OSPF for Interconnecting Cisco Networking Devices Part 1 (100-105) CCENT CCNA

[weasleman]

1) link speed thats is calculated in the LSDB gatehred from LSA's
4) Will need to look this up.
5) If you had a ip address duplicate on a loopback interface i am gathering that it would go to the next physical highest IP address. If you have two IP addresses the same on a physical interface you are going to have problems with routing.
6) Multi-access networks are the bounderies of the OSPF instance this includes all areas and can only have 1 Designated Router and 1 backup router at a time. If an organisations has more than one DR it would suggest that they have multipe multi-access networks.
7) Yes it was designed to see if i understood the process of router ID generation. ?
It allows you to manual setup your router IDS in a way they allows you to have control of the Router ID. ?

But i have a question am i right in saying that the router ID would not change until the OSPF instance is restarted ?

[deanwebb]

5 would be a good thing to lab up so that you could be sure.

Spot on with 8. The loopbacks allow for organizational structure to be created, independent of the IPs assigned by an ISP for an interface.

I think you are correct in your assumption. Best to test it in a quick lab, so you'll be sure.

[weasleman]

Well brought this up in a lab.

Added the following

Interface loopback1
Ip address 1.1.1.1 255.0.0.0
ip ospf 1 area 0

this was added to both routers then restarted the router ID moved from physical ID to loopback ID. But they both now have the same router ID.

Now is this what i should have expected and would this not cause problems.

Output from Show ip ospf 1

[deanwebb]

Well brought this up in a lab.

Added the following

Interface loopback1
Ip address 1.1.1.1 255.0.0.0
ip ospf 1 area 0

this was added to both routers then restarted the router ID moved from physical ID to loopback ID. But they both now have the same router ID.

Now is this what i should have expected and would this not cause problems.

Output from Show ip ospf 1

I do believe that, yes, having two equally authoritative routers in an OSPF area is something that would cause problems. Cisco seems to agree on that account: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/23862-duplicate-router-id-ospf.html

Reading is one thing, you nod your head and move on. But labbing it... you begin to remember those lessons in your bones.

This is what you will look like after many hours of successful labbing:

[weasleman]

I totally agree my only frustration is that there is just so much to learn and know. Thanks for the time and i am enjoying this. its helping me get my head round things like you said reading and doing are totally different.

Here are my other answers.

9. RouterA has OSPF configured correctly and a default route that directs traffic to RouterB. RouterB has a default route that directs traffic to RouterA. What happens if RouterA sends a packet to RouterB that RouterB does not have a specific route defined for?

Destination unreachable ?

10. If you wanted all interfaces on a router to be in the same OSPF area, what command in config mode would you use?

interface range fa0/x-x ip ospf area x

11. In OSPF, what is the maximum number of parallel routes it can support? What is the default number of parallel routes? What command can be used to change the default setting?

The maxium number of parrellel routes is four default is 1. Command maximum-path x

12. An admin wants his OSPF process ID to be 66666 because he likes sixes. What specific limitation of OSPF will just break his heart?

the range value is from 1 to 65535

13. What is the largest number that can be assigned to an OSPF area?

4294967295

14. What is special about OSPF Area 0?

Its the backbone area

15. If OSPF Area 57883 is to share information with OSPF Area 9871, what must be true in the OSPF topology for that to succeed?

they must have some type of bridge. Just looked it up inter area routing.


16. If you enter the command "network 10.0.0.128 0.0.0.63 area 0", will an interface with an IP of 10.0.0.130 participate in OSPF? What about an interface with an IP of 10.0.0.126?

No it will not yes 10.0.0.126 will


17. What are OSPF LSAs? What are the different types of LSAs? If a network is set up to have just one OSPF area, what kind of LSAs will not be used?

Link state advertisements

    LSA Type 1: Router LSA.
    LSA Type 2: Network LSA.
    LSA Type 3: Summary LSA.
    LSA Type 4: Summary ASBR LSA.
    LSA Type 5: Autonomous system external LSA.
    LSA Type 6: Multicast OSPF LSA.
    LSA Type 7: Not-so-stubby area LSA.
    LSA Type 8: External attribute LSA for BGP.

Will we be expected to know all of these for the CCNA

18. How can routers be connected to the OSPF backbone area 0 through a non-backbone area?

Inter vlan routing. This is a guess and takes a cue from joining two autonomous systems in question 15

19. What abbreviation describes a router that joins an OSPF area to Area 0?

stub network

20. What is the benefit of having multiple OSPF areas in a large network? (three main benefits)

Breaks up the network into bits
Allows managebility

21. What is the default setting for the hello timer for OSPF on a Cisco router? What command can be used to change that value to 60 seconds? What two different commands will return the router to its default state, once you realize that having one set for 60 seconds means it won't work well with the other routers still on default settings?

Default setting 40
IP ospf hello-timer x
no ip ospf hello-interval x
clear ip ospf hello interval (guess)

22. In a network, the core router has a loopback address of 10.254.254.254, and all other routers have loopback addresses configured in the 10.0.0.0 /24 range. What happens as far as DRs are concerned if someone in a branch office attaches a router with a loopback of 10.200.200.200? What happens if, after that, someone attaches a router with a loopback of 10.255.255.254?

The DR will still be the same as it looks at the highest IP which is 10.254.254.254 the new router added will become the new BDR when OSPF is restarted. If the next router is added this will become the DR after ospf instance has been restarted as the IP is greater than the DR at present.

