Internet Engineering Task Force (IETF)
Request for Comments: 8249
Updates: 6325, 7177, 7780
Category: Standards Track
ISSN: 2070-1721
M. Zhang
X. Zhang
D. Eastlake 3rd
R. Perlman
Dell EMC
S. Chatterjee
September 2017

Transparent Interconnection of Lots of Links (TRILL):

MTU Negotiation


The base IETF TRILL (Transparent Interconnection of Lots of Links) protocol has a TRILL campus-wide MTU feature, specified in RFCs 6325 and 7177, that assures that link-state changes can be successfully flooded throughout the campus while being able to take advantage of a campus-wide capability to support jumbo packets. This document specifies recommended updates to that MTU feature to take advantage, for appropriate link-local packets, of link-local MTUs that exceed the TRILL campus MTU. In addition, it specifies an efficient algorithm for local MTU testing. This document updates RFCs 6325, 7177, and 7780.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
   2. Link-Wide TRILL MTU Size ........................................4
      2.1. Operations .................................................5
   3. Testing Link MTU Size ...........................................6
   4. Refreshing Sz ...................................................8
   5. Relationship between Port MTU, Lz, and Sz .......................9
   6. LSP Synchronization ............................................10
   7. Recommendations for Traffic Link Testing of MTU Size ...........10
   8. Backward Compatibility .........................................11
   9. Security Considerations ........................................11
   10. Additions to Configuration ....................................12
      10.1. Per-RBridge Configuration ................................12
      10.2. Per-RBridge Port Configuration ...........................12
   11. IANA Considerations ...........................................12
   12. References ....................................................12
      12.1. Normative References .....................................12
      12.2. Informative References ...................................14
   Acknowledgements ..................................................14
   Authors' Addresses ................................................14

1. Introduction

[RFC6325] describes the way RBridges agree on the campus-wide minimum acceptable inter-RBridge MTU (Maximum Transmission Unit) size (called "Sz") to ensure that link-state flooding operates properly and all RBridges converge to the same link state. For the proper operation of TRILL (Transparent Interconnection of Lots of Links) IS-IS, all RBridges format their Link State Protocol Data Units (LSPs) to fit in Sz.

[RFC7177] diagrams the state transitions of an adjacency. If MTU testing is enabled, "Link MTU size is successfully tested" is part of an event (event A6) causing the transition from the "2-Way" state [RFC7177] to the "Report" state for an adjacency. This means that the link MTU testing of size x succeeds, and x is greater than or equal to Sz [RFC6325]. If this link cannot support an MTU of Sz, it will not be reported as part of the campus topology.

In this document, a new RECOMMENDED link-wide minimum inter-RBridge MTU size, "Lz", is specified. As further discussed in Section 2, by calculating and using Lz as specified herein, link-scoped Protocol Data Units (PDUs) can be formatted greater than Sz, up to the link-wide minimum acceptable inter-RBridge MTU size, potentially improving the efficiency of link utilization and speeding link-state convergence.

An optional TRILL MTU size-testing algorithm is specified in Section 3 as an efficient method to update the old MTU testing method described in Section 4.3.2 of [RFC6325] and in [RFC7177]. The new MTU size-testing method specified in this document is backward compatible with the old one. Multicasting the MTU-probes is recommended when there are multiple RBridges on a link responding to the probing with an MTU-ack [RFC7177]. The testing method and rules of this document are devised in a way that minimizes the number of MTU-probes for testing, therefore reducing the number of multicast packets for MTU testing.

This document updates RFCs 6325, 7177, and 7780. The update to [RFC6325] and [RFC7177] is specified in Section 3. The update to [RFC7780] is specified in Section 4.

1.1. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Link-Wide TRILL MTU Size

This document specifies a new value "Lz" for the minimum acceptable inter-RBridge link MTU size on a local link. Link-wide Lz is the minimum Lz supported and agreed upon amongst all RBridges on a specific link. If the link is usable, Lz will be greater than or equal to Sz.

Some TRILL IS-IS PDUs are exchanged only between neighbors instead of throughout the whole campus. They are confined by the link-wide Lz instead of Sz. Complete Sequence Number PDUs (CSNPs) and Partial Sequence Number PDUs (PSNPs) are examples of such PDUs. These PDUs are exchanged only on the local link. (While TRILL IS-IS Hellos are also link local, they are always limited to 1470 bytes for robustness.)

