  United States Court of Appeals
      for the Federal Circuit
               ______________________

            ARISTA NETWORKS, INC.,
                   Appellant

                         v.

              CISCO SYSTEMS, INC.,
                   Cross-Appellant
               ______________________

                2017-1525, 2017-1577
               ______________________

   Appeals from the United States Patent and Trade-
mark Office, Patent Trial and Appeal Board in No.
IPR2015-00978.
               ______________________

             Decided: November 9, 2018
              ______________________

    LAUREN ANN DEGNAN, Fish & Richardson, PC, Wash-
ington, DC, and MATTHEW D. POWERS, Tensegrity Law
Group LLP, Redwood Shores, CA, argued for appellant.
Also represented by MICHAEL J. MCKEON, ADAM
SHARTZER, LINHONG ZHANG, Fish & Richardson, PC,
Washington, DC; ROBERT LEWIS GERRITY, WILLIAM P.
NELSON, Tensegrity Law Group LLP, Redwood Shores,
CA.

   JOHN C. O’QUINN, Kirkland & Ellis LLP, Washington,
DC, argued for cross-appellant. Also represented by
CALVIN ALEXANDER SHANK, JASON M. WILCOX, WILLIAM H.
2               ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



BURGESS; JON WRIGHT, DANIEL S. BLOCK, LORI A. GORDON,
Sterne Kessler Goldstein & Fox, PLLC, Washington, DC.
                ______________________

      Before PROST, Chief Judge, SCHALL and CHEN,
                     Circuit Judges.
PROST, Chief Judge.
    Arista Networks, Inc. (“Arista”) petitioned for an inter
partes review (“IPR”) of certain claims of U.S. Patent No.
7,340,597 (“the ’597 patent”), which is owned by Cisco
Systems, Inc. (“Cisco”). After instituting an IPR, the
Patent Trial and Appeal Board (“Board”) upheld some of
those challenged claims as patentable but invalidated
others. Both Arista and Cisco appeal various aspects of
the Board’s decision. Having considered the parties’
arguments, we reverse and remand as to Arista’s appeal,
and we affirm Cisco’s cross-appeal.
                             I
    Computer networks are made up of various network
devices (e.g., computers, servers, routers, and switches)
that are connected to each other. Within a network,
devices can easily access information and services provid-
ed by other devices in the network. This convenience of
access has drawbacks, however, as it increases the risk of
an external attack on one or more network devices. For
example, if an external attacker compromises one net-
work device, the security of all connected network devices
is threatened. As explained below, the patent at issue in
this case relates to securing network devices from such
attacks by using a logging module to communicate any
configuration changes to a device.
                             A
   The ’597 patent, titled “Method and Apparatus for
Securing a Communications Device using a Logging
Module,” relates generally to ensuring network device
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.             3



security by using a logging module with restricted config-
urability to detect and communicate changes to a network
device’s configuration. ’597 patent col. 2 ll. 35–37, 45–47,
col. 3 ll. 43–49.
    The ’597 patent includes four independent claims.
Claim 1 and dependent claim 29 are illustrative. Claim 1
states:
    1. An apparatus comprising:
        a communications device comprising:
            a subsystem; and
            a logging module, coupled to said subsys-
            tem, and configured to detect a change to
            a configuration of said subsystem of said
            communications device, and communicate
            information regarding said change to said
            configuration of said subsystem of said
            communications device.
Id. at claim 1. Claim 29 states:
    29. The communications device of claim 1, where-
    in the logging module is configured to communi-
    cate the change to the configuration of the
    subsystem by broadcasting the change to the con-
    figuration of the subsystem.
Id. at claim 29.
    In one embodiment, a network communications device
includes a “logging module” that monitors and reports
configuration changes. Id. at col. 3 ll. 43–48, col. 6 ll. 7–
10. When the logging module detects a configuration
change, it can indicate that change in various ways, for
example, by way of “an indicator lamp, a message to a
display, a message to another network device, broadcast
message to specially-configured security devices, or other
such mechanisms.” Id. at col. 7 ll. 25–30.
4               ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



      In some embodiments, the logging module communi-
cates a configuration change by “broadcast[ing] the
change in the configuration of communications interface
. . . to one or more security monitors on the network.” Id.
at col. 7 ll. 39–41; see also id. at col. 8 ll. 52–54. Such
broadcasting occurs by way of a multicast address. Id. at
col. 11 ll. 45–50. To monitor these broadcasts, in some
embodiments, a given security monitor must “subscribe to
this multicast address.” Id. at col. 11 ll. 50–51, col. 13
ll. 57–67 (describing the process of configuring a security
monitor, including “subscribing to a logging module’s
multicast address in order to receive broadcasts from the
logging module”).
                            B
    The named inventor of the ’597 patent is Dr. David
Cheriton, who at the time of the invention was employed
by Cisco as a technical advisor and chief product archi-
tect. Dr. Cheriton assigned “the entire right, title and
interest throughout the world in [his] invention” to Cisco,
requesting that the U.S. Patent and Trademark Office
(“PTO”) “issue all patents granted for said invention” to
Cisco. J.A. 598; see also ’597 patent (listing Cisco as the
assignee). In the same assignment document, Dr. Cheri-
ton agreed “generally to do everything possible to aid said
assignee, their successors, assigns and nominees, at their
request and expense, in obtaining and enforcing patents
for said invention in all countries.” J.A. 598. Cisco com-
pensated Dr. Cheriton for his employment and, according
to Cisco, provided additional compensation for the as-
signment of inventions he developed during his tenure at
Cisco.
    Shortly thereafter, Dr. Cheriton and at least thirteen
other Cisco employees left Cisco to found Arista. Dr.
Cheriton served as Arista’s Chief Scientist for several
years. He also served as a director of Arista and was one
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.            5