23. In OSPF, what happens to the routing table when a router stops getting hello packets from a neighbor?

The route will be removed and if more than one route this route will become the successor

24. In OSPF, what happens to the routing table when a router receives an LSA?

Nothing happens unless there is a change. If there is a change the database table will recalculate and populate the routing table.

25. What information can be used by a router running a link-state protocol to build and maintain its topological database? (Choose two.)

Link speed
and distance

26. An admin is examining a trace of network traffic and sees packets bound for 224.0.0.5. What OSPF packet corresponds with that destination?

Hello packet

27. Which command is used to show all the OSPF link states?

Show ip OSPF x

28. An admin wants to add 172.16.100.128 /25 to OSPF area 0.
Assuming he is starting at a > prompt, what are the commands that need to be entered to accomplish this task?

enable
config t
router ospf x
network 172.168.100.128 0.0.0.255 area 0

29. Below is what someone typed in for an OSPF configuration. Without looking at any other part of the device config, what potential problem can you spot in these lines of code?

Router(config)# router ospf 1

Router(config-router)# network 192.168.1.0 255.255.255.0 area 0

I would not have espected a subnet mask 0.0.0.255 area 0.0.0.255

[deanwebb]

9 - it's much worse than that...  Much worse... Think about it: Router B gets a packet for a particular network and its default route points back to A...

16. Check your numbers again - are you sure on that one?

17. Know the ones that are used and then all the others are lumped in the "not used" category... you'll need to know them all for CCNP.

21. Don't guess on the second one. Best to look up the first line and see if there's an article that mentions a second, equivalent command as an alternative.

Good job on 22!

Also good job on 28! Very important to remember ALL the commands to use! Getting the proper order and usage on the simulations will be important.

[weasleman]

9) Would this not cause a routing loop ? Ahhh now i have read the question

Router A is running OSPF Router B maybe or maybe not. Now the default AD for a default route is 1 which even if OSPF has on router B would take precedent over OSPF unless the route has been specified with a greater AD than OSPF. So the packet will ping from router A to Router B


16) no the 0.0.0.63 would not cover the address ranges. Apologies IPV4 still pickles my brain.

17) Will be looking at CCNP after CCNA. But it is useful to know.

21. What is the default setting for the hello timer for OSPF on a Cisco router? What command can be used to change that value to 60 seconds? What two different commands will return the router to its default state, once you realize that having one set for 60 seconds means it won't work well with the other routers still on default settings?

default setting is 40
ip ospf hello-interval 60
No ip ospf hello-interval x
The other command i cannot find. You could delete all config on the router this would change everything to default though 

this stuff is starting to stick in my brain. THanks for the questions its helping I will wait for the EIGRP one next  hint hint.

[Otanx]

I will throw in some questions because I had to deal with this today.

9 - Does the packet ever stop going back and forth between router A and B? Why or why not? What happens if instead of 1 packet I send 1,000 packets, or 10,000 packets?

-Otanx

[weasleman]

Hi Otanx,

I would expect that in time the packet would expire.....Yes just looked this up it counts down its ttl. As for sending more packets i would expect that bandwidth on the lines to shrink to a point where its the equivalent of a broadcast storm in effect. unresponsive network.

[deanwebb]

this stuff is starting to stick in my brain. THanks for the questions its helping I will wait for the EIGRP one next  hint hint.

I've got 16 years of experience as a teacher, you better believe I can ask some good questions!

[weasleman]

I have a lot of information floating around in my brain. this Forum is now joining the dots.  Well some of it.

Still stuck on

IP OSPF hello-interval

Ebveryone seems to use no ip OSPF hello-interval x

[icecream-guy]

17. What are OSPF LSAs? What are the different types of LSAs? If a network is set up to have just one OSPF area, what kind of LSAs will not be used?

Link state advertisements

    LSA Type 1: Router LSA.
    LSA Type 2: Network LSA.
    LSA Type 3: Summary LSA.
    LSA Type 4: Summary ASBR LSA.
    LSA Type 5: Autonomous system external LSA.
    LSA Type 6: Multicast OSPF LSA.
    LSA Type 7: Not-so-stubby area LSA.
    LSA Type 8: External attribute LSA for BGP.

Will we be expected to know all of these for the CCNA

for IPv4 there are only 7 LSA type,  I don't recall the External attribute LSA for BGP]

for IPv6 there are 2 additional LSA types

LSA Type 8 – Link LSA

Link-LSAs have link-local flooding scope.  A router originates a separate link-LSA for each attached link that supports two or more (including the originating router itself) routers.  Link-LSAs have three purposes:

1.  They provide the router's link-local address to all other routers attached to the link.
2.  They inform other routers attached to the link of a list of IPv6 prefixes to associate with the link.
3.  They allow the router to advertise a collection of Options bits in the network-LSA originated by the Designated Router on a broadcast or NBMA link.

LSA Type 9 – Intra-Area Prefix LSA

An intra-area-prefix-LSA has one of two functions:
1.  It either associates a list of IPv6 address prefixes with a transit network link by referencing a network-LSA...
2.  Or associates a list of IPv6 address prefixes with a router by referencing a router-LSA.

[weasleman]

I am just getting to IPV6. IPV4 still causing me a headache at times but i will get there.