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EIGRP & OSPF Questions

March 16th, 2020 in ENCOR 350-401 Go to comments

Quick OSPF Overview

OSPF router ID selection:

OSPF uses the following criteria to select the router ID:
1. Manual configuration of the router ID (via the “router-id x.x.x.x” command under OSPF router configuration mode).
2. Highest IP address on a loopback interface.
3. Highest IP address on a non-loopback and active (no shutdown) interface.

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
+ Area ID
+ Authentication type and password
+ Stub Area flag
+ Subnet ID and Subnet mask

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 Outdated or missing entries are also requested to be resent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:
+ Neighbors are discovered using multicast hello packets.
+ DR and BDR are elected for every multi-access network to optimize the adjacency building process. All the routers in that segment should be able to communicate directly with the DR and BDR for proper adjacency (in the case of a point-to-point network, DR and BDR are not necessary since there are only two routers in the segment, and hence the election does not take place).
For a successful neighbor discovery on a segment, the network must allow broadcasts or multicast packets to be sent.

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:

Area Restriction
Normal None
Stub No Type 5 AS-external LSA allowed
Totally Stub No Type 3, 4 or 5 LSAs allowed except the default summary route
NSSA No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse
NSSA Totally Stub No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed

OSPF Summarization
OSPF offers two methods of route summarization:
1) Summarization of internal routes performed on the ABRs
2) Summarization of external routes performed on the ASBRs

1) To summarize routes at the area boundary (ABRs), use the command:
area area-id range ip-address mask [advertise | not-advertise] [cost cost]

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:
summary-address {{ip-address mask} | {prefix mask}} [not-advertise] [tag tag]
The ASBR will summarize external routes before injecting them into the OSPF domain as type 5 external LSAs.

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.

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 will exist. If the last specific route will disappear, summary route also will fade out. The metric used by EIGRP manual summary route is the minimum metric of the specific routes.

Question 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.

Reference: https://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/ip-routing/whitepaper_C11-730404.html

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.

OSPF_LSAs_Types_4.jpg

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.

Comments
  1. mofumofu
    April 30th, 2020

    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

  2. Gre47
    May 19th, 2020

    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.

  3. Tosha
    May 29th, 2020

    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

  4. Tosha
    May 29th, 2020

    got it, by adjusting hello/dead timers. answer is A, as they are compatible, cause elect DR/BDR

  5. Lost again
    May 29th, 2020

    I can not see the Q’s I only see the explanation please help

  6. Chromatica
    May 30th, 2020

    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.

  7. jlai
    May 31st, 2020

    @Chromatica I agree answer should be A

  8. jlai
    June 2nd, 2020

    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. :(

  9. jlai
    June 2nd, 2020

    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

  10. rmar
    June 17th, 2020

    @Digitaltut please, could you update this answer in pdf ENCOR_Mar_2020.pdf?

    Thanks

  11. Zaa
    August 3rd, 2020

    Where are the questions??, only see explanations!

  12. schukowski
    September 19th, 2020

    @Q6 OTP isnt part of the official 350-401 ENCOR OCG , so CISCO is giving exam questions which are out of range?

  13. Ban7
    September 24th, 2020

    @digitaltut simulated the question 1 the only answer that works is B. point-to-multipoint to nonbroadcast, the others does not work.

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