of its largest shareholders. He resigned from Arista in
March 2014.
                              C
    In response to Arista’s IPR petition, which Arista filed
on April 1, 2015, the Board instituted review of certain
claims of the ’597 patent. In its Final Written Decision,
the Board upheld claims 29, 63, 64, 73, and 86 as patent-
able, but invalidated claims 1, 14, 39–42, 71, 72, 84, and
85 as anticipated or obvious. Arista Networks, Inc. v.
Cisco Sys., Inc., IPR2015-00978, Paper 32 at 26 (P.T.A.B.
Sept. 28, 2016) (“Final Written Decision”). In doing so, the
Board declined to apply the doctrine of assignor estoppel,
which in Cisco’s view should have prevented Arista from
challenging the patent’s validity.
    Arista timely appealed with respect to the claims
upheld by the Board, and Cisco timely cross-appealed
regarding the invalidated claims. We have jurisdiction
pursuant to 28 U.S.C. § 1295(a)(4)(A).
                              II
    In its appeal, Arista contends that the Board erred in
construing the term “broadcast,” and that this error
caused the Board to improperly reject Arista’s obvious-
ness challenge to claims 29, 63, 64, 73, and 86 of the ’597
patent. Meanwhile, Cisco’s cross-appeal requires us to
address the reviewability of the Board’s refusal to apply
the doctrine of assignor estoppel and, if reviewable,
whether the Board should have analyzed the merits of
Cisco’s arguments regarding assignor estoppel.          We
address each issue in turn.
                              A
    On appeal, Arista argues that the Board erred in its
construction of the term “broadcast” as used in the ’597
6              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



patent. 1 The parties dispute the construction of “broad-
cast” as used the ’597 patent’s claims. The Board con-
strued the term as requiring a broadcast transmission to
be delivered to all network devices. Both parties argue
that the Board’s construction was incorrect. 2
    The ultimate construction of a claim is a question of
law reviewed de novo. SimpleAir, Inc. v. Sony Ericsson
Mobile Commc’ns AB, 820 F.3d 419, 425 (Fed. Cir. 2016).
Where the Board looks beyond the intrinsic evidence and
consults extrinsic evidence, we review the Board’s subsid-
iary factual findings concerning such extrinsic evidence



    1   Cisco argues that we need not decide the claim
construction issue presented on appeal because Arista
allegedly waived its argument that European Patent
Application No. 1,033,844 A2, published Sept. 6, 2000
(“Iwayama”), discloses “broadcasting” under the construc-
tion Arista proposes on appeal. Specifically, Cisco con-
tends that Arista’s IPR petition argued that Iwayama
discloses “broadcasting” only in the context of the claim
construction the Board subsequently adopted (i.e., a
transmission to all devices), but did not present Iwayama
in the context of Arista’s proposed construction (a trans-
mission to one or more devices). But Arista’s petition
explained that Iwayama describes “a variety of broadcast-
ing scenarios,” including “a mailing list system that
distributes mail in a broadcasting manner to a predeter-
mined group of email addresses,” as well as “transmitting
a notification to all other members of the group in a
broadcasting manner.”       J.A. 70 (IPR Pet.) (emphasis
added); see also J.A. 97. We decline to find waiver in
these circumstances.
    2   Although Cisco did not appeal the claim construc-
tion, Cisco proposes an alternative construction in re-
sponse to Arista’s appeal challenging the claim
construction. Cross-Appellant’s Br. 77–85.
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.            7



for substantial evidence. Praxair Distribution, Inc. v.
Mallinckrodt Hosp. Prods. IP Ltd., 890 F.3d 1024, 1031
(Fed. Cir. 2018); cf. Teva Pharm. USA, Inc. v. Sandoz,
Inc., 135 S. Ct. 831, 841 (2015).
    Before the Board, Arista argued that “broadcast”
should be construed as “a transmission to one or more
devices.” J.A. 65 (IPR Pet.). Meanwhile, Cisco proposed
construing the term to mean “transmitting data to one or
more devices without specifying what device(s) will ulti-
mately receive the data.” J.A. 531 (Cisco’s Prelim. Resp.).
    The Board, in its Institution Decision, “decline[d] to
adopt either party’s proposed construction.” Arista Net-
works, Inc. v. Cisco Sys, Inc., IPR2015-00978, Paper 7 at 6
(P.T.A.B. Oct. 6, 2015) (“Institution Decision”). Instead,
the Board construed “broadcast” according to what it
concluded was the “ordinary and customary meaning to
one of skill in the art”—namely, “a transmission of a
message simultaneously to all destinations in a network.”
Id.
    After receiving the construction provided in the Insti-
tution Decision, Cisco argued that the Board’s construc-
tion would exclude the multicast address embodiment of
broadcasting described in the patent. 3 J.A. 1231–32
(Cisco’s Resp.). Cisco reiterated this concern during the
oral hearing. J.A. 4083 l. 23–4084 l. 3, 4084 ll. 10–12
(Hr’g Tr.). For its part, Arista’s counsel stated during the
oral hearing that Arista would “accept” the Board’s con-
struction and “not continue to maintain [Arista’s] con-