[RFC7356] defines the PDUs that support flooding scopes in addition to area-wide scopes and domain-wide scopes. As specified in [RFC8139], RBridges support the Extended L1 Circuit Scope (E-L1CS) Flooding Scope LSP (FS-LSP) [RFC7780]. The originatingSNPBufferSize for a port is the minimum of the following two quantities but not less than 1470 bytes: (1) the MTU of the port and (2) the maximum LSP size that the TRILL IS-IS implementation can handle. They use that flooding to exchange their maximum supported value of "Lz". The smallest value of the Lz advertised by the RBridges on a link, but not less than Sz, is the link-wide Lz. An RBridge on a local link will be able to tell which other RBridges on that link support E-L1CS FS-LSPs because, as required by [RFC7780], all RBridges include the Scope Flooding Support TLV [RFC7356] in their TRILL Hellos.

The maximum size for a level-1 link-local PDU (such as a PSNP or CSNP) that may be generated by a system is controlled by the value of the management parameter originatingL1SNPBufferSize. This value determines Lz. The TRILL APPsub-TLV shown in Figure 1 SHOULD be included in a TRILL GENINFO TLV [RFC7357] in an E-L1CS FS-LSP fragment zero. If it is missing from an E-L1CS FS-LSP fragment zero or there is no E-L1CS FS-LSP fragment zero, it is assumed that its originating IS is implicitly advertising its originatingSNPBufferSize value as Sz octets.

E-L1CS FS-LSPs are link local and can also be sent up to a size of Lz but, for robustness, E-L1CS FS-LSP fragment zero MUST NOT exceed 1470 bytes.

              | Type = 21                     |   (2 bytes)
              | Length = 2                    |   (2 bytes)
              | originatingSNPBufferSize      |   (2 bytes)

Figure 1: The originatingSNPBufferSize APPsub-TLV

   Type: Set to the originatingSNPBufferSize APPsub-TLV (TRILL
      APPsub-TLV type 21).  Two bytes, because this APPsub-TLV appears
      in an extended TLV [RFC7356].


Set to 2.

originatingSNPBufferSize: The local value of

originatingL1SNPBufferSize as an unsigned integer, limited to the range from 1470 to 65,535 bytes. (A value less than 1470 will be ignored.)

2.1. Operations

Lz MAY be reported using an originatingSNPBufferSize APPsub-TLV that occurs in fragment zero of the RBridge's E-L1CS FS-LSP. An originatingSNPBufferSize APPsub-TLV occurring in any other fragment is ignored. If more than one originatingSNPBufferSize APPsub-TLV occurs in fragment zero, the one advertising the smallest value for originatingSNPBufferSize, but not less than 1470 bytes, is used.

Even if all RBridges on a specific link have reached consensus on the value of link-wide Lz based on advertised originatingSNPBufferSize, it does not mean that these RBridges can safely exchange PDUs between each other. Figure 2 shows such a corner case. RB1, RB2, and RB3 are three RBridges on the same link and their Lz is 1800, so the link-wide Lz of this link is 1800. There is an intermediate bridge (say B1) between RB2 and RB3 whose port MTU size is 1700. If RB2 sends PDUs formatted in chunks of size 1800, those PDUs will be discarded by B1.

                         Lz:1800               Lz:1800
                          +---+         |         +---+
                          +---+         |         +---+
                  Lz:1800               |
                   +---+               +--+
                   +---+               +--+

Figure 2: Link-Wide Lz = 1800 vs. Tested Link MTU Size = 1700

Therefore, the link MTU size SHOULD be tested. After the link MTU size of an adjacency is successfully tested, those link-local PDUs, such as CSNPs, PSNPs, and E-L1CS FS-LSPs, will be formatted no greater than the tested link MTU size and will be safely transmitted on this link.

As for Sz, RBridges continue to propagate their originatingL1LSPBufferSize across the campus through the advertisement of LSPs as defined in Section 4.3.2 of [RFC6325]. The smallest value of Sz advertised by any RBridge, but not less than 1470, will be deemed as Sz. Each RBridge formats their "campus-wide" PDUs -- for example, LSPs -- no greater than what they determine as Sz.

3. Testing Link MTU Size

[RFC7177] defines event A6 as indicating that the MTU test was successful if MTU testing is enabled. As described in Section 4.3.2 of [RFC6325], this is a combination of the following event and condition:

  • Event: The link MTU size has been tested.
  • Condition: The link can support Sz.

