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To be able to route packets, a router must know, at a minimum, the following: (give 3)
- destination address
- neighbor routers from which it can learn about remote networks
- possible routes to all remote networks
- the best route to each remote network
- how to maintain and verify routing information
If a router receives a ICMP packet that is destined for an IP address that is not in it's routing table, what will the router do with the packet and how will ICMP respond?
the router will discard the packet and send an ICMP destination unreachable message back out the interface that the source LAN packet originated from.
How does the router learn about remote networks?
c, d. the router learns about remote networks from neigbors (cdp neighbors) OR from an administrator
In this type of routing, a protocol on one router communicates with the same protocol running on the neighbors routers. The routers then update each other about all the networks they know about and place it in the routing table.
In this type of routing, the administrator is responsible for updating all changes by hand into all routers.
The "C" in the routing table output means that the networks listed are?
Sends broadcast every 30 seconds
What are the 3 classes of routing protocols?
- distance vector
- link state
This routing protocol finds the best path to a remote network by judging distance.
True or False: MAC addresses are always local on the LAN and always go through past a router.
False. MAC addresses NEVER go through past a router
True or False: IP runs a complete CRC as the Data Link layer does.
False. IP does NOT run a complete CRC - it only checks the header for errors
What's the first step in the IP Routing Process?
Internet Control Message Protocol (ICMP) creates an echo request payload which is just the alphabet in the data field.
What's the 2nd step in IP routing process?
ICMP hands the payload to the IP, and creates a packet.
At minimum, what does a packet contain?
- IP source address
- IP destination address
- Protocol field with 01h (or 0x01)
What is the 3rd step in the IP routing process?
Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one.
What is the 4th step in the IP Routing Process?
After it determines if the IP is local or remote, it sends the packet to the default gateway.
What is the 5th step in the IP Routing Process?
The packet is handed down to the Data Link layer, framed and sent to the router's interface that's connected to that network.
What is the 6th step in the IP routing process?
ARP - Address Resolution Protocol cache of the host is checked to see if the IP address of the default gateway has already been resolved to the hardware address.
What happens if the hardware address isn't already in the ARP cache of a host?
An ARP broadcast is sent to out onto the local network to search for the hardware address. the router responds to the request and provides the hardware address of the interface and the host caches this address.
What command do you use to check the ARP table?
What is CRC?
Cyclic Redundancy Check
What happens in the 7th step of the IP Routing Process?
- the LAN driver is used to provide media access via the type of LAN being used.
- a frame is then generated, encapsulating the packet with control information.
What is contained within the frame?
- hardware destination and source addresses
- ether-type field
- frame check sequences (FCS) that houses the results of the cyclic redundancy check (CRC)
What happens in the 8th step of the IP routing process?
when the frame is completed, it is handed down to the physical layer (L1) to be put on the physical medium one bit at a time.
What is the 9th step of the IP Routing Process?
Every device in the collision domain receives the bits and builds the frame and each run a CRC and check the answers in the FCS field.
During a CRC, what happens if the answers in the FCS field don't match?
the frame is discarded
What happens during the 10th step of the IP Routing Process?
- the packet is pulled from the frame, and what is left of the frame is discarded.
- the packet is handed to the protocol listed in the Ether-Type field, it's given to IP.
What happens at the 11th step of the IP Routing Process?
IP receives the package and checks the IP destination address
In step 12 of the IP Routing process what must the routing table have?
- the routing table must have an entry for the network
What happens if there is no entry in the routing tables?
- the packet will be discarded immediately and an ICMP message will be sent back to the originating device with an a destination network unreachable message.
What will the destination MAC address be if your packets are destined for outside the LAN?
- the destination MAC addess will always be the router's interface
What are some of the benefits of static routing?
- no overhead on the router cpu
- no bandwidth usage between routers
- adds security to allow routing access to certain networks only
What are some disadvantages of static routing?
- admin must really understand the internetwork and how each router is connected to configure routers correctly
- if a network is added to the internetwork, the admin has to add a route to it on all routers-by hand.
- not feasible in larger networks due to maintaining it
What is the command syntax to add a static route to a routing table?
ip route [destination_network] [mask] [next-hop_address or exitinterface] [administrative_distance] [permanent]
What command do you use to create a static route?
Describe the routing when protocols are used to find networks and update routing tables on routers.
On what type of networks should you only use default routing?
stub networks - those with only one exit path out of the network
What type of protocols use aspects of both distance vector and link state?
Hybrid protocols, like EIGRP
True or False: If a network is directly connected, the router will always use the interface connected to the network.
What happens in the 13th step of IP Routing Process?
if the router finds an entry for the destination network in its table, the packet is switched to the exit interface
What happens during the 14th & 15th step of the IP Routing Process?
- 14th - the router packet-switches the packet to the interface buffer.
- 15th - the interface buffer needs to know the hardware address of the destination host and first checks the ARP cache
Name the two types of routing protocols used in internetworks:
- IGPs - interior gateway protocols
- EGPs - exterior gateway protocols
True of False: Dynamic routing is when protocols are used to find networks and update routing tables on routers.