   3    Cisco also argued that the Board’s construction
was inconsistent with the plain and ordinary meaning of
“broadcasting” because a fundamental aspect of broad-
casting is not specifying the ultimate destination of a
message. J.A. 1228–29 (Cisco’s Resp.); see also J.A. 1324
¶ 103 (Decl. of Cisco’s expert, Dr. Stephen Wicker).
8               ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



struction” if the Board’s construction “includes the only
disclosure of broadcasting in the ’597 patent, which is
using a multicast address.” J.A. 4052 ll. 2–9 (Hr’g Tr.).
Despite the concern raised by both parties regarding
excluding the multicast address embodiment, the Board’s
Final Written Decision maintained the Board’s same
construction. Final Written Decision at 7–8. 4
    In an IPR, “[a] claim in an unexpired patent that will
not expire before a final written decision is issued shall be
given its broadest reasonable construction in light of the
specification of the patent in which it appears.” 37 C.F.R.
§ 42.100(b) (2017); Cuozzo Speed Techs., LLC v. Lee, 136
S. Ct. 2131, 2146 (2016). 5 Under this standard, “words of
the claim must be given their plain meaning, unless such
meaning is inconsistent with the specification and prose-
cution history.” Trivascular, Inc. v. Samuels, 812 F.3d



    4   Although the Board stated that “[a]t oral hearing,
[Arista] indicated its lack of objection to our construction
in the Decision to Institute,” Final Written Decision at 7,
Arista’s statement cannot be viewed as agreeing with the
Board’s construction, as the Board’s construction required
a message to be transmitted to all destinations in a
network. Arista specifically stated during the hearing
that the construction should include “the only disclosure
of broadcasting in the ’597 patent, which is using a mul-
ticast address.” J.A. 4052 ll. 4–5.
    5   We note that the PTO recently changed the claim
construction standard used in IPR proceedings. See
Changes to the Claim Construction Standard for Inter-
preting Claims in Trial Proceedings Before the Patent
Trial and Appeal Board, 83 Fed. Reg. 51,340, 51,340 (Oct.
11, 2018) (to be codified at 37 C.F.R. pt. 42). As stated in
the Federal Register notice, however, the new rule applies
only to petitions filed on or after November 13, 2018, and
therefore does not impact this case. Id.
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.            9



1056, 1062 (Fed. Cir. 2016). Moreover, we have held that
“[e]ven when guidance is not provided in explicit defini-
tional format, the specification may define claim terms by
implication such that the meaning may be found in or
ascertained by a reading of the patent documents.” In re
Abbott Diabetes Care Inc., 696 F.3d 1142, 1150 (Fed. Cir.
2012) (quoting Irdeto Access, Inc. v. Echostar Satellite
Corp., 383 F.3d 1295, 1300 (Fed. Cir. 2004)); see also
Homeland Housewares, LLC v. Whirlpool Corp., 865 F.3d
1372, 1377 (Fed. Cir. 2017) (“[W]hen a patentee uses a
claim term throughout the entire patent specification, in a
manner consistent with only a single meaning, he has
defined that term ‘by implication.’” (quoting Bell Atl.
Network Servs., Inc. v. Covad Commc’ns Grp., Inc., 262
F.3d 1258, 1271 (Fed. Cir. 2001))), cert. denied, 138 S. Ct.
2602 (2018).
    In this case, both parties agree that: (1) the patent
includes multicasting via a multicast address as a type of
broadcasting; (2) this is the only disclosed example of
broadcasting in the ’597 patent; (3) this does not neces-
sarily transmit a message to all destinations on a net-
work; and (4) the Board’s construction, which requires a
transmission to all network destinations, therefore ex-
cludes this embodiment.
    The specification supports the parties’ understanding
of the term “broadcast” as used here. First, the specifica-
tion points to multicasting using a multicast address as
the only example of broadcasting. It explains that “[t]he
multicast address is used by the logging module to broad-
cast security information (such as changes in the configu-
ration of the network device) to the one or more security
monitors on the network.” ’597 patent col. 11 ll. 46–50
(emphasis added). The specification also makes clear that
this does not necessarily require transmitting a message
to all network devices, given that “[a] security monitor
must subscribe to this multicast address in order to
monitor the logging module’s broadcasts.” Id. at col. 11
10              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