This condition can be efficiently tested by the following "binary search algorithm" and rules. This updates [RFC6325] and [RFC7177].

x, lowerBound, and upperBound are local integer variables. The MTU-probe and MTU-ack PDUs are specified in Section 3 of [RFC7176]. It is RECOMMENDED that one Round-Trip Time (RTT) between the two adjacent RBridges be used as the minimum interval between two successive probes. Note that RTT estimation is out of scope for this document. If operators cannot estimate the RTT, the default value of 5 milliseconds should be assumed.

Step 0: RB1 sends an MTU-probe padded to the size of link-wide Lz.

  1. If RB1 successfully receives the MTU-ack from RB2 to the probe of the value of link-wide Lz within k tries (where k is a configurable parameter whose default is 3), the link MTU size is set to the size of link-wide Lz. Stop.
  1. RB1 tries to send an MTU-probe padded to 1470 bytes.

a) If RB1 fails to receive an MTU-ack from RB2 after k tries

(an MTU-ack should be considered to have failed two RTTs after the probe is sent out), RB1 sets the "failed minimum MTU test" flag for RB2 in RB1's Hello. Stop.

b) The link MTU size is set to 1470; lowerBound is set to 1470;

upperBound is set to the link-wide Lz; x is set to [(lowerBound + upperBound) / 2], rounded down to the nearest integer.

Step 1: RB1 tries to send an MTU-probe padded to the size x.

  1. If RB1 fails to receive an MTU-ack from RB2 after k tries:

upperBound is set to x - 1; x is set to
[(lowerBound + upperBound) / 2], rounded down to the nearest integer.

  1. If RB1 receives an MTU-ack to a probe of size x from RB2:

The link MTU size is set to x; lowerBound is set to x; x is set to [(lowerBound + upperBound) / 2], rounded down to the nearest integer. If lowerBound equals upperBound - 1, then x is set to upperBound.

  1. If lowerBound >= upperBound or Step 1 has been repeated n times (where n is a configurable parameter whose default value is 5), stop.
  1. Repeat Step 1.

After the testing, the two connected RBridges agree on the value of the link MTU size. MTU testing is only done in the Designated VLAN [RFC7177]. Since the execution of the above algorithm can be resource consuming, it is RECOMMENDED that the Designated RBridge (DRB) [RFC7177] take the responsibility to do the testing. Multicast MTU-probes are used instead of unicast when multiple RBridges are desired to respond with an MTU-ack on the link. The binary search algorithm given here is a way to minimize the probing attempts; it reduces the number of multicast packets for MTU-probing.

The following rules are designed to determine whether the aforementioned "Condition" holds.

RBridges have figured out the upper bound and lower bound of the link MTU size from the execution of the above algorithm. If Sz is smaller than the lower bound or greater than the upper bound, RBridges can directly judge whether the link supports Sz without MTU-probing.

(a) If lowerBound >= Sz, this link can support Sz.

(b) Else if upperBound <= Sz, this link cannot support Sz.

Otherwise, RBridges SHOULD test whether the link can support Sz as in item (c) below. If they do not, the only safe assumption will be that the link cannot support Sz. This assumption, without testing, might rule out the use of a link that can, in fact, handle packets up to Sz. In the worst case, this might result in unnecessary network partition.

(c) lowerBound < Sz < upperBound. RBridges probe the link with

MTU-probe messages padded to Sz. If an MTU-ack is received within k tries, this link can support Sz. Otherwise, this link cannot support Sz. Through this test, the lower bound and upper bound of the link MTU size can be updated accordingly.

4. Refreshing Sz

RBridges may join or leave the campus; this may change Sz.

  1. Joining

a) When a new RBridge joins the campus and its

originatingL1LSPBufferSize is smaller than the current Sz, reporting its originatingL1LSPBufferSize in its LSPs will cause other RBridges to decrease their Sz. Then, any LSP greater than the reduced Sz MUST be split, and/or the LSP contents in the campus MUST be otherwise redistributed so that no LSP is greater than the new Sz.

b) If the joining RBridge's originatingL1LSPBufferSize is greater

than or equal to the current Sz, reporting its originatingL1LSPBufferSize will not change Sz.

  1. Leaving

a) From the specification of the Joining process, we know that if

an RBridge's originatingL1LSPBufferSize is smaller than Sz, this RBridge will not join this campus.

b) When an RBridge leaves the campus and its

originatingL1LSPBufferSize equals Sz, its LSPs are purged from the remainder of the campus after reaching MaxAge [IS-IS]. Sz MAY be recalculated and MAY increase. In other words, while in most cases RB1 ignores link-state information for IS-IS unreachable RBridge RB2 [RFC7780], originatingL1LSPBufferSize is meaningful. Its value, even from IS-IS unreachable RBridges, is used in determining Sz. This updates [RFC7780].

c) When an RBridge leaves the campus and its

originatingL1LSPBufferSize is greater than Sz, Sz will not be updated, since Sz is determined by another RBridge with a smaller originatingL1LSPBufferSize.

