EIGRP & OSPF Questions
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Quick OSPF Overview OSPF router ID selection: OSPF uses the following criteria to select the router ID: OSPF forms neighbor relationship with other OSPF routers on the same segment by exchanging hello packets. The hello packets contain various parameters. Some of them should match between neighboring routers. These include: + Hello and Dead intervals When OSPF neighbor relationship is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below: Down: no information (hellos) has been received from this neighbor Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval Init: specifies that the router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet 2-Way: indicates bi-directional communication has been established between two routers Exstart: Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR Exchange: OSPF routers exchange and compare database descriptor (DBD) packets Loading: In this state, the actual exchange of link state information occurs. Outdated or missing entries are also requested to be resent Full: routers are fully adjacent with each other When OSPF is run on a network, two important events happen before routing information is exchanged: In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically. OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered. Neighbors must be configured manually to overcome these problems Each OSPF area only allows some specific LSAs to pass through. Below is a summarization of which LSAs are allowed in each OSPF area:
Or this table will help you grasp it:
OSPF Summarization 1) To summarize routes at the area boundary (ABRs), use the command: An internal summary route is generated if at least one subnet within the area falls in the summary address range and the summarized route metric is equal to the lowest cost of all the subnets within the summary address range. Interarea summarization can only be done for the intra-area routes of connected areas, and the ABR creates a route to Null0 to avoid loops in the absence of more specific routes. 2) To summarize external routes on the domain boundary (ASBRs), use the command: Note: An exception of using the “summary-address” is at the boundary of a NSSA area. In both methods of route summarization described above, a summarized route is only generated if at least one subnet in the routing table falls in the summary address range. OSPF point-to-point network type Setting OSPF to point-to-point mode results in advertised routes containing the actual subnet mask instead of the default behavior of advertising /32 for a loopback interface. Summarization in EIGRP and OSPF Unlike OSPF where we can summarize only on ABR or ASBR, in EIGRP we can summarize anywhere. Manual summarization can be applied anywhere in EIGRP domain, on every router, on every interface via the ip summary-address eigrp as-number address mask [administrative-distance ] command (for example: ip summary-address eigrp 1 192.168.16.0 255.255.248.0). Summary route will exist in routing table as long as at least one more specific route exists. If the last specific route disappears, summary route will also fade out. The metric used by EIGRP manual summary route is the minimum metric of the specific routes. The example below shows how to configure EIGRP manual summarization:
If you are not sure about OSPF LSA Types, please read our OSPF LSA Types Tutorial. OSPF area filtering The command “area area-number filter-list prefix … in“: Prevent prefixes from entering this area (in keyword here means “into”) |
R1(config)#interface fa1/0Question 1
Explanation
The following different OSPF types are compatible with each other:
+ Broadcast and Non-Broadcast (adjust hello/dead timers)
+ Point-to-Point and Point-to-Multipoint (adjust hello/dead timers)
Broadcast and Non-Broadcast networks elect DR/BDR so they are compatible. Point-to-point/multipoint do not elect DR/BDR so they are compatible.
Question 2
Explanation
On Ethernet interfaces the OSPF hello intervl is 10 second by default so in this case there would be a Hello interval mismatch -> the OSPF adjacency would not be established.
Question 3
Explanation
This combination of commands is known as “Conditional debug” and will filter the debug output based on your conditions. Each condition added, will behave like an ‘And’ operator in Boolean logic. Some examples of the “debug ip ospf hello” are shown below:
| *Oct 12 14:03:32.595: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet1/0 from 192.168.12.2 *Oct 12 14:03:33.227: OSPF: Rcv hello from 1.1.1.1 area 0 on FastEthernet1/0 from 192.168.12.1 *Oct 12 14:03:33.227: OSPF: Mismatched hello parameters from 192.168.12.1 |
Question 4
Explanation
If we configured an EIGRP stub router so that it only advertises connected and summary routes. But we also want to have an exception to this rule then we can configure a leak-map. For example:
| R4(config-if)#router eigrp 1 R4(config-router)#eigrp stub R4(config)#ip access-list standard R4_L0opback0 R4(config-std-nacl)#permit host 4.4.4.4 R4(config)#route-map R4_L0opback0_LEAKMAP R4(config-route-map)#match ip address R4_L0opback0 R4(config)#router eigrp 1 R4(config-router)#eigrp stub leak-map R4_L0opback0_LEAKMAP |
As we can see the leak-map feature goes long with ‘eigrp stub’ command.