ll. 50–51 (emphasis added); see also J.A. 1306 ¶ 64 (Decl.
of Cisco’s expert, Dr. Stephen Wicker) (“Multicasting
differs from network broadcasting in that a multicast
message is not necessarily sent to all hosts on a network,
and instead can be sent to a group of hosts.”). We there-
fore agree with the parties that the Board’s construction
cannot stand. Not only does it ignore the patentee’s use of
“broadcast,” it excludes the only disclosed embodiment of
broadcasting (i.e., multicasting by way of a multicast
address), which allows for transmission to fewer than all
devices. See GE Lighting Sols., LLC v. AgiLight, Inc., 750
F.3d 1304, 1311 (Fed. Cir. 2014) (“We normally do not
construe claims in a manner that would exclude the
preferred embodiment, especially where it is the only
disclosed embodiment.”).
    With this in mind, we are nonetheless faced with
competing proposed constructions on appeal.        Arista
contends that “broadcast” should be construed to mean “a
transmission to one or more devices.” Meanwhile, Cisco
proposes: “transmitting data to one or more devices
without specifying which devices will ultimately receive
the data.” Cross-Appellant’s Br. 77 (emphasis added). We
address each construction in turn.
    Arista argues that its proposed construction—“a
transmission to one or more devices”—tracks the lan-
guage of the specification. Appellant’s Br. 34 (citing, e.g.,
’597 patent col. 11 ll. 45–50 (“The multicast address is
used by the logging module to broadcast security infor-
mation . . . to the one or more security monitors on the
network.” (emphasis in Arista’s brief))). While this is
true, Arista’s construction is still too broad. As the Board
noted, this construction could encompass many types of
network transmissions, such as simply sending an email.
See Institution Decision at 6. We therefore reject Arista’s
construction as being overly broad, even under the broad-
est reasonable interpretation standard.
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.             11



     Cisco’s proposed construction fares only slightly bet-
ter. Cisco contends that its construction reflects the key
difference between a broadcast and other network com-
munications—namely, that in a broadcast, the sender
does not know who ultimately receives the sender’s
transmission. Specifically, Cisco explains that because
intermediate network devices maintain the lists of devices
subscribed to a particular group address, the device that
initially sends a broadcast to that group address will not
know which devices ultimately receive the broadcast. But
we are not convinced that the additional limitation Cisco
proposes should be incorporated into the construction.
Cisco’s argument relies heavily on its expert’s testimony,
but that testimony is not based on the ’597 patent’s speci-
fication. In short, nothing in the specification suggests
that Cisco’s proposed negative limitation is a required
part of the contemplated “broadcast.” We therefore see no
reason to limit the term as Cisco proposes.
     With this in mind, we reject Cisco’s proposed limita-
tion and instead construe the term based on the specifica-
tion’s consistent focus on broadcasting via a multicast
address. See ’597 patent col. 11 ll. 45–51, col. 14 ll. 15–17,
col. 15 l. 66–col. 16 l. 1. We therefore construe the terms
“broadcast” and “broadcasting” to mean, respectively, “a
transmission to one or more devices using a multicast
address” and “transmitting to one or more devices using a
multicast address.” 6
   Given that the Board conducted its patentability
analysis under an incorrect claim construction, we find it
appropriate to remand this case for the Board to consider,


    6    Although the ’597 patent was the subject of a re-
lated appeal from the International Trade Commission
(“ITC”), this court’s opinion in that case did not address
the construction of the term “broadcast.” See Cisco Sys.,
Inc. v. ITC, 873 F.3d 1354 (Fed. Cir. 2017).
12              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



in the first instance, the patentability of the claims raised
in Arista’s appeal, in light of the construction provided
herein.
                             B
    In its cross-appeal, Cisco argues that the Board’s deci-
sion to invalidate some claims should be reversed based
on assignor estoppel. In Cisco’s view, the facts presented
here represent a textbook case of assignor estoppel.
Indeed, in the related ITC investigation, the ITC held
that assignor estoppel barred Arista’s challenge to the
’597 patent’s validity. See In re Certain Network Devices,
Related Software and Components Thereof (I), Inv. No.
337-TA-944, USITC Pub. 575521 at 132 (Feb. 2, 2016)
(Initial Determination), available at Fed. Cir. Appeal No.
16-2563, ECF No. 133 (App. 888.143–888.149), aff’d, 873
F.3d 1354 (Fed. Cir. 2017). 7
                             1
    Before reaching the question of whether assignor es-
toppel should apply in the IPR context, we must first
ensure that this issue is reviewable in light of the Su-
preme Court’s analysis of 35 U.S.C. § 314(d) in Cuozzo
Speed Technologies v. Lee, 136 S. Ct. 2131 (2016), and this
court’s recent en banc application of Cuozzo in Wi-Fi One,
LLC v. Broadcom Corp., 878 F.3d 1364 (Fed. Cir. 2018)
(en banc).
    Under § 314(d), “[t]he determination by the Director
whether to institute an inter partes review under this
section shall be final and nonappealable.” In Cuozzo, the
Supreme Court analyzed this “no appeal” provision in the
context of a challenge to the Board’s decision to institute
review of two claims based on a certain prior art combina-