Frequent LSP "resizing" is harmful to the stability of the TRILL campus, so, to avoid this, upward resizing SHOULD be dampened. When an upward resizing event is noticed by an RBridge, it is RECOMMENDED that a timer be set at that RBridge via a configurable parameter -- LSPresizeTime -- whose default value is 300 seconds. Before this timer expires, all subsequent upward resizing will be dampened (ignored). Of course, in a well-configured campus with all RBridges configured to have the same originatingL1LSPBufferSize, no resizing will be necessary. It does not matter if different RBridges have different dampening timers or if some RBridges resize upward more quickly than others.

If the refreshed Sz is smaller than the lower bound or greater than the upper bound of the tested link MTU size, the issue of resource consumption from testing the link MTU size can be avoided according to rule (a) or (b) as specified in Section 3. Otherwise, RBridges test the link MTU size according to rule (c).

5. Relationship between Port MTU, Lz, and Sz

When the port MTU of an RBridge is smaller than the local originatingL1SNPBufferSize of an RBridge (an inconsistent configuration), that port SHOULD be disabled, since, in any case, an adjacency cannot be formed through such a port. On the other hand, when an RBridge receives an LSP or E-L1CS FS-LSP with size greater than the link-wide Lz or Sz but not greater than its port MTU size, this LSP is processed normally. If the size of an LSP is greater than the MTU size of a port over which it is to be propagated, this LSP MUST NOT be sent over the port and an LSPTooLargeToPropagate alarm shall be generated [IS-IS].

6. LSP Synchronization

An RBridge participates in LSP synchronization on a link as soon as it has at least one adjacency on that link that has advanced to at least the 2-Way state [RFC7177]. On a LAN link, CSNPs and PSNPs are used for synchronization. On a point-to-point link, only PSNPs are used.

The CSNPs and PSNPs can be formatted in chunks of size (at most) link-wide Lz but are processed normally if received having a larger size. Since the link MTU size may not have been tested in the 2-Way state, link-wide Lz may be greater than the supported link MTU size. In that case, a CSNP or PSNP may be discarded. After the link MTU size is successfully tested, RBridges will begin to format these PDUs with a size no greater than that MTU; therefore, these PDUs will eventually get through.

Note that the link MTU size is frequently greater than Sz. Link-local PDUs are limited in size by the link MTU size rather than Sz, which, when Lz is greater than Sz, promises a reduction in the number of PDUs and a faster LSP synchronization process.

7. Recommendations for Traffic Link Testing of MTU Size

Sz and link-wide Lz are used to limit the size of most TRILL IS-IS PDUs. They are different from the MTU size restricting the size of TRILL Data packets. The size of a TRILL Data packet is restricted by the physical MTU of the ports and links the packet traverses. It is possible that a TRILL Data packet successfully gets through the campus but its size is greater than Sz or link-wide Lz values.

The algorithm defined for testing the link MTU size can also be used in TRILL traffic MTU size testing; in that case, the link-wide Lz used in that algorithm is replaced by the port MTU of the RBridge sending MTU-probes. The successfully tested size x MAY be advertised as an attribute of this link, using the MTU sub-TLV defined in [RFC7176].

Unlike RBridges, end stations do not participate in the exchange of TRILL IS-IS PDUs; therefore, they cannot grasp the traffic link MTU size from a TRILL campus automatically. An operator may collect these values using network management tools such as TRILL ping or TraceRoute. Then, the path MTU can be set as the smallest tested link MTU on this path, and end stations should not generate frames that -- when encapsulated as TRILL Data packets -- exceed this path MTU.

8. Backward Compatibility

There can be a mixture of Lz-ignorant and Lz-aware RBridges on a link. This configuration will behave properly, although it may not be as efficient as it would be if all RBridges on the link are Lz aware.

For an Lz-ignorant RBridge, TRILL IS-IS PDUs are always formatted no greater than Sz. Lz-aware RBridges as receivers can handle these PDUs, since they cannot be greater than the link-wide Lz.

