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Exam 300-410 topic 1 question 534 discussion

Actual exam question from Cisco's 300-410
Question #: 534
Topic #: 1
[All 300-410 Questions]

How does MPLS Layer 3 VPN function?

  • A. When an EIGRP internal route is redistributed into BGP by one PE and then back into EIGRP by another PE, the originating router ID for the route is changed to the router ID of the first PE.
  • B. When a destination PE device receives a labeled packet, it pops the label and uses it to forward the packet to the correct CE device.
  • C. When a PE device forwards a packet received from a CE device across the provider network, it labels the packet with the label learned from the source PE device.
  • D. When a VPN route is learned from a CE device and injected into IGP, a VPN route distinguisher attribute is associated with it.
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Suggested Answer: B 🗳️

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Highly Voted 1 year, 4 months ago
Selected Answer: B
I think B may be correct- the label in question being the VPN label NOT the MPLS label which would have being removed by the previous router in the LSP.
upvoted 8 times
Pietjeplukgeluk
1 year, 1 month ago
I think C is to vague, so i agree, the outer label is removed still having the inner (VPN) label. Anyway, C sounds best to me
upvoted 1 times
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[Removed]
Most Recent 9 months, 3 weeks ago
Selected Answer: B
B is correct C is incorrect, since the PE labels the packet based on its own label distribution with the labels it has learned for the VPN routes, not from the source PE device.
upvoted 1 times
bk989
8 months, 3 weeks ago
The PE imposes an LDP label from the egress (destination) PE
upvoted 2 times
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Pietjeplukgeluk
10 months, 1 week ago
Selected Answer: B
Guess B is ok as bk989 specified in the cisco doc
upvoted 3 times
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bk989
1 year, 1 month ago
Answer is C according to cisco documentation: hen the destination PE device receives the labeled packet, it pops the label and uses it to direct the packet to the correct CE device https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l3_vpns/configuration/15-mt/mp-l3-vpns-15-mt-book/mp-bgp-mpls-vpn.html
upvoted 1 times
Pietjeplukgeluk
10 months, 1 week ago
Indeed, the doc link clarifies using B as an answer.
upvoted 2 times
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bk989
1 year, 1 month ago
Actually this looks like B
upvoted 2 times
bk989
9 months ago
To add to this I kept looking at C over and over again. However C would be correct if this was LDP. For L3VPN, the VPNLabel, this is the locallly significant RD, so has nothing to do with PE on other side. D makes no sense. I was also looking at A, bit this doesn't seem true. If this was eBGP between CE and PE, and a route-reflector in middle the RID of first PE is preserved, other than that after redistribution into EIGRP I don't see how the BGP router-id of first PE is preserved. The EIGRP router-id definately isn't preserved. B seems like the best answer.
upvoted 1 times
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Bombbear_W
1 year, 2 months ago
Selected Answer: C
MPLS Layer 3 VPN operates by using MPLS labels to distinguish and forward packets between Customer Edge (CE) devices across the Provider Edge (PE) devices. When a packet is received by a PE device from a CE device, the PE device labels the packet with a label that corresponds to the VPN forwarding table entry learned from the source PE device. This labeled packet is then forwarded across the provider network to the destination PE device, which uses the label to determine the correct egress interface towards the destination CE device. So, option C accurately describes the function of MPLS Layer 3 VPN.
upvoted 2 times
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Community vote distribution
A (35%)
C (25%)
B (20%)
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