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Forum Replies

  1. Hi Diana,

    Glad to hear you like it!

    There's not really a short answer to your question. Technically, ND doesn't require MLD to work.

    IPv6 ND uses multicast and the solicited node multicast addresses.

    Using multicast instead of broadcast sounds effective but in reality, your ND traffic is probably still broadcasted since your L2 switches don't know where to forward the multicast traffic to.

    To improve this, you could enable MLD snooping on the switches. Your hosts will have to report what they want to receive through MLD and then the switch will be able to deliver multicast traffic only on the required interfaces.

    This sounds great but in reality, it doesn't work. Each host will have a unique solicited multicast address so if you have 1000 hosts then your switch has to keep track of 1000 multicast groups. Depending on the switch, it might be unable to do this.

    It's best to keep MLD snooping disabled, your NICs will drop multicast traffic that they are not interested in.

    Rene

  2. andrew says:

    Itai,
    The solicited node multicast address is known because the IPv6 RFC standards have established a "rule" in how to form the solicited node multicast address based on the IPv6 addresses that is needing to be queried. The rule states that solicited node multicast address is: FF02::1:FFXX:XXXX where X is the last 24 bits of the IPv6 target address. This means you take the last "half" of the next to last hextet, and the entire last hextet and append it to FF01::1:FF. See example below.

    The last thing you would need to know is that there is also an IPv6 RFC rule for creating the layer 2 address for any IPv6 multicast address. That rule states the address is 3333:FFXX:XXXX where X is the last 24 bits of the multicast address

    Example: Let's say Host A has just booted up, and wants to use the local IPv6 address of FE80::0200:0BFF:FE0A:2D51. Host A needs to determine whether another host is using this address before it is allowed to start using it. In this case, a Neighbor Solicitation called DAD (duplicate address detection) is used. Here are the layer 3 and layer 2 addresses this DAD would use:

    Layer 3
    Source Address: :: <--------- Host A isn't allowed to use a layer 3 address yet
    Destination Address: FF02::1:FF0A:2D51 <--------- This is the result of the IPv6 RFC rule for crafting a solicited node multicast address

    Layer 2
    Source Address: <Host A's MAC address> <--------- Normal Ethernet operation here
    Destination Address: 3333:FF0A:2D51 <--------- This is the result of the IPv6 RFC rule for crafting layer 2 addresses from an IPv6 multicast


    --Andrew

  3. Hello Dionisis.

    I was unable to find the quote you are referring to so I do not know the context of your question. Can you be more specific as to the quote you mentioned?

    Thanks!

    Laz

  4. Hi,
    My topo:
    [R1]----[R2]
    When both router booted up, I first configured R1 interface as 2001::1/64 IPv6, and hasn't configured R2 yet. But I can see from packet capture that R1 is sending NS and NA both. Without configuring R2, why R1 sent NA? And after that, if I configure R2 with 2001::2/64 interface, I notice the same behaviour. Please find the pcap and interface show o/p. Please let me know the correct behaviour?
    ==============================
    PCAP: https://www.cloudshark.org/captures/623abeaa7dc5
    ==============================

    R1:
    ESW1#sh ipv6 int f0/0
    FastEthernet0/0 is up, line protocol is up
      IPv6 is enabled, link-local address is FE80::C001:8FFF:FEEF:0
      No Virtual link-local address(es):
      Global unicast address(es):
        2001::1, subnet is 2001::/64
      Joined group address(es):
        FF02::1
        FF02::1:FF00:1
        FF02::1:FFEF:0
      MTU is 1500 bytes
      ICMP error messages limited to one every 100 milliseconds
      ICMP redirects are enabled
      ICMP unreachables are sent
      ND DAD is enabled, number of DAD attempts: 1
      ND reachable time is 30000 milliseconds

    =================================================

    R2:
    ESW2#show ipv6 int f0/0
    FastEthernet0/0 is up, line protocol is up
      IPv6 is enabled, link-local address is FE80::C002:8FFF:FEF7:0
      No Virtual link-local address(es):
      Global unicast address(es):
        2001::2, subnet is 2001::/64
      Joined group address(es):
        FF02::1
        FF02::1:FF00:2
        FF02::1:FFF7:0
      MTU is 1500 bytes
      ICMP error messages limited to one every 100 milliseconds
      ICMP redirects are enabled
      ICMP unreachables are sent
      ND DAD is enabled, number of DAD attempts: 1
      ND reachable time is 30000 milliseconds

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