Discuss Ericsson ECP-206 Exam Topic 4 Question 25

Which mismatched field would cause an IS-IS adjacency between two routers to fail?

AMTU

BSEL

CContext

DSysID

Suggested Answer: A

MTU mismatch would cause an IS-IS adjacency between two routers to fail. MTU stands for maximum transmission unit, which is the maximum size of a packet that can be transmitted on a link. IS-IS hellos are padded to the full MTU size, partly to allow a router to implicitly communicate its MTU to its neighbors.If two routers have different MTUs on their interfaces, they will not be able to exchange hellos properly, and their adjacency will not form or will be dropped34. To solve this problem, the MTUs on both interfaces must be configured to match.

SEL, context, and SysID are not fields that would cause an IS-IS adjacency failure. SEL stands for selector, which is a one-octet field that identifies the network layer service to which a packet should be sent.For IS-IS routers, the SEL must always be 005.Context is not a field in IS-IS packets, but rather a term used to describe the scope of routing information in different levels or areas6. SysID stands for system identifier, which is a six-octet field that uniquely identifies a router within an area.SysID does not have to match between two routers for them to form an adjacency7.

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35 comments · newest first

Gearldine 10 months ago

100% agree, MTU is the usual suspect here.

Eveline 11 months ago

Wait, really? I didn't know MTU was that critical.

Suzi 11 months ago

SEL mismatch can also mess things up!

Elly 11 months ago

I thought it was SysID that caused problems?

Deane 11 months ago

Definitely MTU, it's a common issue.

Shanice 12 months ago

Context seems less likely to affect adjacency, but I could be wrong. I need to double-check my notes on that.

Leonora 12 months ago

I practiced a similar question where SysID was mentioned, and I feel like that one is crucial for establishing adjacency.

Mary 12 months ago

I think SEL might be important too, but I can't recall if it's a critical factor for adjacency.

Remedios 1 year ago

I remember studying that MTU mismatches can definitely cause issues, but I'm not sure about the others.

Karan 1 year ago

The context field seems like an interesting option, but I'm not as familiar with how that impacts IS-IS adjacency. I'll have to research that a bit more.

Nancey 1 year ago

I'm pretty confident this has to do with the System ID (SysID) field. That's a key parameter for establishing IS-IS adjacency, right?

Krystal 1 year ago

Hmm, this seems like a tricky one. I'll need to think through the different IS-IS adjacency requirements carefully.

Tenesha 1 year ago

I'm a bit unsure on this one. Could it also be something like the MTU or SEL field? I'll have to review my notes to be sure.

Dana 1 year ago

This seems like a classic high availability scenario. I'd go with option D and use an Auto Scaling group across two AZs, setting the minimum capacity to 4 to ensure the 60% availability is maintained. That way, the system can automatically scale up and down as needed.

Markus 1 year ago

Okay, I've got this. The slope-policy and the WRED-policy are the two that will allow dynamic queue rate allocation when combined.

Stephen 2 years ago

I think SysID is also important for identifying the router, so that could be a potential mismatch causing the adjacency to fail.

Benedict 2 years ago

MTU all the way! I once had a colleague who thought changing the MTU would make the network 'go faster'. Needless to say, we had to roll back that change pretty quickly.

Mose 2 years ago

↳ replying to Benedict

B) SEL

Emogene 2 years ago

↳ replying to Benedict

Oh no, changing the MTU can definitely cause issues with IS-IS adjacencies.

Casie 2 years ago

↳ replying to Benedict

A) MTU

Laurene 2 years ago

Gotta be the Context field. I mean, who even uses that anyway? Sounds like something straight out of a networking trivia game.

Flo 2 years ago

↳ replying to Laurene

Definitely, it's not a common field to come across in networking.

Nana 2 years ago

↳ replying to Laurene

Yeah, I agree. Context seems like the odd one out here.

Wendell 2 years ago

↳ replying to Laurene

D) SysID

Ernie 2 years ago

↳ replying to Laurene

C) Context

Blair 2 years ago

↳ replying to Laurene

B) SEL

Annmarie 2 years ago

↳ replying to Laurene

A) MTU

Lenna 2 years ago

But MTU affects the maximum size of packets that can be transmitted, so it seems like a critical field to match.

Lorrine 2 years ago

SysID, for sure. Can't have two routers with the same system ID, that's just asking for trouble. Although, I did once work with a team that thought changing the SysID was a good way to 'optimize' the network. Needless to say, that didn't end well.

Hannah 2 years ago

I disagree, I believe it's Context that would cause the adjacency to fail.

Florinda 2 years ago

Definitely the MTU! I learned that one the hard way during a network troubleshooting session. Reminds me of the time I accidentally set the MTU to 1400 on a link - instant connectivity loss!

Wilda 2 years ago

↳ replying to Florinda

B) SEL

Elfrieda 2 years ago

↳ replying to Florinda

Oh no, that must have been frustrating!

Leatha 2 years ago

↳ replying to Florinda

A) MTU

Lenna 2 years ago

I think the mismatched field that would cause an IS-IS adjacency to fail is MTU.

Ericsson ECP-206 Exam - Topic 4 Question 25 Discussion