     7 This court’s affirmance in Cisco Systems, Inc. v.
ITC did not address assignor estoppel. 873 F.3d at 1364.
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.            13



tion, where the challenger raised that combination with
respect to a third, dependent claim. Cuozzo, 136 S. Ct. at
2138–39. There, the patent owner argued that by insti-
tuting review on grounds not cited for those claims, but
instead based on a conclusion that the petitioner had
implicitly challenged the claims on that ground, the Board
violated the requirement that an IPR petition “may be
considered only if . . . the petition identifies, in writing
and with particularity, each claim challenged, the
grounds on which the challenge to each claim is based,
and the evidence that supports the grounds for the chal-
lenge to each claim.” § 312(a)(3).
    In finding the Board’s institution decision unreviewa-
ble, the Court explained that “where a patent holder
merely challenges the Patent Office’s ‘determin[ation]
that the information presented in the petition . . . shows
that there is a reasonable likelihood’ of success ‘with
respect to at least 1 of the claims challenged,’ § 314(a), or
where a patent holder grounds its claim in a statute
closely related to that decision to institute inter partes
review, § 314(d) bars judicial review.” Cuozzo, 136 S. Ct.
at 2142 (emphasis added). The Court reasoned that the
challenge in that case—that the petition was not pleaded
“with particularity” under § 312—was “little more than a
challenge to the Patent Office’s conclusion, under § 314(a),
that the ‘information presented in the petition’ warranted
review.” Id. The Court also noted, however, that it was
not deciding “the precise effect of § 314(d) on appeals that
implicate constitutional questions, that depend on other
less closely related statutes, or that present other ques-
tions of interpretation that reach, in terms of scope and
impact, well beyond ‘this section.’” Id. at 2141 (emphasis
added).
    Just three months after Cuozzo, a panel of this court
in Husky applied the decision to the question of whether
assignor estoppel could bar a party from filing an IPR
petition. Husky Injection Molding Sys. Ltd. v. Athena
14               ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



Automation Ltd., 838 F.3d 1236, 1241 (Fed. Cir. 2016).
The panel concluded that “the question whether assignor
estoppel applies in full force at the [PTO] does not fall into
any of the three categories the Supreme Court specifically
mentioned as reviewable,” including that the question
does not “depend on other less closely related statutes.”
Id. at 1245. In doing so, the panel explained that this
question “necessarily requires an interpretation of 35
U.S.C. § 311(a),” and that an interpretation of § 311 “to
either include or foreclose assignor estoppel is very ‘closely
related’ to any decision to initiate inter partes review.” Id.
at 1246 (emphasis in original). 8
     Since Husky, however, this court, sitting en banc, ad-
dressed the reviewability standard articulated in Cuozzo.
Wi-Fi One, 878 F.3d at 1372–75. In Wi-Fi One, we ex-
plained that Cuozzo held that the bar on judicial review in
§ 314(d) applies where a patent owner challenges the
Board’s determination under § 314(a) that there is a
“reasonable likelihood” of success with respect to at least
one claim, as well as where a patent owner challenges the
Board’s determination based on a statute “closely related
to that decision to institute inter partes review.” Id. at
1373 (emphasis in Wi-Fi One) (quoting Cuozzo, 136 S. Ct.
at 2142); see id. at 1370 (noting that Cuozzo “tied the
‘closely related’ language to the specific ‘reasonable likeli-
hood’ determination made under § 314(a)”); see also SAS
Inst., Inc. v. Iancu, 138 S. Ct. 1348, 1359 (2018) (“Cuozzo
concluded that § 314(d) precludes judicial review only of
the Director’s ‘initial determination’ under § 314(a) that



     8   Section 311(a) states, in relevant part:
     (a) In General.—Subject to the provisions of this
     chapter, a person who is not the owner of a patent
     may file with the Office a petition to institute an
     inter partes review of the patent. . . .
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.              15



‘there is a “reasonable likelihood” that the claims are
unpatentable on the grounds asserted’ and review is
therefore justified.” (quoting Cuozzo, 136 S. Ct. at 2140)). 9
In short, we are to “examine the statutory scheme in
terms of what is ‘closely related’ to the § 314(a) determi-
nation.” Wi-Fi One, 878 F.3d at 1373. Looking to
§ 314(a), we then explained that § 314(a) “does only two
things: it identifies a threshold requirement for institu-
tion, and . . . it grants the Director discretion not to insti-
tute even when the threshold is met.” Id. at 1372 (citing
Cuozzo, 136 S. Ct. at 2140). This “threshold” requirement
“focuse[s] on the patentability merits of particular
claims.” Id. We thus concluded that Cuozzo “strongly
points toward unreviewability being limited to the Direc-
tor’s determinations closely related to the preliminary
patentability determination or the exercise of discretion
not to institute.” Id. at 1373; see also Cuozzo, 136 S. Ct.
at 2140 (noting the “‘strong presumption’ in favor of
judicial review that we apply when we interpret statutes,
including statutes that may limit or preclude review”).
    Husky’s application of Cuozzo’s “less closely related
statutes” exception cannot be reconciled with the reason-
ing of this court’s subsequent decision in Wi-Fi One. We
therefore must rely on the reasoning of the en banc court




