Routing Flashcards
router
any hardware or software that forwards packets based on their destination IP address
layers in each model where routers belong
OSI - network layer
TCP/IP - Internet layer
interfaces
at least 2 - 1 internal, 1 external
can have more
2 criteria in each column of the routing table
which packets are for the route
which port to send them out
destination LAN IP
defined network ID
every network ID directly connected to one of the router’s ports is always listed here
subnet mask is needed to define the network
gateway
IP address for next hop router
where the packet should go
gateway when directly connected to network
gateway not needed
value set to 0.0.0.0 or IP address or directly connected port
gateway when not directly connected to network
gateway column of the routing table tells the router the IP address of a router to send packet to
interface
tells router which of its ports to use
LAN / WAN
Port IP
fa0/0 & fa0/0 - Cisco
0 in routing table
means “anything”
default route (example)
[0.0.0.0] [0.0.0.0] [70.30.4.1] [WAN]
any destination address
with any subnet mask
forward to 70.30.4.1
using WAN port
default route (importance)
tells router what to do with every incoming packet unless another line in the routing table gives another route
2 places where routers do not have default routes
private internetworks - every router knows every network
tier 1 Internet backbone
other devices with routing tables
all TCP/IP capable devices
command to display routing table
netstat -r (OS X & Windows)
route print (Windows)
metric
relative value that defines the cost of using a particular route
adding interfaces to a router
removable modules are available for more powerful routers
NAT
network address translation
enables routers to hide internal IP addresses while still allowing communication with the broader Internet
how NAT functions
router replaces the source IP address of a computer is its outside interface address on outgoing packets
basic NAT
translates the private / internal IP address to the global IP address on 1-1 basis
primary reason NAT was created & conception of what it does
address the problem of limited IPv4 addresses
security / anonymity
PAT
port address translation
uses port numbers to route traffic from specific machine in the network
most common form of NAT that handles 1-to-many connections
how PAT functions
internal machine initiates a session with an external machine
source & destination IP addresses & port numbers for TCP segment / UDP datagram are recorded in PAT’s translation table
private IP address is swapped for public IP address on each packet
port number used by the internal computer for the session is also translated into a unique port number that is also recorded by router
receiving system returns packet - reverses IP addresses & ports
router compares incoming destination IP address & port number and compares to translation table - puts correct IP back on packet
sends to correct computer
PAT limitation
only works for outgoing communication, not incoming communication
SNAT
static NAT
maps a single routable (not private) IP address to a single machine
enables access to that machine from outside the network
port forwarding
designate a specific local address for various network services
request a service using the public IP address of the router & the port number of the desired service
how port-forwarding routers work
examines the packet & compares to list of services mapped to local addresses
sends packet to appropriate computer
hiding hosted services inside network
change default port number for that service
DNAT
many computers can share a pool of routable IP addresses that number fewer than the number of computers
LAN traffic uses internal IP
NAT distributes routable IP from the pool for external requests
metrics only work when
there are direct connections between network IDs
hop
each time a packet goes through a router
metric
relative value that routers use when they have more than one route to get to another network
router always chooses route with lowest metric
most common criteria for determining a metric
hop count
bandwidth
latency
cost
MTU
how many hops with 3 intermediate routers
4
latency
collective term that describes the large number of issues that may occur between routers that slow down network connections
MTU
maximum transmission unit
largest frame a particular technology can handle
Ethernet frame size
1,500 bytes
fragmentation
packet is too large for a technology to handle
broken down to fit in network’s protocol
slows down movement of packets
way to limit fragmentation
set optimal MTU size before sending packets
administrative distance
metric used by routers that use multiple protocols & thus can have multiple paths to the same destination
SPB
shortest path bridge
routing metric standard to provide true shortest-path forwarding within Ethernet mesh topology
supports large layer 2 networks by providing fast convergence & multiple, equal cost paths
distance vector routing protocols (definition)
calculate the total cost to a network ID & compare to the totals cost of all other routes to that network ID
