MTU issues (and TCP MSS clamping) in residential IPv6 deployments

Numerous residential access technologies face path MTU discovery issues. PPPoE connections (with MTU = 1492 bytes instead of 1500 bytes) is the best-known example, and we’ll see more of them as various tunneling-based IPv4-to-IPv6 transition mechanisms (6rd, DS-Lite, MAP-E) become more popular.

Obviously you could use the same old MSS clamping tricks in the brave new IPv6 world or decide (like DS-Lite) to deal with IP fragmentation in underlay access networks ... but there’s another option in the IPv6 world: reduce client-side MTU with router advertisement messages.

How does it work?

IPv6 router advertisement messages (defined in RFC 4861) could include MTU option to “... ensure all nodes on a link use the same MTU value in the cases where the link MTU is not well known.”

As is usually the case in networking, that option is commonly misused for a TCP MSS clamping kludge: if the router advertises an MTU lower than 1500 bytes on client-facing interfaces, the clients will pick up the lower MTU setting and start advertising lower MSS value in TCP SYN packets.

Examples: the IPv6 MTU advertised on Ethernet interfaces should be 1492 bytes in PPPoE environments and 1480 bytes in 6rd environments.

How good is this solution?

The reduced LAN MTU trick works only if:

  • The end-hosts listen to the MTU option in router advertisement messages (usually they do);
  • All routers attached to the LAN advertise the same value (or you have a single router, as is usually the case in residential deployments);
  • You don’t mind that intra-LAN communication uses reduced MTU value (no jumbo frames).

Not exactly the best solution there is, but it’s good enough for residential deployments ... and in many cases it’s the best you can do. There’s no ipv6 tcp adjust-mss command in Cisco IOS (and I wasn’t able to find one in Junos either).

In a recent e-mail exchange Trevor Warwick claimed ip tcp adjust-mss command modifies MSS option for both IPv4 and IPv6 TCP sessions since IOS release 15.2(4)M ... without considering the larger size of IPv6 headers. An IOS developer obviously did a great job ;)

How is it configured?

Cisco IOS always advertises the link MTU in router advertisement messages. You can change the MTU with the ipv6 mtu command and inspect the contents of router advertisement messages with the debug ipv6 nd command.

Sample RA message with IPv6 MTU set to 1400

ICMPv6-ND: Request to send RA for FE80::C800:8FF:FE04:6
ICMPv6-ND: Sending RA from FE80::C800:8FF:FE04:6 to FF02::1 on FastEth0/1
ICMPv6-ND: MTU = 1400
ICMPv6-ND: prefix = 2001:DB8:0:1::/64 onlink autoconfig
ICMPv6-ND: 120/60 (valid/preferred)

Some low-end CPE devices automatically reduce the LAN side MTU to reflect the WAN connectivity (PPPoE or 6rd).

More information

Building large IPv6 service provider networks webinar describes the fine points of IPv6 router advertisement mechanisms and related Cisco IOS configuration; 6rd, DS-Lite and MAP-E are described in IPv6 transition mechanisms webinar. You get access to both of them (and numerous others) with the yearly subscription.


  1. Hi Ivan

    Seems that the gap between IPv4 and IPv6 header with ip 'tcp adjust-mss' is solved on 15.2(4)M2.
  2. Not sure I get it entirely... if your CPE runs some sort of a tunnel (6rd, DS-Lite, MAP-E, whatever) and uses the MTU lower than 1500 on the "wan" side, where's the issue that pMTUd would not work? The ICMPv6 packet with message "MTU too big, here's my MTU" is sent from you on-link-neighbour device... I agree with the pMTUd issues where the "bottleneck" is somewhere remotely in the path, no doubt - but locally? Do you expect the CPE vendors to be so short-sight not to make this work?
  3. You might want to watch this video and think about what's going on when you establish an HTTP connection to a remote web server. Who is going to receive the first "packet too big" ICMP reply?
  4. Is it possible within IPv6 to have differently scoped addresses receive different MTU values? For example: the router advertisement for globally scoped addresses informs clients of an 1480B path but a locally scoped prefix is also advertised with an MTU of 9000B (or whatever the local LAN can handle)? I realize this probably doesn't make much sense for residential subscriber networks but could be used in the enterprise.
    1. No. There's a single MTU field in RA messages.
    2. But wouldn't the router send two RAs?.. one for the UGA and one for the ULA? Or does the router only send a single RA that includes both?
  5. how can we solve the mtu problem if the same clients are on the same vlan and there is smaller mtu sized connection between two devices?
    1. MTU should be the same throughout a L2 domain, or you have a severely broken design.
    2. How can we handle if vlan extended via OTV or EOL2TP? If we have applications that marks DF bit as 1. Hoe can we change DF bit to 0 on L2 environment.
    3. As I expected - broken design.

      You CANNOT do fragmentation over OTV or EoL2TP, you MUST have MTU larger than 1500 in transit network.

      Even if you'd manage to get it to work (breaking PMTUD in the process), you'd kill the CPU in the receiving router doing the reassembly.
    4. It would seem that IOS XE does now support it.
      We are using the CSR1Ks to put in a temporary stretched VLAN for migration purposes over a 1500byte MTU transport network.

      the CSR1K BU has validated the design:
    5. Hi Adam,

      Thank you for the response. otv fragmentation will solve our problems..
  6. some ISPs do not provide more than 1500 byte MTU line. In that case, we have to trust PMTUD to handle mtu process.
    1. "some ISPs do not provide more than 1500 byte MTU line" ... in which case you SHOULD NOT use OTV or EoL2TP but something (the device formerly know as A ROUTER) that can actually send an ICMP report back to the sending host.
  7. MSS clamping is a hack, has been and always will be. The correct solution is to not break pMTUd, if you have a 1492MTU then the router should be advising of that - if it's capable of sending out RAs, I don't see why an ICMPv6 Fragmentation Needed should be any different. Of course, setting your RAs to advertise the MTU is definitely a Good Thing. since it will save hosts from periodically re-probing the link MTU.
    1. Nobody said it's not a hack. As for router sending ICMPv6 messages - think about a typical web browsing scenario: who will send the first oversized packet, who should get the first ICMPv6 packet and will it get the packet?

      Sending RAs with reduced MTU is always an option, but every time someone suggests that, there's an ivory tower crowd jumping up and down and mourning the loss of performance.
  8. Just bear in mind to explicilty have "ipv6 mtu" in IOS XR, otherwise you may get wrong MTU in RA messages....apparently it is a feature, not a bug
  9. FWIW - IOS IP Application Services Command Reference claims that "ipv6 tcp adjust-mss" was introduced with IOS XE 3.11S.
    ( )

    The IPv6 experience on my 6RD residential access had deteriorated considerably while I was away for 2 months (probably some changes upstream) and the symptoms just reeked of MTU problems (endless delays, missing packets in TLS negotiations, timeouts, the lot!).

    "ipv6 tcp adjust-mss 1400" on the 6RD tunnel interface cured it for good.
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