Question 5
Explanation
EIGRP provides a mechanism to load balance over unequal cost paths (or called unequal cost load balancing) through the “variance” command. In other words, EIGRP will install all paths with metric < variance * best_metric into the local routing table, provided that it meets the feasibility condition to prevent routing loop. The path that meets this requirement is called a feasible successor. If a path is not a feasible successor, it is not used in load balancing.
Note: The feasibility condition states that, the Advertised Distance (AD) of a route must be lower than the feasible distance of the current successor route.
Question 6
Explanation
OTP leverages existing LISP encapsulation which:
+ Allows dynamic multi-point tunneling (-> Answer A is correct)
+ Provides instance ID field to optionally support virtualization across WAN (see EVN WAN Extension section)
OTP does NOT use LISP control plane (map server/resolver, etc.) (-> Therefore answer B is not correct) instead it uses EIGRP to exchange routes and provide the next-hop (-> answer C and answer D are not correct), which LISP encapsulation uses to reach remote prefixes.
Question 7
Explanation
When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. The states are Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full. Short descriptions about these states are listed below:
Down: no information (hellos) has been received from this neighbor.
Attempt: only valid for manually configured neighbors in an NBMA environment. In Attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval.
Init: specifies that the router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet
2-Way: indicates bi-directional communication has been established between two routers.
Exstart: Once the DR and BDR are elected, the actual process of exchanging link state information can start between the routers and their DR and BDR.
Exchange: OSPF routers exchange database descriptor (DBD) packets
Loading: In this state, the actual exchange of link state information occurs
Full: routers are fully adjacent with each other
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f0e.shtml)
Neighbors Stuck in Exstart/Exchange State
The problem occurs most frequently when attempting to run OSPF between a Cisco router and another vendor’s router. The problem occurs when the maximum transmission unit (MTU) settings for neighboring router interfaces don’t match. If the router with the higher MTU sends a packet larger that the MTU set on the neighboring router, the neighboring router ignores the packet.
Question 8
Explanation
EIGRP support unequal-cost load balancing via the “variance …” while OSPF only supports equal-cost load balancing.
Question 9
Explanation
The Broadcast network type is the default for an OSPF enabled ethernet interface (while Point-to-Point is the default OSPF network type for Serial interface with HDLC and PPP encapsulation).
Reference: https://www.oreilly.com/library/view/cisco-ios-cookbook/0596527225/ch08s15.html
Question 10
Explanation
Summary ASBR LSA (Type 4) – Generated by the ABR to describe an ASBR to routers in other areas so that routers in other areas know how to get to external routes through that ASBR. For example, suppose R8 is redistributing external route (EIGRP, RIP…) to R3. This makes R3 an Autonomous System Boundary Router (ASBR). When R2 (which is an ABR) receives this LSA Type 1 update, R2 will create LSA Type 4 and flood into Area 0 to inform them how to reach R3. When R5 receives this LSA it also floods into Area 2.
In the above example, the only ASBR belongs to area 1 so the two ABRs (R2 & R5) send LSA Type 4 to area 0 & area 2 (not vice versa). This is an indication of the existence of the ASBR in area 1.
Note:
+ Type 4 LSAs contain the router ID of the ASBR.
+ There are no LSA Type 4 injected into Area 1 because every router inside area 1 knows how to reach R3. R3 only uses LSA Type 1 to inform R2 about R8 and inform R2 that R3 is an ASBR.
Question 5
Why is Answer A the correct answer?
In the end, the only routes that can be used are the Successors and Feasible Successors, so I guess it will be “EIGRP supports 2 unequal-cost paths”.
It seems to me that Answer E is correct.
https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html#feasibleandreported
Mofumofu
Here says:
EIGRP Load Balancing
Every routing protocol supports equal cost path load balancing. In addition, Interior Gateway Routing Protocol (IGRP) and EIGRP also support unequal cost path load balancing. Use the variance n command in order to instruct the router to include routes with a metric of less than n times the minimum metric route for that destination. The variable n can take a value between 1 and 128. The default is 1, which means equal cost load balancing. Traffic is also distributed among the links with unequal costs, proportionately, with respect to the metric.
Note: If a path is not a feasible successor, the path is not used in load balancing.
So A and B are indeed the correct answers.
Question 1
Which OSPF networks types are compatible and allow communication through the two peering devices?
