip
network 192.168.1.0
How is RIPv2 enabled?
using the ‘version 2’ command
What is auto sumarisation
Also called supernetting
Borrows bits from the network ID to represent networks
How is auto sumarisation disabled
Using the ‘no auto-summary’ command
What is a passive interface
Receives routing updates but doesn’t send any
How is a passive interface configured
router rip
passive-interface g0/0
end
What side of a serial cable DTE and DCE requires the clock speed to be set
DCE
What are the dynamic routing protocols used with IPv6
MP-BGP4
RIPng
OSPFv3
When configuring RIP, what command generates a default route
default-information originate
What is the default interface setting on a router
admin down
How would you identify a default route in the route table
marked with an *