For an Lz-aware RBridge, in the case that link-wide Lz is greater than Sz, larger link-local TRILL IS-IS PDUs can be sent out to increase efficiency. Lz-ignorant RBridges as receivers will have no problem handling them, since the originatingL1LSPBufferSize value of these RBridges had been tested and the link-wide Lz is not greater than that value.

An Lz-ignorant RBridge might not support the link MTU size-testing algorithm defined in Section 3 but could be using some algorithm just to test for the Sz MTU on the link. In any case, if an RBridge per [RFC6325] receives an MTU-probe, it MUST respond with an MTU-ack padded to the same size as the MTU-probe.

9. Security Considerations

This document raises no significant new security issues for TRILL. In TRILL, RBridges are generally considered to be trusted devices. Protection against forged TRILL IS-IS PDUs, including forged Hellos containing originatingSNPBufferSize APPsub-TLVs, can be obtained through IS-IS PDU cryptographic authentication [RFC5310]. The worst that an RBridge can do by reporting an erroneous originatingSNPBufferSize is reduce Lz to Sz and thus make unavailable the optimization of being able to use link MTUs that exceed the campus-wide MTU for link-local TRILL IS-IS PDUs.

   For general and adjacency-related TRILL security considerations, see
   [RFC6325] and [RFC7177].

10. Additions to Configuration

Implementation of the features specified in this document adds two RBridge configuration parameters, as follows:

10.1. Per-RBridge Configuration

Each RBridge implementing the RECOMMENDED LSP resizing damping strategy specified in Section 4 has an LSPresizeTime parameter that is an integer in the range of 0-65,535 and that defaults to 300. It is the number of seconds for which an RBridge determines that Sz has increased before it will create any LSP or E-L1FS FS-LSP fragments.

10.2. Per-RBridge Port Configuration

Each RBridge port on which the calculation and use of Lz are implemented has an originatingL1SNPBufferSize parameter that is an integer in the range of 1470-65,535. This parameter defaults to the minimum of the size that the port can accommodate and the link-local IS-IS PDU size that the TRILL implementation can accommodate.

11. IANA Considerations

IANA has assigned a new APPsub-TLV type for the TRILL originatingSNPBufferSize APPsub-TLV defined in Section 2 of this document. This new type has been assigned from the range less than 256 in the "TRILL APPsub-TLV Types under IS-IS TLV 251 Application Identifier 1" registry. The entry is as follows:

      Type  Name                      Reference
      ----  ------------------------  ---------
      21    originatingSNPBufferSize  RFC 8249

12. References

12.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310,
              February 2009, <>.
   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
   [RFC7176]  Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
              D., and A. Banerjee, "Transparent Interconnection of Lots
              of Links (TRILL) Use of IS-IS", RFC 7176,
              DOI 10.17487/RFC7176, May 2014,
   [RFC7177]  Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., and
              V. Manral, "Transparent Interconnection of Lots of Links
              (TRILL): Adjacency", RFC 7177, DOI 10.17487/RFC7177,
              May 2014, <>.
   [RFC7356]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,
   [RFC7357]  Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
              Stokes, "Transparent Interconnection of Lots of Links
              (TRILL): End Station Address Distribution Information
              (ESADI) Protocol", RFC 7357, DOI 10.17487/RFC7357,
              September 2014, <>.
   [RFC7780]  Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
              Ghanwani, A., and S. Gupta, "Transparent Interconnection
              of Lots of Links (TRILL): Clarifications, Corrections, and
              Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,

12.2. Informative References

   [IS-IS]    International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
              November 2002.
   [RFC8139]  Eastlake 3rd, D., Li, Y., Umair, M., Banerjee, A., and F.
              Hu, "Transparent Interconnection of Lots of Links (TRILL):
              Appointed Forwarders", RFC 8139, DOI 10.17487/RFC8139,
              June 2017, <>.


The authors would like to thank Vishwas Manral for his comments and suggestions.

Authors' Addresses

   Mingui Zhang
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095
   Phone: +86-13810702575
   Xudong Zhang
   Huawei Technologies
   No. 156 Beiqing Rd. Haidian District
   Beijing  100095


   Donald Eastlake 3rd
   Huawei Technologies
   155 Beaver Street
   Milford, MA  01757
   United States of America
   Phone: +1-508-333-2270
   Radia Perlman
   Dell EMC
   505 1st Ave South
   Seattle, WA  98104
   United States of America
   Somnath Chatterjee
   Cisco Systems
   SEZ Unit, Cessna Business Park
   Outer Ring Road
   Bangalore  560087