Exam 350-501 topic 1 question 348 discussion

Actual exam question from Cisco's 350-501

Question #: 348
Topic #: 1

Refer to the exhibit. Routers R1 and R2 are connected via a serial link and use the IS-IS routing protocol for route exchange. After a configuration change on R2, IS-IS connectivity is interrupted. A network engineer confirmed that the interfaces are in the UP state and connectivity exists between the two routers. Which two actions must the engineer perform to resolve the problem? (Choose two.)

A. Change the hello interface timer to 10 seconds on R1
B. Change the MTU to 1500 bytes on R2
C. Change R2 to an IS-IS Level 1 router
D. Disable padding for hello packets under the serial interface on R2
E. Enable hello packet padding globally on R1

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Suggested Answer: BD 🗳️

by chst at March 22, 2023, 5:34 p.m.

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sushil_bhattacharjee

10 months, 2 weeks ago

Answer: BD -1) R2 is sending hello with "1699 bytes," whereas the maximum MTU is "1500 bytes.". The following link is explained very well. -2) If we change the MTU to "1500 bytes" on the R2 interface, then it will send hello with "1499 bytes," and it will build the neighborship if the link is not over MPLS. -3) With Hello padding disabled on the interface Serial 2/0, the size of the Hello packet to the minimum. -4) Thus, if you only disable the IS-IS Hello padding, it is not enough to bring up the IS-IS adjacency. The MTU must be low enough so that the MTU-sized IS-IS Hello packets are sent and received properly by the routers on either side of the link.

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cerifyme85

1 year, 2 months ago

Selected Answer: BD

Sorry guys but it seems like answer is B, D Adjust MTU Disable Hello padding so that ISIS does not detect MTU mismatches, as there wont be any need to

upvoted 1 times

cerifyme85

1 year, 2 months ago

... need to enable hello padding

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cerifyme85

1 year, 4 months ago

Selected Answer: AE

I will go for A and E https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/119399-technote-isis-00.html#:~:text=Note%3A%20Cisco%20recommends%20that%20you%20do%20not%20disable%20the%20IS%2DIS%20Hello%20padding%20in%20order%20to%20ensure%20that%20two%20routers%20form%20an%20IS%2DIS%20adjacency%20on%20a%20link%20that%20has%20mismatched%20MTU%20values%20on%20either%20side.

upvoted 2 times

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chst

1 year, 9 months ago

https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/119399-technote-isis-00.html The MTU on the serial interfaces for the routers R1 and R2 are the default 1,500 bytes. The IS-IS adjacency fails because the IS-IS Hello packets are 1,499 bytes in size. The MPLS network only allows 1,500-byte packets, minus eight bytes (two MPLS labels for the MPLS service), which equals 1,492 bytes (the packet size that is allowed to pass through). For transport of L2 over MPLS, the size of the L2 header must be subtracted from the 1,492 bytes that result as well.

upvoted 2 times

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