    9   For reference, the full text of § 314(a) reads:
    (a) Threshold.—The Director may not authorize
    an inter partes review to be instituted unless the
    Director determines that the information present-
    ed in the petition filed under section 311 and any
    response filed under section 313 shows that there
    is a reasonable likelihood that the petitioner
    would prevail with respect to at least 1 of the
    claims challenged in the petition.
16              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



in Wi-Fi One to analyze whether the issue presented in
this cross-appeal is reviewable.
     Applying the now-governing reasoning in Wi-Fi One,
it is clear that we may review the Board’s decision as to
whether § 311(a) contemplates application of assignor
estoppel. 10 The question of whether assignor estoppel
applies in IPRs stands in stark contrast to the statutory
provision before the Court in Cuozzo, § 312(a)(3), which
deals with pleading an IPR petition with particularity.
Further, unlike the statutory provision at issue in Cuozzo,
assignor estoppel does not relate to the patentability
merits of an IPR petition. See Wi-Fi One, 878 F.3d at
1373–74 (distinguishing the statute in Cuozzo from
§ 315(b) by noting that the § 315(b) time-bar does not go
to the merits of a petition). And, like the § 315(b) time-
bar, assignor estoppel “is not focused on particular claims,
whereas § 314(a)’s threshold determination is.” See id. at
1373 (discussing § 315(b)). Instead, assignor estoppel,
like the § 315(b) time-bar, “is unrelated to the Director’s
preliminary patentability assessment or the Director’s
discretion not to initiate an IPR even if the threshold
‘reasonable likelihood’ is present.” See id. (discussing
§ 315(b)). In short, whether § 311(a) contemplates appli-
cation of assignor estoppel is not “closely related to the
preliminary patentability determination or the exercise of
discretion not to institute.” Id. 11 We therefore conclude



     10 As explained in the next section of this opinion,
we view § 311(a) as dispositive of the question whether
assignor estoppel should apply in the IPR context.
    11  Although Wi-Fi One stated in passing that
§§ 311–13 relate more closely to the Director’s institution
decision, we do not view this broad characterization as
dispositive of the question here. See Wi-Fi One, 878 F.3d
at 1373–74. Instead, it is clear that when referring to
§§ 311–13, the court was referring to the aspects of those
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.           17



that, like the time-bar in § 315(b), the issue of whether
§ 311(a) contemplates application of assignor estoppel is
reviewable.
                               2
    We next turn to the merits of Cisco’s argument that
assignor estoppel should apply in the IPR context. “As-
signor estoppel prevents a party who assigns a patent to
another from later challenging the validity of the assigned
patent.” Mentor Graphics Corp. v. Quickturn Design Sys.,
Inc., 150 F.3d 1374, 1377 (Fed. Cir. 1998); see also West-
inghouse Elec. & Mfg. Co. v. Formica Insulation Co., 266
U.S. 342, 349 (1924) (“[A]n assignor of a patent right is
estopped to attack the utility, novelty or validity of a
patented invention which he has assigned or granted as
against any one claiming the right under his assignment
or grant.”). The doctrine often arises in factual scenarios
similar to the facts of this case, where an employee in-
vents something during his or her tenure with a company,
assigns the rights to that invention to his or her employer,
then leaves the company to join or found a competing
company. See, e.g., Westinghouse, 266 U.S. at 345–46;
Shamrock Techs., Inc. v. Med. Sterilization, Inc., 903 F.2d
789, 790 (Fed. Cir. 1990); Diamond Sci. Co. v. Ambico,
Inc., 848 F.2d 1220, 1222 (Fed. Cir. 1988). In such situa-
tions, the employee’s new company may be estopped


sections that “relate to the Director’s ability to make an
informed preliminary patentability determination pursu-
ant to § 314(a).” Id. at 1374 (“The time-bar provision
contrasts with many of the preliminary procedural re-
quirements stated in §§ 311–13, which relate to the
Director’s ability to make an informed preliminary pa-
tentability determination pursuant to § 314(a).”). As we
have explained above, determining whether a particular
petitioner may petition for inter partes review does not
relate to the Board’s patentability analysis.
18              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



