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Exam 350-501 topic 1 question 328 discussion

Actual exam question from Cisco's 350-501

Question #: 328
Topic #: 1

Refer to the exhibit. The customer that owns the CE-1, CE-2, and CE-3 routers purchased point-to-point E-Line services from the Carrier Ethernet provider. The service provider is delivering multiplexed UNI at the customer HQ location on PE-1 and untagged UNIs at the PE-2 and PE-3 locations. Additionally, the customer provided these VLAN to EVC mapping requirements:

• EVC 1 between CE-1 and CE-2 must be provisioned with C-VLAN 12 at the HQ location.
• EVC 2 between CE-1 and CE-3 must be provisioned with C-VLAN 13 at the HQ location.

Which configuration must the network engineer implement on the PE routers to provide end-to-end Carrier Ethernet service to the customer?

Show Suggested Answer

Suggested Answer: D 🗳️

by chst at Jan. 22, 2023, 6:27 p.m.

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rans3001

1 year ago

Selected Answer: A

D does not make sense: The traffic sent from CPE2 and CPE3 is not tagged. The symmetric on PE1 means that the service instances will try to remove vlan 12 or vlan 13 when sending traffic to CPE1. C is wrong because the xconnect VC IDs don't match. B The rewrite actions on PE2 and PE3 does not make sense (no vlan tag arrives from CPE2, CPE3, PE1)

upvoted 1 times

rans3001

1 year ago

Correction: The right answer is D because on PE1 is removing vlan tags 12 and 13 at ingress, so the packets are going untagged to CEP2 and CPE3. CPE2 and CPE3 are sending untagged frames, so they are tagged at PE1 with vlan 12 or vlan 13, before being sent to CPE1 (because of symmetric keyword)

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sushil_bhattacharjee

1 year, 5 months ago

Selected Answer: B

Answer: B In my previous comment, I mentioned Answer: D which is wrong. PE1 has to differentiate traffic between CE2 and CE3. Therefore, PE2 and PE3 have to push the tag in the direction of PE1. PE1 can be untagged.

upvoted 2 times

cocopost

1 year, 4 months ago

Agree. the push is needed. I believe the 10011301 or 10011301 is only locally significant and cannot help to multiplex the packets on the far end . trying to find a reference to confirm that.

upvoted 1 times

cocopost

1 year, 4 months ago

Answer D: My apologies, I am changing my vote on this. I did a lab today in GNS3. (this reminded me one thing) The lines: xconnect 172.20.20.1 1001201 encapsulation mpls xconnect 172.20.20.1 1001301 encapsulation mpls which appear with different vc-id in PE-2/PE-3 are sufficient to differentiate the traffic between CE2 and CE3. It is not necessary to push a tag in the direction of PE1! VLAN ID rewrite takes effect automatically. Answer D works fine without push. for VLAN-ID rewrite see: https://sites.google.com/site/amitsciscozone/atom/eompls-carrying-one-vlan. Furthermore as pointed out by user nottoday2077, Answer B is missing service instance. Correct answer: D.

upvoted 1 times

cocopost

1 year, 1 month ago

I labbed it again in GNS3. Answers B and D both work. Both correctly push the vlan tags for PE1 but in different places (D: PE1 rewrite ingres tag pop 1 symmetric) or (B: PE2,PE3, rewrite ingress tag push dot1q 12/13 symmetric) Both B & D are correct.

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cocopost

1 year, 1 month ago

But still answer B is missing a service instance. D is more correct.

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sushil_bhattacharjee

1 year, 5 months ago

Answer D: "symmetric". Without "symmetric" word, it will not add VLAN-tag at egress. https://community.cisco.com/t5/networking-knowledge-base/understanding-ethernet-virtual-circuits-evc/ta-p/3108219#:~:text=rewrite%20ingress%20tag%20pop%201%20symmetric%20Since%20the,on.%20The%20rewrite%20ingress%20command%20does%20just%20that.

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12_sdeEQ

2 years, 4 months ago

Selected Answer: D

answer is D

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DUsoo

2 years, 5 months ago

Selected Answer: D

PE1 should have pop 1 "symmetric "

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nottoday2077

2 years, 5 months ago

Selected Answer: D

This one was bugging me because both B and D work. However figured out the missing piece. B - PE3 is missing the service instance. Would work if the service instance was there. D - PE1 - pops tag > PE2/PE3 egress untagged packet, ingress untagged traffic > PE1 egress tags based on set encapsulation (symmetric). A/C seem to be the same and don't work anyways.

upvoted 3 times

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chst

2 years, 6 months ago

I think the correct is "C" "pop 1 symmetric " https://www.cisco.com/c/en/us/td/docs/wireless/asr_901/Configuration/Guide/b_asr901-scg/b_asr901-scg_chapter_01011.pdf

upvoted 2 times

thejag

2 years, 4 months ago

I think you mean D, at least now that is the one with "pop 1 symmetric" on PE1.

upvoted 1 times

chst

2 years, 3 months ago

Yes, I meant D

upvoted 1 times

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