router chooses lowest cost route
distance vector routing protocols
total cost = hops between router & network
1 hop = cost of 1
can manually set cost of slower routes to 10
distance vector routing protocols - transfer of routing tables
routers transfer their entire routing table to other routes in the WAN
each protocol has a maximum number of hops that it will send its routing table to limit traffic
convergence
steady state
process of updating routing tables has been completed
distance vector routing protocols - convergence
routing tables are shared & updated
all but lowest total cost route are deleted from table
updated tables are shared again
all routers only have the lowest total cost routes saved
distance vector routing protocols - limitation
smaller networks - less than 10 routers
convergence would take too long
distance vector routing protocols - specific protocols
RIPv1
RIPv2
BGP
RIPv1
routing information protocol
max hop count = 15
every router sent updates every 30 seconds - bogged down networks
no authentication used
RIPv2
routing information protocol
current version of RIP
incorporates VLSM & uses authentication
otherwise works the same as RIPv1
VLSM
variable-length subnet masking
networks connected via a router use different subnet masks
AS
autonomous system
1 or more networks that are governed by a single dynamic routing protocol
use ASN rather than IP addresses
2 16-bit numbers separated by a dot (1.33457)
ASN
autonomous system number
globally unique identifier assigned by IANA
EGP
exterior gateway protocol
used by AS’s to communicate with each other
IGP
interior gateway protocol
used by the network(s) within an AS to communicate with each other
border gateway protocols
many used within AS’s i.e. RIP
BGP-4 is standard for communication between AS’s of the Internet
exam classification of BGP
hybrid protocol
more accurately a path vector protocol
configuration of BGP routers
manual
don’t go down often
edge routers
AS to AS routers
pass / advertise information i.e. ASN & other non-IP items
route aggregation
backbone routers track location of routers that connect to subsets of locations
way to simplify routing tables into manageable levels
supported by BGP
iBGP & eBGP
internal BGP & exterior BGP
BGP can be used to connect routers within an AS
link state dynamic routing protocol
only announce & forward individual route changes instead of entire routing table
OSPF & IS-IS
OSPF
open shortest path first
most commonly used IGP on Internet
designed to work within a single AS
much faster convergence
LSA
link state advertisements
packets sent by OSPF-capable routers to find other OSPF routers
“hello packets”
used to update link state databases
flooding
transmission of a lot of LSA packets when a router first comes online
determining OSPF cost
speed of the link
100,000,000 / bandwidth in bps
10BaseT 100,000,000 / 10,000,000 = 10
areas
groupings of routers to limit excessive broadcast while scaling up a network
backbone
central area in a network of interconnected areas
assigned area ID
all traffic between areas goes through backbone
area ID
0 or 0.0.0.0
ABRs
area border routers
special routers that interconnect areas
frequency of hello LSAs upon convergence
~ 30 minutes
unless break is detected in link state
break in link state is detected
routers attempt to reconnect > unsuccessful
routers send LSA announcing broken connection
each router updates table to remove broken route
benefits of OSPF
provides authentication
prevents loops
protocol of choice for large enterprise networks
IP version support in OSPF
OSPFv2 supports IPv4
OSPFv3 supports IPv6
IS-IS
intermediate system to intermediate system
very similar to OSPF
IPv6 compatible from the start
standard for ISPs - rare to see in practice
EIGRP
enhanced interior gateway routing protocol
developed by Cisco
“advanced distance vector protocol”
aspects of distance vector & link state protocols
route redistribution
router takes a route it has learned from 1 protocol & announces that route over another protocol
routers can’t communicate with different protocols
can use multiple protocols simultaneously to perform route redistribution
connecting to a router
serial connection - Cisco - rollover or Yost cable
usb
RJ-45
managed devices
advanced switches & routers that require configuration
method of configuring router once connected
terminal emulation programs i.e. PuTTY & HyperTerminal
basic settings for serial connections
9,600 baud
8 data bits
1 stop bit
no parity
console
screen you type into in a terminal emulation program
NMS
network management software
software that interfaces with network hardware to provide an overall view of the network
[blank] are configured to use protocols
individual NICs - not entire routers
protocols must match
common source of routing problems
error in routing table
commands for investigating routes
tracert - Windows
traceroute - Unix/Linux
traceroute no longer works
something wrong between your router and the next router upstream
mtr
my traceroute
continuous Linux tool
pathping
Windows tool that computes performance over a set amount of time & displays summary
