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CCNA - Routing Final

QuestionAnswer
What is a routing loop? A packet is continuously transmitted within a series of routers without ever reaching its intended destination network.
Common causes of routing loops? Incorrect: Static, Route Redistribution (between routing protocols) and Inconsistent Routing Tables (slow convergence).
Problems caused by routing loops? Uses bandwidth + CPU, routing updates could be lost or processed slower leading to more loops, packets might get lost without reaching intended destination.
Solutions to Routing Loops? #1. Max Metric (RIP uses 15, prevents counts to infinity) #2. Hold-down timers #3. Split Horizon #4. Route Poisoning or Poison Reverse #5. Triggered Updates.
What is Count-To-Infinity? A routing loop causes the hop count metric of RIP to continually go higher, max of 15 prevents a Count to Infinity.
What is a Hold-Down Timer? If a route goes down there is a set amount of time when the router will not accept any information on a route unless it's a better metric than before. This allows time for the information to propagate the network.
What is Split-Horizon? Routers will not send information about a route back from the same interface it received the update.
What is Route Poisoning? Showing a route is unreachable by sending a route update at the MAX metric (16 hops in RIP).
What is Poison Reverse? Combination of Route Poisoning with Split Horizon, send poison update out the interface where the route was learned from.
How does TTL affect routing loops? TTL will get the packet dropped once it reaches zero, will not loop forever.
What are Alex Zinin's Routing Table Principles? #1. Routing decisions made from routers routing table #2. Just because information is in one routing table doesn't mean it's in another #3. Routing information to a destination does not guarantee a return path.
In RIPv1 if one router has ip 192.168.185.0/28 what will be advertised out and interface with ip 192.168.182.16/29? Due to VLSM it will not be advertised.
In RIPv1 if one router has ip 192.168.185.0/28 what would be advertised out an interface with ip 192.168.182.16/28? Since the mask is the same it will be advertised.
In which network layer does CDP operate? Layer 2, so any directly connected devices become neighbors, no IP (layer 3) information required.
Commands to show CDP informaion? show cdp neighbors OR show cdp neighbors detail.
How do you turn off CDP advertisements? ENTIRE ROUTER: Router(config)# no cdp run SINGLE INTERFACE: Router(config-if)# no cdp enable
When calculating a static summary route where do you draw the boundary line? In between where all the bits are the same and where there are any different bits.
In RIPv1 a router has an interface with ip 192.168.185.0/28 it sends an update out an interface with a 172.18.0.0/22 mask, what happens? RIPv1 will automatically change the 192.168.185.0/28 to 192.168.185.0/24 which is the classful address. Automatic Summary.
What Metric does RIP use? Hop count.
What Metric does EIGRP use? Bandwidth and Delay. Could also use Load and Reliability if configured.
What Metric does OSPF use? Cost (Bandwidth on cisco devices).
What is the Administrative Distance of RIP? 120
What is the Administrative Distance of EIGRP? 90, EIGRP Summary = 5, External = 170
What is the Administrative Distance of OSPF? 110
What is the Administrative Distance of Directly Connected? 0
What is the Administrative Distance of Static? 1
What are the three types of routes in a routing table? Directly Connected, Static, Dynamic
What are level 1 routes? Default, Supernet and Network (parent) routes.
What are level 2 routes? Subnet or Child Routes.
What is an Ultimate Route? A route that includes a next-hop IP or an exit interface.
What if a route doesn't have an exit interface? Recursive route look-up must be done to find the exit interface.
How is the parent route listed if the child routes all have the same mask? The parent will be given the mask and all the child routes will be listed without one
How is a parent route listed if the child routes have VLSM The parent route will be given a classful mask and the child routes will all have their own individual masks.
How does classful routing behavior affect the routing table lookup process? If the route found a best match was a parent route but there were no children routes that match it, packet gets dropped. If it checks a parent route it will not go back to default route for a lesser match.
How to set up classless routing behavior? ip classless (no ip classless to turn it off)
When did ip classless become the default behavior (which version) 11.3 or > = ip classess < 11.3 = no ip classess
What algorithm does EIGRP use? DUAL
How often does EIGRP send out periodic routing updates? It doesn't use periodic routing updates, they are triggered.
How does DUAL work? It maintains a topology table where it holds on to not only the best route (Successor) but also all loop-free routes (feasible successor)
How does EIGRP remain loop-free? EIGRP only adds routes that have a REPORTED DISTANCE lower than the current paths FEASIBLE DISTANCE to be sure there is no loop.
How does autonomous system number work? When setting up EIGRP it is required to be the same for the routers to form adjacency
How does EIGRP form adjacency? Sends out Hello packets.
What is a PDM? Protocol Dependent Module, used in EIGRP, lets it use other protocols besides IP
What is RTP? Reliable Transport Protocol, like UDP or TCP for EIGRP but different so that it can be used with other protocols. Even though says Reliable can be unreliable.
What is EIGRP Query used for? When EIGRP needs to find a route to a network, query used to ask other routers for routes, other routers respond with REPLY.
Default Hello and Hold times on EIGRP? 5 seconds Hello, 15 seconds Hold Time....On slow networks (<1.544Mbps) 60 and 180seconds
What happens in EIGRP when hold timer runs out? route declared as down DUAL searches for new path in topology table or queries get sent out.
What is a Bounded Update? EIGRP sends partial updates only to routers affected by the change, only routers that need the info get updated
What is a Partial Update? Used by EIGRP, only includes the information on the route changes. Might be called Incremental.
Big name people behind DUAL? Dijikstra, Scholten, Garcia-Luna-Aceves
What is an autonomous system? Collection of networks under the administrative control of a single entity, presents common routing policy to internet. Could be called PROCESS ID (16 bit 0-65,535).