A. broadcast to nonbroadcast
B. point-to-multipoint to nonbroadcast
C. broadcast to point-to-point
D. point-to-multipoint to broadcast
Broadcast
Hello: 10 Wait: 40 Dead: 40
Non-broadcast
Hello: 30 Wait: 120 Dead: 120
Point-to-point
Hello: 10 Wait: 40 Dead: 40
Point-to- multipoint
Hello: 30 Wait: 120 Dead: 120
Two options suit, B, C
If im not correct, please let me know
got it, by adjusting hello/dead timers. answer is A, as they are compatible, cause elect DR/BDR
I can not see the Q’s I only see the explanation please help
The correct answer of question 6 is: A. OTP uses LISP encapsulation for dynamic multipoint tunneling. EIGRP OTP uses EIGRP for control plane and LISP for data plane exclusively. As LISP is used for the traffic in the WAN, indeed: the IP-in-IP/UDP LISP encapsulation makes dynamic multipoint between the different RLOCs between all the CEs-PEs links connected to the WAN in order to transport data between different points across the ISP. It is true that next-hop addresses and neighborships are on EIGRP control plane functions, but it is false thar OTP maintains LISP control plane.
@Chromatica I agree answer should be A
I hope they will update the answers on the site and on the composite quizzes as well. its getting confusing now whenever i practice on the quiz. if thats the case then my money is not worth it. :(
This was already updated by digitaltut correct answer is A.
Question 6
Which statement about LISP encapsulation in an EIGRP OTP implementation is true?
A. OTP uses LISP encapsulation for dynamic multipoint tunneling
B. OTP maintains the LISP control plane
C. OTP uses LISP encapsulation to obtain routes from neighbors
D. LISP learns the next hop
Answer: A
@Digitaltut please, could you update this answer in pdf ENCOR_Mar_2020.pdf?
Thanks
Where are the questions??, only see explanations!
@Q6 OTP isnt part of the official 350-401 ENCOR OCG , so CISCO is giving exam questions which are out of range?
@digitaltut simulated the question 1 the only answer that works is B. point-to-multipoint to nonbroadcast, the others does not work.
@schukowski: Part V, Chapter 16, Page 464ff: Cisco Location/ID Separation Protocol (LISP) in ENCOR 350-401 Official Cert Guide; Edgeworth, …
Question 2 and 6 about Leak Maps and OTP EIGRP are no where to be found in the Official CERT for 350-401. There are legacy books. Were these topic really found in ENCOR?
@Confused: They are from the current ENCOR exam. Legacy exam does not have LISP.
Q5
Correct answers BE
Because -> Note: Using max-paths, you can configure EIGRP to use up to six routes of equal cost.
Answer A isn’t correct.
Q5
Answers A not correct.
Note: Using max-paths, you can configure EIGRP to use up to six routes of equal cost.
Correct answers BE
Q5 = A, B & E are the correct answers.
Equal cost= maximum 4 paths
Unequal cost – maximum 6 paths.
CEF does loadbalancing per packet or per destination regardless of the interfaces.
I would choose B & E If I have to :-)
Question 6 is the reason I hate Cisco exam system…
Cisco Express Forwarding (CEF) is an advanced Layer 3 switching technology which can be used for load balancing in routers. By default, CEF uses per-destination load balancing. If it is enabled on an interface, per-destination load balancing forwards packets based on the path to reach the destination. If two or more parallel paths exist for a destination, CEF takes the same path (single path) and avoids the parallel paths. This is a result of the default behavior of CEF. CEF takes the single path in cases when load sharing is done simultaneously on interfaces of different physical types, such as serial and tunnel. The hash algorithm determines the path to be chosen. In order to utilize all the parallel paths in CEF and load balance the traffic, you must enable per-packet load balancing when you have different physical interfaces like serial and tunnel. So, on the basis of the configuration and topology (serial or tunnel), load sharing can fail to work correctly with the default CEF load balancing mode.
looks like for Q5 there are 3 correct answers – A B E
Pass4 surexams.com exam dumps helped me achieve a score of 92%.
In Q5 A is not correct unless there is a typo
EIGRP supports 16 ( Not 6 ) unequal-cost paths
Sh Sh
https://www.ciscopress.com/articles/article.asp?p=2101519&seqNum=2#:~:text=EIGRP%20supports%20up%20to%20six%20unequal%2Dcost%20paths.
Explanation Q7 states: Init: specifies that the router has received a hello packet from its neighbor, but the receiving router’s ID was not included in the hello packet.
The receiving router’s ID must be the ‘sending’ router.
Question 5 :
Equal cost maximum is 16
The maximum number of routes over which EIGRP can load balance is set with the
maximum-paths paths command. paths may be any number from 1 to 16 in IOS 12.3(2)T
and later 12.3(T) releases and any number from 1 to 6 in earlier versions. The default for
all versions is 4.