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Exam 300-410 topic 1 question 462 discussion

Actual exam question from Cisco's 300-410
Question #: 462
Topic #: 1
[All 300-410 Questions]



Refer to the exhibit. An engineer applied an IPv6 traffic filter on R1. The interface flapped between R1 and R2 and clearing the BGP session did not restore the BGP session and failed. Which action must the engineer take to restore the BGP session from R2 to R1?

  • A. ICMPv6 must be permitted by the IPv6 traffic filter.
  • B. Swap the source and destination IP addresses in the IPv6 traffic filter.
  • C. Enable the BGP session, which went down when the session was cleared.
  • D. Apply the IPv6 traffic filter in the outbound direction on the interface.
Show Suggested Answer Hide Answer
Suggested Answer: A 🗳️
by HungarianDish_111 at June 4, 2023, 3:51 p.m.

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HungarianDish_111
Highly Voted 1 year, 11 months ago
Selected Answer: A
Solution "A" makes sure that neighbor advertisement packets are not discarded. (Usually only required after adding "deny ipv6 any any"). https://alexandremspmoraes.wordpress.com/2012/02/17/how-ios-ipv6-acls-handle-icmpv6-neighbor-discovery-messages/
upvoted 9 times
inteldarvid
1 year, 9 months ago
Brothers u r really usefull, i hope if u have time to contact me here, i need to discuss some tasks with u kindly, +5491173651673 whats
upvoted 1 times
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MEDO95
1 year, 11 months ago
Brother u r really usefull, i hope if u have time to contact me here, i need to discuss some tasks with u kindly, +15749999611 whats
upvoted 1 times
inteldarvid
1 year, 9 months ago
Brothers u r really usefull, i hope if u have time to contact me here, i need to discuss some tasks with u kindly, +5491173651673 whats
upvoted 1 times
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MasterMatt
Highly Voted 1 year, 9 months ago
BGP use ICMP for keepalive. Thus why ICMPv6 is required.
upvoted 5 times
Pietjeplukgeluk
1 year, 1 month ago
BGP does NOT use ICMP for keepalive. Anyway this questions seems stupid and most likely "the best" answer might be A. But this questions seems WRONG, put your time in other questions that are fully correctly written.
upvoted 3 times
bk989
8 months, 3 weeks ago
the neighbor discovery packets would be required if R2 didn't have an IPv6 interface address, and R1 was blocking the R2 neighbor discovery messages.
upvoted 1 times
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bk989
Most Recent 8 months, 2 weeks ago
My buest guess is the BGP flapped, due to a downed link, and neighbor solicitation messages weren't sent through to discover the link-local address of the other side. BGP uses update packets for keepalives, so if the answer is A, it is because of the reason I mentioned, not because BGP needs ICMPv6. B doesn't solve anything. Enabling the peering on the other BGP may help, but the exhibit shows we don't have connectivity. Why don't we have connectivity? The link went down, we lost the IPv6 address bindings, now we need neighbor sollicitation messages. We need ICMPv6 for Neighbor Sollicitation messages to find out link-local addresses of other router. This is my best guess.
upvoted 2 times
XBfoundX
7 months, 3 weeks ago
you are right, basically is like blocking arp resolution in ipv4 the result will be after the shutdown that we cannot resolve the arp and the result is no tcp session equal no bgp session: *Sep 17 04:21:53.550: ICMPv6: Sent N-Solicit, Src=2001:DB8::1, Dst=FF02::1:FF00:2 *Sep 17 04:21:53.559: ICMPv6: ipv6_check_if_anycast address 2001:DB8::2 *Sep 17 04:21:53.559: ICMPv6: Sent Unreachable code 5, Src=2001:DB8::1, Dst=2001:DB8::2 *Sep 17 04:21:54.601: ICMPv6: Sent N-Solicit, Src=2001:DB8::1, Dst=FF02::1:FF00:2 R1#
