Interesting IPv6/NDP observation

By on 30 Jan 2023

Category: Tech matters

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The following observations and tests were made by me in collaboration with ExtremeIX via AS149794.

First, a quick shout out to Devendra Singh (Network Operation Centre Manager of AS132559) and the folk at ExtremeIX who have both collaborated with me in having this non-profit and non-commercial network setup and connectivity for the purpose of research, observations, and improvements.

We will start with a quick primer on Neighbour Discovery Protocol (NDP), which is the IPv6 replacement of Address Resolution Protocol (ARP) in IPv4. It is used for discovering the link-layer addresses of other nodes on the same link, discovering other nodes that are neighbors, and determining the reachability of other nodes.

NDP is a key component of IPv6 as it provides the functionality required for a node to determine the link-layer address of its default router and to discover the addresses of other nodes on the local link. It also provides a mechanism for routers to advertise their presence and the services they provide. Overall, NDP plays a crucial role in enabling IPv6 nodes to communicate with each other on a local link and to discover routes to other networks.

There are several RFCs that discuss NDP, but for the purpose of this article, we will refer to RFC 4861 and, in particular, to sections 4.3 and 4.4, which discuss the format used for Neighbor Solicitation (NS) and Neighbor Advertisement (NA).

Network setup on AS149794

I have a single gigabit L2 path to my upstream provider’s (AS132559) nearest L2 termination switch where I send two separately tagged VLANs, one for transit and one for Internet Exchange Point (IXP) connectivity, which in this case, is ExtremeIX Bangalore. The hardware is a MikroTik CCR1036-8G-2S+ running RouterOS 7.6 along with a matching version for RouterBOARD firmware.

In my router’s firewall configuration, I use strict firewall rules in the raw table (pre-routing chain) and one of those rules is the following:

If a packet has a Global Unicast Address (GUA) source address and the destination address is not a GUA (such as Link-Local Addresses (LLA), Unique Local Addresses (ULAs), and so on) — drop the traffic.

In my opinion, the above rule made sense at the time because:

  1. I did not see a valid reason to permit traffic originating from external networks to reach my non-GUA addresses whatsoever from a GUA source.
  2. The same concept is used in IPv4 with no problems, where I drop any packets destined towards RFC 6890 ranges from external networks where it is from a public address block source.

The issue

With the firewall rule in place, I started observing that my IPv6 BGP sessions with ExtremeIX’s Route Servers were flapping and unstable. Of course, I have tens of firewall rules in place that are complex and tied together, so it took quite a time to narrow it down to one single rule.

When I realized that disabling the rule solved the issue with BGP sessions flapping, I decided to dig deeper with WireShark and discovered NDP NS/NA packets where the source address is GUA and the destination address is link-local.

Figure 1 — ICMPv6 packets for NDP showing source/destination addresses matching the GUA<>LLA pattern.


On the ExtremeIX side

We initially assumed this GUA<>LLA traffic pattern was vendor/OS specific. The original route server was running CentOS with kernel version 3.10.0. So, we tried to move my peering to an experimental route server (RS) running Linux Kernel version 6.0.6, but that changed absolutely nothing and hence suggested the behaviour is inherent to specific OSes/vendors at the very least.

On my side

I decided to check this out in my own LAN. I have macOS, iOS, Windows 11, and Debian 11 as hosts on a VLAN, where of course, LLA is enabled along with a /64 GUA prefix via SLAAC.

However, even though I observed for over 48 hours with WireShark, and with my hosts/router sending/receiving NA/NS packets 24/7 — I could not find a single packet that matched the GUA<>LLA pattern. All the packets were strictly the LLA<>LLA pattern as I originally expected.

This is when my research led me to RFC 4861 sections 4.3 and 4.4 where I noticed ambiguity. It does not explicitly say whether we can use GUA as the source address/destination for NS and NA when LLA is working as intended. I initially assumed that for NDP NS/NA both source and destination would be only LLA, but as we can see, that’s certainly not the case and hence this behaviour is technically RFC compliant as the RFC is not clear.