because “[a]ssignor estoppel also prevents parties in
privity with an estopped assignor from challenging the
validity of the patent.” Mentor Graphics, 150 F.3d at
1379; see also Diamond Sci., 848 F.2d at 1224 (“The
estoppel also operates to bar other parties in privity with
the assignor, such as a corporation founded by the assign-
or.”).
     As discussed above, Dr. Cheriton invented the subject
matter of the ’597 patent while employed by Cisco, and he
assigned his rights to that invention to Cisco. He then
left Cisco to co-found Arista, a competitor to Cisco.
    Despite the factual similarities between this case and
our other assignor estoppel cases, the Board declined to
apply the doctrine in this case, suggesting that the doc-
trine is unavailable in the IPR context. Institution Deci-
sion at 7–8; Final Written Decision at 10–11. The Board
provided two rationales for declining to apply assignor
estoppel.   First, it quoted a now-precedential Board
decision, which held that § 311(a) “presents a clear ex-
pression of Congress’s broad grant of the ability to chal-
lenge the patentability of patents through inter partes
review.” Institution Decision at 7 (quoting Athena Auto-
mation Ltd. v. Husky Injection Molding Sys. Ltd.,
IPR2013-00290, Paper 18 at 12–13 (P.T.A.B. Oct. 25,
2013)). Second, the Board reasoned that Congress has not
expressly provided for assignor estoppel in the IPR con-
text, where it has in other contexts. Institution Decision
at 8 (citing Redline Detection, LLC v. Star Envirotech,
Inc., IPR2013-00106, Paper 40 at 4 (P.T.A.B. Oct. 1,
2013)).
                            3
    The question before us, as is often the case, is one of
congressional intent: did Congress intend for assignor
estoppel to apply in IPR proceedings?
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.            19



    Cisco’s primary argument in favor of applying assign-
or estoppel is that assignor estoppel is a well-established
common-law doctrine that should be presumed to apply
absent a statutory indication to the contrary. With this
principle in mind, Cisco particularly takes issue with the
Board’s reasoning that Congress would have expressly
provided for application of equitable defenses if it so
desired. 12
    There is some merit to Cisco’s argument. In Westing-
house, the Court characterized assignor estoppel as “a
rule well settled by 45 years of judicial consideration and
conclusion” in the district and circuit courts, reaching
back as early as 1880. Westinghouse, 266 U.S. at 349; see
also Scott Paper Co. v. Marcalus Mfg. Co., 326 U.S. 249,
260 (1945) (Frankfurter, J., dissenting) (“The principle of
fair dealing as between assignor and assignee of a patent
whereby the assignor will not be allowed to say that what
he sold as a patent was not a patent had been part of the
fabric of our law throughout the life of this nation.”). But,
in Lear, Inc. v. Adkins, the Supreme Court appeared to
cast some doubt on the doctrine’s continued viability. 395
U.S. 653, 664–66 (1969). And although this court has
held that the doctrine survived Lear, we did so recogniz-
ing that court decisions post-Lear “reveal[ed] some uncer-
tainty about the continued vitality of the doctrine.”
Diamond Sci., 848 F.2d at 1223; see also id. (“Although
Lear involved the licensing, rather than the assignment,
of a patent, the opinion reviewed the history of ‘patent
estoppel’ in general, and indicated that the Court’s previ-



    12   Although Cisco contends that this was the Board’s
only basis for declining to apply assignor estoppel in the
IPR context, this is not so. As noted above, the Board also
pointed to earlier Board decisions that explained that
§ 311(a) allows any person who is not the patent owner to
file an IPR petition.
20              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



ous decisions had sapped much of the vitality, if not the
logic, from the assignment estoppel doctrine as well.”).
      With this history in mind, we recognize that “Con-
gress is understood to legislate against a background of
common-law adjudicatory principles.” Astoria Fed. Sav.
& Loan Ass’n v. Solimino, 501 U.S. 104, 108 (1991).
“Thus, where a common-law principle is well established,
. . . the courts may take it as given that Congress has
legislated with an expectation that the principle will
apply except ‘when a statutory purpose to the contrary is
evident.’” Id. (quoting Isbrandtsen Co. v. Johnson, 343
U.S. 779, 783 (1952)); see also Impression Prods., Inc. v.
Lexmark Int’l, Inc., 137 S. Ct. 1523, 1536 (2017). But see
United States v. Craft, 535 U.S. 274, 288 (2002) (“The
common-law rule was not so well established . . . that we
must assume that Congress considered the impact of its
enactment on the question now before us.”). But even
assuming that assignor estoppel could be considered such
a well-established common law principle, we nonetheless
conclude that, here, “a statutory purpose to the contrary
is evident.” Astoria, 501 U.S. at 108 (quoting Isbrandtsen,
343 U.S. at 783). In particular, we view § 311(a) as gov-
erning the question of whether Congress intended assign-
or estoppel to apply in the IPR context. 13



     13  Cisco briefly attempts to draw a distinction be-
tween filing an IPR petition under § 311(a) and obtaining
relief on the merits under § 316(e), arguing that
“[a]ssignor estoppel does not prevent Arista from filing a
petition under § 311(a), just as it would not prevent Arista
from filing an invalidity counterclaim in district-court
litigation. Instead, assignor estoppel simply forecloses
relief on the merits, preventing Arista from meeting its
burden under § 316(e) . . . .” Cross-Appellant’s Br. 48. We
are unconvinced. Section 316(e) states, in full: “(e) Evi-
dentiary Standards.—In an inter partes review instituted
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.           21