upvoted 1 times
XBfoundX
7 months, 3 weeks ago
R1#debug bgp ipv6 *Sep 17 04:23:00.746: BGP: 2001:DB8::2 open failed: Connection timed out; remote host not responding *Sep 17 04:23:00.746: BGP: 2001:DB8::2 Active open failed - tcb is not available, open active delayed 6144ms (35000ms max, 60% jitter) *Sep 17 04:23:00.746: BGP: ses global 2001:DB8::2 (0xC772DC20:0) act Reset (Active open failed). *Sep 17 04:23:00.751: BGP: 2001:DB8::2 active went from Active to Idle *Sep 17 04:23:00.751: BGP: nbr global 2001:DB8::2 Active open failed - open timer running *Sep 17 04:23:00.751: BGP: nbr global 2001:DB8::2 Active open failed - open timer running
upvoted 1 times
XBfoundX
7 months, 3 weeks ago
after this acl applied everthing was fine: ipv6 access-list inbound-acl permit tcp host 2001:DB8::2 eq bgp host 2001:DB8::1 permit tcp host 2001:DB8::2 host 2001:DB8::1 eq bgp permit icmp any any nd-ns permit icmp any any nd-na permit icmp any any router-solicitation permit icmp any any router-advertisement permit icmp any any redirect sequence deny ipv6 2001:DB8::/32 any permit ipv6 any any
upvoted 1 times
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[Removed]
9 months, 2 weeks ago
Selected Answer: A
A is corerct
upvoted 1 times
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[Removed]
1 year, 5 months ago
Honestly completely missed that BGP uses ICMP for keepalives.
upvoted 3 times
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HungarianDish_111
1 year, 11 months ago
The connection never came up with source and destination swapped. So not "B". R1(config)#do sh ip bgp all su For address family: IPv6 Unicast BGP router identifier 1.1.1.1, local AS number 1 BGP table version is 1, main routing table version 1 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 2001:DB8::2 4 2 0 0 1 0 0 00:04:22 Active R1(config)#do sh ipv6 access IPv6 access list INBOUND permit tcp host 2001:DB8::1 eq bgp host 2001:DB8::2 sequence 10 permit tcp host 2001:DB8::1 host 2001:DB8::2 eq bgp sequence 20 deny ipv6 2001:DB8::/32 any (131 matches) sequence 30 permit ipv6 any any (21 matches) sequence 40 R1(config)#
upvoted 1 times
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HungarianDish_111
1 year, 11 months ago
I applied this config in the lab (+reset bgp peers), and the result was that bgp dd not get blocked by this ACL. Am I missing something? R1(config)#do sh ipv6 access-l IPv6 access list INBOUND permit tcp host 2001:DB8::2 eq bgp host 2001:DB8::1 (10 matches) sequence 10 permit tcp host 2001:DB8::2 host 2001:DB8::1 eq bgp sequence 20 deny ipv6 2001:DB8::/32 any sequence 30 permit ipv6 any any (1 match) sequence 40 R1(config)# R1(config)#do sh run | sec GigabitEthernet0/0 interface GigabitEthernet0/0 ipv6 address 2001:DB8::1/64 ipv6 traffic-filter INBOUND in R1(config)#
upvoted 1 times
HungarianDish_111
1 year, 11 months ago
After applying the ACL bgp connection is still OK. Also received one prefix from neighbor. R1(config)#do sh ip bgp all su For address family: IPv6 Unicast BGP router identifier 1.1.1.1, local AS number 1 BGP table version is 2, main routing table version 2 1 network entries using 168 bytes of memory 1 path entries using 108 bytes of memory 1/1 BGP path/bestpath attribute entries using 160 bytes of memory 1 BGP AS-PATH entries using 24 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 460 total bytes of memory BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 2001:DB8::2 4 2 9 9 2 0 0 00:04:39 1
upvoted 1 times
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C (25%)
B (20%)
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