What is used to rate the trustworthiness of routing information received on a router from a neighbor router?
AD - Administrative Distance
An administrative distance is an integer from 0 to?
(this multiple choice question has been scrambled)
What integer is the most trusted and which means no traffic will be passed via this route?
A. 0 & 192
B. 0 & 255
C. 0 & 254
D. 255 & 0
E. 0 & 254
F. 1 & 225
B. 0 is the most trusted and 255 means no traffic will be passed
(this multiple choice question has been scrambled)
What is the first thing the router checks if it receives two updates listing the same network?
it checks the AD
True or False: If an advertised route has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.
What is the default administrative distance for a static route?
What is the default administrative distance for EIGRP?
What is the default administrative distance for IGRP?
What is the default administrative distance for OSPF?
What is the default administrative distance for RIP?
What is the default administrative distance for External EIGRP?
What happens at the 16th step of the IP Routing process?
The Data Link layer creates a frame with the destination and source hardware address, Ether-Type field, and FCS field at the end and then handed to the Physical layer to be sent out one bit at a time
Which statement about RIPng is true?
A. RIPng uses broadcasts to exchange routes.
B. RIPng allows for routes with up to 30 hops.
C. RIPng is enabled on each interface separately.
D. There can be only one RIPng process per router.
C. - RIPng is similar to RIPv2 but is used for IPv6. But unlike RIPv1 and RIPv2, RIPng is enabled on each interface separately.
(this multiple choice question has been scrambled)
What are two characteristics of RIPv2? (Choose two)
A. classful routing protocol
B. variable-length subnet masks
C. broadcast addressing
D. manual route summarization
E. uses SPF algorithm to compute path
B and D
Which (config-router) command will allow the network represented on the interface to be advertised by RIP?
no auto summary
ip address 10.12.6.1 255.255.0.0
A. redistribute ethernet0
B. network ethernet0
C. redistribute 10.12.0.0
D. network 10.12.0.0
Describe the following command in the string:
- destination_network - the network you are placing in the routing table
- mask - the subnet mask being used on the network
- next-hop_address - the address of the next-hop router that will receive the packet and forward it to the remote network
- exitinterface - used in place of the next-hop address or directly connected route
- administrative_distance - used to rate the trustworthiness of routing information received on a router from a neighbor router
- permanent - keeps the entry in the routing table no matter what happens
Describe routing by rumor
a router receiving an update from a neighbors router believes the information about the remote networks without actually finding out for itself.
What is checked first if you have a network that has multiple links to the same remote network? And what happens if the AD is the same?
- the administrative distance of each received update is checked first
- the protocols will use other metrics to determine the best path to use to that remote network
What does RIP use to determine the best path to a network?
What happens if RIP finds more than one link with the same hop count to the same remote network?
it automatically performs a round-robin load balancing
How many equal-cost links can RIP perform load balancing for? And what is the default?
up to six and four by default
How does distance-vector routing protocols keep track of any changes to the internetwork?
by broadcasting periodic routing updates out all active interfaces
What do the broadcast's include?
the complete routing table
Why do routing loops occur?
because every router isn't updated simultaneously
What causes counting to infinity?
when wrong information being communicated and propagated throughout the internetwork, or gossip broadcasts
What is the maximum hop count that RIP permits?
15, anything more than 15 is deemed unreachable
Reducing incorrect routing information and routing overhead in a distance-vector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received is known as what?
a split horizon
What is a split horizon?
not advertising the route back out the same interface that it was learned on
What is a holddown?
preventing regular update messages from reinstating a route that is going up and down or flapping
Which routing protocol is considered a true distance-vector routing protocol? And why?
Routing Information Protocol (RIP) because it sends the complete routing table out to all active interfaces every 30 seconds and only uses hop count to determine the best way to a remote network.
What type of routing does RIPv1 use? Explain.
It uses classful routing meaning all devices in the network must use the same subnet mask.
What type of routing does RIPv2 use? Explain.
It uses classless routing, and unlike RIPv1 it sends subnet mask information with route updates
What is the command sequence to configure rip on a router?
- config t
- router rip
- network 10.0.0.0
What happens if a router receives a routing update that contains a higher-cost path to a network that's already in its routing table?
the update will be ignored
What command would you use to prevent RIP update broadcasts from being sent out a specific interface but still receive RIP updates on that same interface?
use the passive-interface command and specify the interface
What command sequence do you use to configure RIPv2 on a router?
- config t
- router rip
- network [network_address]
- version 2
What must your routers be, in order to use IGRP in your network? Explain.
all routers on your network must be Cisco routers because IGRP is a Cisco-proprietary distance-vector routing protocol
What is the main difference between RIP and IGRP configuration?
When you configure IGRP you supply the autonomous system number.
Describe some advantages of using IGRP over RIP.
- can be used in large internetworks
- uses an autonomous system number for activation
- gives full route table update every 90 seconds
- administrative distance of 100 instead of 120
- uses bandwidth and delay of the line as metric, with a max houp count of 255 (15 for RIP)