    Section 311(a) states, in relevant part: “(a) In Gen-
eral.—Subject to the provisions of this chapter, a person
who is not the owner of a patent may file with the Office a
petition to institute an inter partes review of the pa-
tent. . . .” § 311(a) (emphasis added).
    Arista contends that § 311(a) unambiguously leaves
no room for assignor estoppel in the IPR context, given
that the statute allows any person “who is not the owner
of a patent” to file an IPR. 14        We agree.   Where
“the statutory language is plain, we must enforce it ac-
cording to its terms.” King v. Burwell, 135 S. Ct. 2480,
2489 (2015); see also Hardt v. Reliance Standard Life Ins.
Co., 560 U.S. 242, 251 (2010). In our view, the plain
language of this statutory provision is unambiguous.




under this chapter, the petitioner shall have the burden of
proving a proposition of unpatentability by a preponder-
ance of the evidence.” § 316(e). This section merely sets
forth the IPR petitioner’s burden to prove the merits of its
unpatentability arguments, such as its arguments based
on anticipation or obviousness. Whether a particular
party should be estopped from challenging a patent’s
validity altogether is a separate question from the sub-
stantive patentability analysis that is the focus of
§ 316(e).
    14  Alternatively, Arista contends that even if
§ 311(a) is ambiguous as to the application of assignor
estoppel, we should defer to the Board’s reasonable inter-
pretation of the statute under Chevron, U.S.A., Inc. v.
Natural Resources Defense Council, Inc., 467 U.S. 837
(1984). Cisco, for its part, argues that Chevron deference
should not apply to the Board’s decision in this case. We
need not address whether and to what extent Chevron
deference may apply here, given our holding that the
statute is unambiguous.
22              ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



     Cisco contends that this statute does not directly
speak to the question of assignor estoppel in IPRs. In-
stead, Cisco views § 311(a) as reflecting two principles:
first, that an IPR must begin as an adversarial proceed-
ing, rather than as a means for a patent owner to confirm
the patentability of certain claims; and second, that there
is no Article III-like standing requirement for filing an
IPR. In our view, however, the statute, by its terms, does
more—it delineates who may file an IPR petition. The
plain language of § 311(a) demonstrates that an assignor,
who is no longer the owner of a patent, may file an IPR
petition as to that patent.
    This conclusion is consistent with Congress’s express
incorporation of equitable doctrines in other related
contexts. For example, a statute governing International
Trade Commission investigations states that “[a]ll legal
and equitable defenses may be presented in all cases.” 19
U.S.C. § 1337(c); cf. 15 U.S.C. § 1069 (providing in the
Lanham Act context that “[i]n all inter partes proceedings
equitable principles of laches, estoppel, and acquiescence,
where applicable may be considered and applied”). And
although such express inclusion of equitable defenses in
other contexts is not dispositive of the issue presented in
this case, it is further evidence of congressional intent.
    Finally, Cisco contends that allowing assignor estop-
pel in other forums, such as in the ITC and in district
court, while not allowing it in the IPR context creates an
inconsistency that invites forum shopping. We, however,
do not view this as an inconsistency, but rather as an
intentional congressional choice. Such a discrepancy
between forums—one that follows from the language of
the respective statutes—is consistent with the overarch-
ing goals of the IPR process that extend beyond the par-
ticular parties in a given patent dispute. See Cuozzo, 136
S. Ct. at 2144 (“[I]nter partes review helps protect the
public’s ‘paramount interest in seeing that patent monop-
olies . . . are kept within their legitimate scope.’” (quoting
ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.            23



Precision Instrument Mfg. Co. v. Auto. Maint. Mach. Co.,
324 U.S. 806, 816 (1945))). Moreover, any policy choices
regarding forum shopping are better left to Congress than
to this court. Cf. SAS Inst., 138 S. Ct. at 1358 (“Policy
arguments are properly addressed to Congress, not this
Court.”); Click-to-Call Techs., LP v. Ingenio, Inc., 899 F.3d
1321, 1350 (Fed. Cir. 2018) (Taranto, J., concurring) (“If
there turns out to be a problem in the statute’s applica-
tion according to its plain meaning, it is up to Congress to
address the problem.”).
    In sum, we conclude that § 311(a), by allowing “a per-
son who is not the owner of a patent” to file an IPR,
unambiguously dictates that assignor estoppel has no
place in IPR proceedings.
                               4
    Separately, Cisco contends that the Board’s decision
not to apply assignor estoppel in the IPR context is arbi-
trary and capricious, given that the Board applies other
equitable and non-statutory doctrines (such as collateral
estoppel, prosecution laches, and obviousness-type double
patenting) in the IPR context. In light of our holding that
§ 311(a) expresses Congress’s clear intent that assignor
estoppel should not apply in the IPR context, however,
this argument is unavailing. The Board’s practice is
consistent with the unambiguous language of the statute.
                              III
    For the foregoing reasons, with respect to Cisco’s
cross-appeal, we affirm the Board’s holding that assignor
estoppel does not bar Arista from challenging the validity
of the ’597 patent in this IPR. With regard to Arista’s
appeal, we reverse the Board’s construction of the term
“broadcast” and remand to the Board for further proceed-
ings consistent with the construction provided above.
 AFFIRMED-IN-PART, REVERSED-IN-PART, AND
               REMANDED
24               ARISTA NETWORKS, INC.   v. CISCO SYSTEMS, INC.



                           COSTS
     The parties shall bear their own costs.
