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Stump The Chump technical screen questions

October 12, 2012[Stump The Chump technical screen questions]

2) Late Collisionincomputer networkingis a type of collision found inCSMA/CDbasedprotocols. Specifically a late collision is one that happens further into the packet than is allowed for by the protocol standard in question.In 10 megabit shared medium ethernet, if a collision error occurs after the first 512 bits of data are transmitted by the transmitting station,[1]a late collision is said to have occurred. Importantly, late collisions are not re-sent by theNICunlike collisions occurring before the first 64 octets; it is left for the upper layers of theprotocol stackto determine that there was loss of data.As a correctly set up CSMA/CD network link should not have late collisions, the usual possible causes are full-duplex/half-duplex mismatch, exceeded Ethernet cable length limits, or defective hardware such as incorrect cabling, non-compliant number of hubs in the network, or a bad NIC.3) Hot Standby Router Protocol(HSRP) is aCiscoproprietaryredundancy protocol for establishing a fault-tolerantdefault gateway, and has been described in detail inRFC 2281.The protocol establishes a framework between network routers in order to achieve default gateway failover if the primary gateway becomes inaccessible,[1]in close association with a rapid-converging routing protocol likeEIGRPorOSPF. By multicasting packets, 4) HSRP sends its hello messages to themulticast address224.0.0.2 (all routers) for version 1, or for version 2[2], usingUDPport 1985, to other HSRP-enabled routers, defining priority between the routers. The primary router with the highest configured priority will act as avirtualrouter with a pre-defined gateway IP address and will respond to the ARP request from machines connected to the LAN with the MAC address 0000.0c07.acXX where XX is the group ID inhex. If the primary router should fail, the router with the next-highest priority would take over the gateway IP address and answer ARP requests with the same mac address, thus achieving transparent default gateway fail-over5) Priority and Preempt, (Load Balancing)

6) Ping the desired IP address, then do a show ARP on the Default gateway of the device, This gives you the MAC Address, Then do a Show Mac-Address Address MAC ADDRESS FROM SHOW ARP To get the interface associated with that MAC.Or if this is a Cisco Device and you are looking for Cisco Device you could do a Show CDP Neighbor.7) Think of it like a highway - the only difference being is the car traveling the highway cannot "choose" which off-ramp to take, it has to be told.

Computer "A" on network "A" sends the messenger in his car down the freeway (ethernet). As he leaves the building (the computer) he travels down a side street (ether cable) and finally comes into contact with the main traffic. On a 24 port switch, see this as being a long "main" street with 24 exist or side-streets. As the car travels past each street (device plugged directly into the switch) the driver calls out "I'm going to such-and-such address!" and if the computer on that branch answers "Hey, that's me!" the car continues on the path.

Now, when there's another switch involved where the destination computer resides, the car travels past the street where the other switch is connected. The driver screams out the intended address, but this time, the network "B" switch says "I'm not him, but he's down this way!"

Once the driver heads onto the other "main street" the same process begins again.

Sorry if the analogy sounds "childish" but it tends to get the correct idea across to packet forwarding, TCP itself and how Ethernet functions.8) Frame Relayis a standardizedwide area networktechnology that specifies the physical and logical link layers of digital telecommunications channels using apacket switchingmethodology. Originally designed for transport acrossIntegrated Services Digital Network(ISDN) infrastructure, it may be used today in the context of many other network interfaces.9) Adata link connection identifier(DLCI) is aFrame Relay10 bit wide link-local virtual circuit identifier used to assign frames to a specificPVCorSVC. Frame Relay networks use DLCIs tostatistically multiplexframes. DLCIs are preloaded into each switch and act as road signs to the traveling frames.[1]The standard allows the existence of 1024 DLCIs, however only numbers from 16 to 991 are available for end users' equipment. The rest are reserved for various management purposes.[2]

10) BECN: BECN (Backward Error Congestion Notification)If device A is sending data to device B across a Frame Relay infrastructure and one of the intermediate Frame Relay switches encounters congestion, congestion being full buffers, over subscribed port, overloaded resources, etc, it will set the BECN bit on packets being returned to the sending device and the FECN bit on the packets being sent to the receiving device. This has the effect of telling the sending router to Back off and apply flow control like traffic Shaping and informs the receiving device that the flow is congested and that it should inform upper layer protocols, if possible, that it should close down windowing etc to inform the sending application to slow down.A FECN tells the receiving device that the path is congested so that the upper layer protocols should expect some delay. The BECN tells the transmitting device that the Frame Relay network is congested and that it should "back off" to allow better throughput.FECN (Forward Error Congestion Notification)


BGP:1. What is BGP?a. Border Gateway Protocol2. What is a BGP Autonomous System?a. A defined routing domain containing some number of subnets also known as prefixes3. What port/ports do BGP use?a. TCP port 1794. What command would you use to look at a brief summary of your BGP peers and their status?a. Show IP BGP Neighbors b. Show IP BGP Summary5. If you see one of the Peers is in Active mode what does that mean? Is it good or Bad?a. Active means the router is trying to establish a BGP session, nothing more.b. It does NOT mean TCP connectivity to the neighbor is OK at the moment.c. I would say that this is not good sign:i. the BGP session is configured on your side and the BGP neighbor state machine for this neighbor has been started.ii. but the configuration may be missing on the other side for example or there is a problem of TTL if it is an eBGP sessiond. The state to look for is established in sh ip bgp neigh.i. in sh ip bgp sum you should see the number of rx prefixes (including 0 special case)6. What are some of the well known BGP Attributes?a. Well-known attributes must be recognized by all BGP implementations. Some of these attributes are mandatory and must be included in every UPDATE message.b. Originc. AS_Pathd. Next_Hope. Multi_Exit_Descriminatorf. Local_Preference (Discretionary)g. Atomic_Aggregate (Discretionary)7. What is AS Path prepending?R6(config)# route-map PREPEND permit 10R6(config-route-map)# set as-path prepend 67 67R6(config-route-map)# router bgp 67R6(config-router)# neighbor route-map PREPEND out