Since we were unable to replicate the behaviour in my LAN, nor stop the behaviour by upgrading the kernel version on the RS, the folks at ExtremeIX decided to dump traffic from their larger locations where they have tens/hundreds of members connected to gain better insights.

From the traffic dumps, these were the findings:

  1. CentOS (and likely many other Linux distros), Cisco, MikroTik and Huawei showed the GUA<>LLA pattern for NDP.
  2. In contrast, vendors like Juniper and Edge-Core did not show any GUA<>LLA pattern and were instead explicitly only LLA<>LLA.

There is no documentation that we could find to explain why different vendors showed different behaviours.

Based on this APNIC Blog article on NDP fundamentals, as per our understanding, it seems to suggest that GUA<>LLA communication in NDP for the purpose of NS/NA is not normal, which, if true, suggests only Juniper/Edge-Core from our findings are following ‘compliant’ or expected behaviour for NDP.


Although NDP-related communication matching the GUA<>LLA pattern likely should not exist, we can confirm that it does exist in the IPv6 implementation of some vendors.

This does not cause any issues with our observations and this can be considered as just an interesting observation and informative finding.

However, the underlying question remains — why does this behavioural difference exist? While we have not observed inter-vendor issues, we cannot entirely rule out the possibility of inter-vendor issues in certain environments or use cases. I believe that this should be documented in the IPv6 Working Group(s) at IETF and the relevant RFCs, which would give solid instructions to vendors about what to do regarding the documented behaviour and prevent possible inter-vendor incompatibility in the future.

I have since modified my firewall rules to permit this behaviour while not compromising the original reason for the firewall rule itself. I have also ensured this firewall rule is reflected in my ‘Edge Router and BNG Optimisation Guide for ISPs’ article as well, which contains the technical configuration aspects if you are interested.

Here is the snippet:

#Here “bgp_peers” is an address lists containing all the /126s-/127s or /64s used for the peering interfaces with external networks#

/ipv6 firewall raw

add action=accept chain=prerouting comment="Accept all ICMPv6 traffic from BGP peers (Required for LL<>GUA packets)" icmp-options=!154:4-5 in-interface-list=WAN protocol=icmpv6 src-address-list=bgp_peers

Daryll Swer is an IT & Networking Enthusiast (AS149794), who currently works at cldin (AS48635) as a System & Network Engineer.

This post is adapted from the original at Daryll’s Blog.

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The views expressed by the authors of this blog are their own and do not necessarily reflect the views of APNIC. Please note a Code of Conduct applies to this blog.


  1. Jen Linkova

    Strictly speaking, if the destination address is considered onlink, then the host is free to use any address assigned to its interface as a source address for Neighbor Solicitation messages.

    Actually if the host is using Default Address Selection (RFC6724), then it would use link-local address as a source. Because, as per Section 5 (ource Address Selection), rule 2:

    “Rule 2: Prefer appropriate scope.
    If Scope(SA) < Scope(SB): If Scope(SA) < Scope(D), then prefer SB and
    otherwise prefer SA. Similarly, if Scope(SB) < Scope(SA): If
    Scope(SB) < Scope(D), then prefer SA and otherwise prefer SB"

    Obviously, the scope (link-local address) < scope(global address). So the implementation which uses link-local fro neighbour discovery is fully compliant with RFC6724.

    If, however, the host decides to use any other approaches to select the source address, it can pick up a GUA.

    IMHO it makes sense to have a firewall rule which allows ND packets *before* you start filtering by source/destination.

  2. Jen Linkova

    Sorry, I don’t know what I was thinking, disregard the previous answer 😉 Actually it’s another way around.

    By *default* the host would be picking up the global source for the global destination because of rule 2.

    However as the destination is considered ‘onlink’ then there is no reason for the host not to use link-local as a source address for NS messages. So link-local source -> GUA destination is also OK.

    So basically if you implement ND yourself you have a choice: always use link-local sources (as ND is always stays onlink) and do not care about using default address selection, or use default address selection (which would lead to GUA appearing in ND packets). IMHO, both approaches are OK.


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