Guide To Surviving The "Stump The Chump" Technical Screening Questions For Sr. Network Engineering Interviews

1. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
2)Late Collision in computer networking is a type of collision found in CSMA/CD based protocols.
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
[1]
transmitted by the transmitting station, a late collision is said to have occurred. Importantly, late
collisions are not re-sent by the NIC unlike collisions occurring before the first 64 octets; it is left for the
upper layers of the protocol stack to 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 a Cisco proprietary redundancy protocol for establishing a
fault-tolerant default gateway, and has been described in detail in RFC 2281.
The protocol establishes a framework between network routers in order to achieve default gateway
[1]
failover if the primary gateway becomes inaccessible, in close association with a rapid-converging
routing protocol like EIGRP or OSPF. By multicasting packets,
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2. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
4) HSRP sends its hello messages to the multicast address 224.0.0.2(all routers) for version 1, or
[2]
224.0.0.102for version 2 , using
UDP port 1985, to other HSRP-enabled routers, defining priority between the routers. The primary router
with the highest configured priority will act as a virtual router with a pre-defined gateway IP address and
will respond to the ARP request from machines connected to the LAN with the MAC
address0000.0c07.acXXwhere XX is the group ID in hex. 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-over
5) Priority and Preempt, (Load Balancing)
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3. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
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.
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4. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
8) Frame Relay is a standardized wide area network technology that specifies the physical and logical
link layers of digital telecommunications channels using a packet switching methodology. Originally
designed for transport across Integrated Services Digital Network (ISDN) infrastructure, it may be used
today in the context of many other network interfaces.
9) A data link connection identifier (DLCI) is a Frame Relay 10 bit wide link-local virtual circuit identifier
used to assign frames to a specific PVC or SVC. Frame Relay networks use DLCIs to statistically
[1]
multiplex frames. DLCIs are preloaded into each switch and act as road signs to the traveling frames.
The standard allows the existence of 1024 DLCIs, however only numbers from 16 to 991 are available for
[2]
end users' equipment. The rest are reserved for various management purposes.
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 offand apply flow control like traffic Shaping and informs the receiving device that the
flow is congestedand 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)
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5. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
BGP:
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6. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
BGP:
1. What is BGP?
a. Border Gateway Protocol
2. What is a BGP Autonomous System?
a. A defined routing domain containing some number of subnets also known as “prefixes
3. What port/ports do BGP use?
a. TCP port 179
4. 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 Summary
5. 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 establisha 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 session
d. The state to look for is established in shipbgp neigh.
i. in shipbgp sumyou 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. Origin
c. AS_Path
d. Next_Hop
e. Multi_Exit_Descriminator
f. Local_Preference (Discretionary)
g. Atomic_Aggregate (Discretionary)
7. What is AS Path prepending?
R6(config)# route-map PREPEND permit 10
R6(config-route-map)# set as-path prepend 67 67
R6(config-route-map)# router bgp 67
R6(config-router)# neighbor 172.31.68.8 route-map PREPEND out
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7. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
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
command neighbor x.x.x.x send-community must be applied.
BGP has default 4 well known communitiesthat 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.
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8. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
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-multihop2 {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 IGPor a Static route
b. Neighbor “IP Address” update-source Loopback”#”
11. Will this work?
a. No
12. Why?
a. Because of BGP Split Horizon…eBGP 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
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9. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
15. Are there any other ways to make this work?
1. Route Reflectors
2. 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.
EIGRP
1. What is the basic components that make up the metric of an EIGRP route
1. Bandwidth, Delay, Load, Reliability
2. 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 Table
4. What is the feasible distance
1. FD = [Cost between the local router and the next-hop router] + (AD)
2. I think FD is rather calculated as below which even the IOS output confirms
3. FD=256* (( 10^7/Lowest BW along path in Kbps) + ( Cumulative Delay in
microsec/10) )
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10. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
OSPF:
1. Can you name 3 differentLSA types and what they are used for?
2.
3. What us the different between a stubby and a not so stubby area?
1. Stub areasexist 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 STUBS
4. Hence the NSSA. An NSSAbehaves 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 ABRconverts 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.
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11. October 12, 2012 [STUMP THE CHUMP TECHNICAL SCREEN QUESTIONS]
7. The “no-summary” still exists, and is only needed on the ABR. So, in addition to the NSSA behaviour, you’re
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: 172.16.1.0/24)
what could be the cause that you are only seeing the summarized route 172.16.0.0/16
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 network
7. 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 BIGIP
1. 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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