8. What are BGP Communities?The BGP community attribute is a numerical value that can be assigned to a specific prefix and advertised to other neighbors. When the neighbor receives the prefix it will examine the community value and take proper action whether it is filtering or modifying other attributes.By default the community attribute is removed from the update before being sent to the neighbor. To allow community values to be sent to a specific neighbor the commandneighbor x.x.x.x send-communitymust be applied.BGP has default 4 well known communities that can be used to mark prefixes; listed as follows: Internet: advertise these routes to all neighbors. Local-as: prevent sending routes outside the local As within the confederation. No-Advertise: do not advertise this route to any peer, internal or external. No-Export: do not advertise this route to external BGP peers.

How to set community attribute values? Network command: the optional route-map at the end of the network command can be used to set the community of the advertised prefix. Aggregate address: the Attribute-map option can be used to set the attributes of the aggregate route including the community value. Neighbor command: a route-map can be used to set the community of specific prefixes received from or sent to the neighbor. Redistribution: you can set the community attribute of the redistributed prefixes using the route-map option of the redistribute command.

9. If you are peering between two different Autonomous systems and using loopback addresses, are there any special neighbor commands that need to be applied for the BGP peers to come up?a. >>>>Answer:b. Neighbor IP Address ebgp-multihop 2 {IF External AS Peer}c. Neighbor IP Address update-source Loopback#10. Are there any specific commands needed if you are peering using loopback addresses between two devices in the same AS?a. Must have connectivity Either IGP or a Static routeb. Neighbor IP Address update-source Loopback#11. Will this work?a. No12. Why?a. Because of BGP Split HorizoneBGP route updates are not past into the iBGP domain because the route source is not changed.13. What needs to be AS 65666 for this to work?1. Neighbor IP Address next-hop-self

15. Are there any other ways to make this work?1. Route Reflectors2. Confederation

16. What is BGP split horizon?1. BGP doesn't really have a split horizon rule. iBGP refuses to talk to anybody except it's friends unless you specifically tell it to via route reflector or confederation. EBGP doesn't care, it'll take any route from anyone unless specifically configured to ignore them, and then just decides what is the best path and drops that into the routing table. If the best path goes away, if it has other routes, it'll insert the next best path into the routing table.

17. Explain BGP synchronization, why would we turn that off?1. BGP requires that IF a prefix is learned from a 1st AS and will transit your AS destined for a 3rd AS THEN the route will not be advertised until all routers within the AS have learned about the route via an IGP.EIGRP1. What is the basic components that make up the metric of an EIGRP route1. Bandwidth, Delay, Load, Reliability2. What is feasible successor?1. It is the second best route to a destination that meets the feasibility condition as calculated by the DUAL / Diffusing Update Algorithm 3. Where can you find a list of feasible successors?1. They are stored in the Topology Table4. What is the feasible distance1. FD = [Cost between the local routerand the next-hop router] + (AD)2. I think FD is rather calculated as below which even theIOS output confirms3. FD=256* (( 10^7/Lowest BW along path in Kbps) + ( Cumulative Delay in microsec/10) )

OSPF:1. Can you name 3 different LSA types and what they are used for?

2. 3. What us the different between a stubby and a not so stubby area?1. Stub areas exist to reduce the number of LSAs that are processed within an area. This is done by sacrificing some information. Practically, this means we filter out various routes that are coming into the area and replace it with a default route.2. Remember that as a link state protocol, all routers within the area have to have the same information, so most of the work is done on the ABR.3. A stubby area ignores external routes (O E1, O E2), and permits both inter and intra area routes (O IA and O). The path to the external routes is replaced by a default route injected by the ABR.4. So an ABR operating in stubby mode filters out type 5 LSAs.5. ASBRs and external routes are not allowed in stubs4. Hence the NSSA. An NSSA behaves the same as a stubby area except that ASBRs are allowed, with some trickery happening to get around the rules.5. In an NSSA, external routes are allowed if they originate inside. The ABRs still filter out the type 5 LSAs at the border. Any ASBR within an NSSA advertises external routes as type 7 routes instead of type 5. Thus, they show up in the routing table as O N1 instead of O E1. The ABR converts the type 7 LSA into a type 5 LSA before it advertises the LSA to the backbone.6. Because of the restriction on the type 5 LSAs, and the need to understand type 7 LSAs, all routers in the area need to be configured as an NSSA.7. The no-summary still exists, and is only needed on the ABR. So, in addition to the NSSA behaviour, youre still allowed to filter out those interarea type 3 LSAs at the border.1. NSSAs are goofy in that you need to originate a default route explicitly in the nssa command.8. Within OSPF why do you need area 0?1. Area 0 is needed to prevent routing loops. 9. If you are redistributing another routing protocol in OSPF (for example: what could be the cause that you are only seeing the summarized route

5. What is the difference between external type 1 and external type 2 OSPF routes.6. What would you do to speed up convergence in an OSPF network7. If all configurations are standard and you have a gig and fastE link to a destination network. Which link would the OSPF process prefer.8. Why?9. How can you influence the decision?F5 BIGIP1. Questions are based on V4.2. What is a VIP?3. What is persistence?4. What command would you use to display the current status of a VIP?5. What command would you use display the persistence settings on a particular VIP configurations?

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