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CCNA - Routing Final
| Question | Answer |
|---|---|
| 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
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