How is Wild Card calculated? 255.255.255.255 - subnet mask = wild card
Command to show connected routers in EIGRP? show ip eigrp neighbors. Shows ip, interface, hold seconds, uptime, etc.
What does passive-interface command do for EIGRP? Turns off the hello packets on the interface.
Why use no auto-summary on EIGRP? Without no auto-summary EIGRP sets the parent addresses to use a null interface making them an ultimate route.
What letter does EIGRP show up as in a routing table? D - for dual
What letter does OSPF show ip in routing table? O
How to check the metric values in EIGRP? show ip protocols
What is Delay? Measure of time it takes for a packet to traverse a route based on type of link. For Fast Ethernet = 100, T1 = 20,000
What is Reliability? Probability that link will fail or how often it has experienced failures. 0-255, 255=good, 0=bad
What is Load? Amount of traffic on link. 0-255, 0=good, 255=bad
How to calculate EIGRP metric? (10,000,000 / slowest bandwidth (kbps) ) * 256 + ( sum of delays / 10 ) * 256
What is Successor? The next router used in route
Feasible Successor? Router which has no possibility of routing loops that also has a route, back up route.
Feasible Distance? The lowest calculated metric of available routes.
Reported Distance? Metric that neighboring routers have to the destination.
Feasibility Condition? Reported Disance < Feasible distance = no routing loops
Topology Database? EIGRP stores the Successor, FD, FS and RD. show ip eigrp topology
Passive State? In EIGRP topology table, displays as P, means the route is stable
Active State? In EIGRP topology table, means recalculating or searching for new path, if stuck in active displays an A (unusual)
FSM? Finite State Machine, defines possible states something can go through, what events cause them and events result from them.
EIGRP manual summarization? R1(config-if)# ip summary-address eigrp (AS#) (summary IP) (summary mask)
Default route in EIGRP? redistribute static, because EIGRP external route = 170 AD (D*EX)
change EIGRP bandwidth utilization? R1(config-if)# ip bandwidth-percent eigrp (AS#) (%)
change EIGRP hello/hold intervals? R1(config-if)# ip hello-interval eigrp (AS#) (seconds) OR R1(config-if)# ip hold-time eigrp (AS#) (seconds)
What is SPF? Shortest Path First algorithm, smallest sum all hops to get to the destination network. (not shortest distance, but smallest metric sum)
What is LSP? Link-State Packet, contains the state of each directly connected link (neighbor ID, link type, bandwidth)
What is flooding? OSPF floods out LSPs as soon as they receive them, do calculations on them later so it has fast convergence. Stores into a database.
What conditions for OSPF to form adjacency? Hello/Dead Timer, Link Type, Area
Advantages of Link-State Routing Protocol? Builds topological map, fast convergence, event-driven updates, hierarchical design (areas)
Whats is paranoid update? OSPF flooding network every 30 minutes without any trigger.
Disadvantages of Link-State Routing Protocol? Memory, CPU, bandwidth (while flooding)
When is router-id important for OSPF? In multiaccess networks like ethernet and frame relay
Multicast Address of OSPF? 224.0.0.5 (Hello packets) or 224.0.0.6 (DROthers use to send LSU/LSA)
Multicast Address of EIGRP? 224.0.0.10
Multicast Address of RIPv2? 224.0.0.9
How does OSPF election process work? Highest Interface Priority > Highest Router-ID > Loopback IP > interface IP ... but the router has to be online to be elected so sometimes the fastest loading router wins
What is DR? OSPF Designated Router, all neighbors in multiaccess network segment send routing into to this router (and BDR) and DR floods it to others.
What is BDR? OSPF Backup Designated Router, all routers in multiaccess network segment send routing info to BDR and DR. Doesn't flood.
What is DROther? Router using OSPF in multiaccess network, doesn't flood, sends info to DR and BDR
How long is the Dead Interval for OSPF? 4 times the length of the hello interval, default 40 seconds
How long is the Hello Interval for OSPF? 10 seconds
Whats is LSU/LSA? Link-State Update/Advertisement, used for sending link-state records
Command to try and reset the router-id? R1# clear ip ospf process
What is a flapping link? network that cycles between up and down states.
What is SPF schedule delay? To prevent SPF algorithm running too frequently there is a 5 second delay and a 10 second hold time after running the algorithm
How to calculate OSPF metric? cost = reference bandwidth / link bps ... default reference bandwidth = 1 billion, can be changed with command auto-cost reference-bandwidth <#>
How to set cost for OSPF on a link? bandwidth (#) or ip ospf cost (#) on an interface
OSPF five network types? 1.Point2Point 2.Broadcast Multiaccess 3.Nonbroadcast Multiaccess 4.Point2Multipoint 5.Virtual Links
How to calculate # of adjacencies in a multiaccess network? n=# of routers ... n(n-1)/2
For OSPF how to set priority on a interface? R1(config-if)# ip ospf priority (0-255) , 0 means cannot be made DR or BDR
Sending default route with OSPF? default-information originate , shows up as 0*E2
What extra information required in OSPF network commands? Area #
Change OSPF hello/dead timer intervals? R1(config-if)#ip ospf hello-interval (seconds) R1(config-if)#ip ospf dead-interval (seconds)
How to check timers on ospf neighbor? show ip ospf neighbor
What 4 things does DUAL provide EIGRP? 1:Loop-free Route 2:Loop-free backup routes 3:Fast convergence 4:Minimum bandwidth usage with bounded updates
In an SPF tree what is a leaf network? A network that does not connect to another router.
What are the 5 steps in route lookup process? 1:level 1 2:Level 2 3:Classful=drop classless=look for lesser match 4:If lesser level 1 found forward 5:If no lesser match drop
Created by: Datheral