ISIS Routing Levels

Answered Question
Feb 20th, 2010

I came across this question on a practice exam.

Which two statements about the isis routing protocol are true? (Choose two.)

A. - isis is capable of supporting ipv4 and ipv6
B. - isis is only capable of supporting ipv6 and clns
C. - isis routers use esis hellos to establish and to maintain neighbor relationships
D. - isis routers run the Bellman-Ford algorithm against LSDB for route selection
E. - level 1 routers learn about paths within the area of which they are a part
F. - level 2 routers learn about paths both within areas and between areas


I chose A and E and that's what the exam shows for an answer, but shouldn't F also be true?

I have this problem too.
0 votes
Correct Answer by Reza Sharifi about 6 years 9 months ago

88ssmontecarlo wrote:

Is the below assumption correct?

If all links in the diagram are L2 links, then RT3 would receive the inter-area routes.  If the link between RT1 and RT2 was changed to a Level 1, then RT3 would NOT receive the inter-area routes.


If this is true, then response F from above is only sometimes true which would then make sense to me as to why it wasn't a correct answer

Gary,

Your assumption is correct, if you change the link between RT1 and RT2 to level-1, since theses 2 routers are in 2 different areas you loose your ISIS adjacency and you will not receive any routes from RT1

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Edison Ortiz Sat, 02/20/2010 - 09:43

No, ABR routers in ISIS are called L1/L2 routers. L2 routers only participate within the L2 area.

Reza Sharifi Sat, 02/20/2010 - 11:36

Hi,

Yes, the only way to connect 2 ISIS areas is by using an L-2 link between them.  For example, you can have R-1 and R-2, R-1 can be in are 0002 and R-2 can be in area 0001, but the link between the 2 areas has to be L-2 link not L-1. Another word, an ISIS router that has L-1 and L-2 links is similar to OSPF ABR.

HTH

Reza

Edison Ortiz Sat, 02/20/2010 - 11:46

Not a L2 'only' router - it has to be a L1/L2 router to participate on both L1 and L2.

Gary Jeffers Sat, 02/20/2010 - 11:58

If we look at the attached diagram, will RT3 (Level 2 only) receive routes for area 49.0001?  If so, I don't understand how answer F isn't true.

Attachment: 
Edison Ortiz Sat, 02/20/2010 - 12:03

RT1 and RT2 are L1/L2 routers.

If you change your topo with RT1 and RT2 as L1 routers, would RT3 get the routes from L1 while configured as L2 router?

Gary Jeffers Sat, 02/20/2010 - 12:14

"RT1 and RT2 are L1/L2 routers."

Yes, RT1 and RT2 are L1/L2, but RT3 is only L2.

"If you change your topo with RT1 and RT2 as L1 routers, would RT3 get the routes from L1 while configured as L2 router?"

No, but as is wouldn't RT3 get routes for 49.0001?

Edison Ortiz Sat, 02/20/2010 - 12:42

That area (49.001) is on both L1 and L2 LSDB.

L1/L2 routers will advertise L1 routes into the backbone (L2) by default. However, without a L1/L2 router - a L2 router won't be able to receive those routes hence that's why answer F does not qualify. F would be correct if instead of L2, it had L1/L2.

You can make the same argument if RT3 was to be a L1 router with RT1 and RT2 being L1/L2, right?

L1/L2 routers will advertise routes from L2 into L1 with redistribution.

Reza Sharifi Sat, 02/20/2010 - 12:37

Gary,

These are L-2 links right?  RT1 has a link to RT2 and that is an L-2 link, because they are in 2 different areas. RT3 is an L-2 router only (according to your diagram). So yes, RT3 will receive routes advertised from area 49.0001.

If still not clear mark each link weather it is L-1, L-2 or both.

Gary Jeffers Sat, 02/20/2010 - 12:53

Is the below assumption correct?

If all links in the diagram are L2 links, then RT3 would receive the inter-area routes.  If the link between RT1 and RT2 was changed to a Level 1, then RT3 would NOT receive the inter-area routes.


If this is true, then response F from above is only sometimes true which would then make sense to me as to why it wasn't a correct answer

Correct Answer
Reza Sharifi Sat, 02/20/2010 - 13:34

88ssmontecarlo wrote:

Is the below assumption correct?

If all links in the diagram are L2 links, then RT3 would receive the inter-area routes.  If the link between RT1 and RT2 was changed to a Level 1, then RT3 would NOT receive the inter-area routes.


If this is true, then response F from above is only sometimes true which would then make sense to me as to why it wasn't a correct answer

Gary,

Your assumption is correct, if you change the link between RT1 and RT2 to level-1, since theses 2 routers are in 2 different areas you loose your ISIS adjacency and you will not receive any routes from RT1

Reza Sharifi Sat, 02/20/2010 - 12:04

Correct, but I am talking about the link itself. In order to establish an adjacency between 2 routers in 2 different areas, the link in between has to be a L-2 link.

Edison Ortiz Sat, 02/20/2010 - 12:05

Reza, I'm not referring to your post on my repy.

In order to establish an adjacency between 2 routers in 2 different areas, the link in between has to be a L-2 link.

We aren't discussing that.

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